Rapid semi-quantitative method of determining water soluble sulphate in some Saskatchewan soils

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Rapid semi-quantitative method of determining water soluble sulphate

in some Saskatchewan soils

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DIVISION OF BUILDING RESEARCH

A RAPID SEMl-QUANTITATIVE METHOD OF DETERMINING WATER SOLUBLE SULPHATE IN SOME SASKATCHEWAN SOILS

by

R. T. Gardiner

ANAL VZED

Internal Report No. 370 of the

Division of Building Research

OTTAWA June 1969

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To combat s ulphat e attack on underground concrete structures the soluble sulphate ion concentration of the soil must be known in advance of construction. Mr. Gardiner, a soil scientist by training, suggests in this report a simple method whereby soluble sulphates can be evaluated with the required accuracy. The report was prepared while Mr. Gardiner was a Research Officer with the Prairie Regional Station of DBR/NRC at Sa skatchewan.

The study is based on chemical analyses of soils sampled and analyzed in connection with the South Saskatchewan River Irrigation Pro-j ect survey which was carried out Pro-jointly by the Saskatchewan Institute of Pedology, the University of Saskatchewan and PFRA Federal Department of Agriculture. The results were made available to the author to carry out this novel study not envisaged at the planning stage of the survey. The Division of Building Research, NRC, expresses its appreciation to the above agencies for cooperating in this way.

Although the suggestions and correlations in the report are pre-liminary in nature they are thought to point to a useful approach worth pur-suing and hence warrant publication in the Internal Report series of the Di-vision for circulation to interested persons.

Ottawa June

1969

R. F. Legget Director

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SOLUBLE SULPHATE IN SOME SASKATCHEWAN SOILS

by

R. T. Gardiner

The choice of an analytical procedure for m ea s ur irig the water soluble sulphate concentration of soils depends on such factors as the reason for rna k ing the rn e a s u r e rn errts , the nurnb e r of s arrrpl e s to be handled, the cost, and the tirri e and effort available for doing the work. Usually the rn o r e accurate rn e tho d s are tirn e c on s urn.irig and this Ii.mit s the nwnber of de te r m ina ti on s that can be rn a d e , EstiITlation of the po-tential sulphate hazard to concrete of the soils at a construction site would not require a high degree of accuracy. The procedure used should, however, cover a wide range of sulphate concentrations and be s irrip l e and able to be done quickly to facilitate the analysis of large nurrib e r s of s arrip l e s , Water soluble sulphate concentrations are generally highly variable, therefore thorough s arrrpl i ng is necessary throughout the en-tire depth as well as at several locations within the construction site. It would be desirable if in addition, the procedure could be used under field conditions.

The water soluble salts in rn o st prairie soils are p r ed orn in an tly sulphates of c a Ic iurn, rna gn e s iurn, and s odiurn . Chloride is present in rn o st soils but usually only in trace arn ounts , Bicarbonate concentrations ITlay be relatively high in the surface horizons but decrease rapidly to low arn ount s with depth (1,2,3). Knowledge of the fact that sulphate salts constitute a large proportion of soil salinity p r ornpt.ed an investigation of the possible application of electrical conductance as a rn e th od of m ea s uring water soluble sulphate concentrations in soils. Electrical conductance has long been established as a rn e an s of m ea s u r irig soluble salt concen-trations in soils (4,5).

The initial phase of the investigation involved establishing the dOITli-nance of the sulphate anion in the soil solution. This was ac c ornp l i sh ed by rneasuring the rna.g nitude of sulphate and chloride concentrations, relative to bicarbonate, over a wide range of soil salinities. In the second phase a statistical evaluation of the extent of correlation between a specific elec-trical conductance and the sulphate content of a saturation extract was rn a d e , The final phase of the study consisted of the calculation of a standard cali-bration curve, using the available soils data, relating specific electrical

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conductance of a saturation extract (mmhos/cm) to sulphate concentration, the results being expressed as a per cent of soil weight.

LITERATURE REVIEW

A number of analytical procedures for the quantitative determination of sulphate concentrations in soils are now in use; some of the more common methods are discuss ed here briefly. In addition, the electrical resistanc e and conductance methods of determining soil salinity are compared.

QUANTITATIVE METHODS OF DETERMINING SULPHATE IN SOILS The concentration of sulphate in the soil solution is generally deter-mined gravimetrically as BaS0_{4 •} An extract of the soil solution is made slightly acid by the addition of HC1; it is boiled to remove carbonates and

ｂ｡ｃｾ is th en added to cause precipitation (6). Precipitation, digestion,fil-tration, washing, ignition, and weighing constitute a tirnevcon s um ing pro-cedure and the precipitate is subject to contamination as a result of the co-precipitation of calcium, potassium and sodium salts. Because of the lengthy nature of the gravimetric method, more rapid titrimetric and turbi-dimetric procedures have been developed. Some of the volumetric pro-cedures described below have certain flaws such as steps that are too time consuming or inaccuracies in the values obtained with low sulphate ion con-centrations (7).

Jackson (5) and Munger, et al (7) describe a titrimetric procedure employing a standard dis odium dihydrogen ethylendiamine tetraacetate so-lution. Addition of a known concentration of BaC1₂ to the soil solutionre-sults in precipitation of BaS0₄ • The concentration of excess Ba

remain-ing after precipitation, is determined through chelation with Versene; an Erichrome black T indicator is used to determine the endpoint. When using the Ba ion alone the initial endpoint is slow in forming and can easily be passed, thus necessitating a back titration with a known quantity of MgC1_{2 •} One advantage of this method over some others is that it is not necessary to remove the BaS0₄ precipitate before titration. A value of the combined Ca and Mg ion concentrations is required for calculating the sulphate concen-tration. Ca , Ca

+

Mg, and excess Ba must be determined in separate ali-quots and this not only requires large samples but a lot of time. Modifica-tions to this procedure (8) allow for the determination of Ca, Mg and Ba on the s arn e aliquot, which should save time and minimize the amount of sample needed. The accuracy of the sulphate determination depends on the accuracy of the preliminary Ca and Mg determinations, as well as titration after ad-dition of BaC1₂ solution. At high sulphate concentrations, 1 per cent accuracy is possible and reasonable recovery is possible with concentrations as low as 5 ppm. This p r oc .sdu r e is useful when determining Ca, Mg and sulphate.

A second titrimetric method involves the addition of acidified benzidine hydrochloride to a clear soil solution extract (9). Sulphate precipitates as

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benzidine sulphate. The precipitate hydrolyzes to fo r rn I-i:eS04 in an amount equivalent to the sulphate originally present in the sample. The resultant I-i:eS04 is then titrated with standard NaOH using phenolphthalein as the indi-cator. Two disadvantages of this method are a quantitative filtration step and the slight solubility of benzidine sulphate both of which make the pro-cedure inaccurate at low sulphate concentrations.

The direct titration of excess standard BaC12 solution in the presence

of an internal indicator, d i s od iurn tetrahydroxyquinone, is another method available (9). Standard BaC 1₂precipita tes sulphate as BaS0_{4 •} Addition of excess Ba compound produces a color change from yellow to red due to the internal indicator. The successful use of this method depends on the ease with which the operator can detect the endpoint. Pseudo endpoints, a re-sult of slow precipitation of BaS04and the localized formation of the red bari-um salt of the indicator, cause the greatest concern (10). The sharpness of the endpoint may be increased by addition of AgN0₃ (9) but rapid stirring is essential (10). Phosphates interfere thus requiring special treatment when present (9). _{Because of the relatively slow formation of the BaS04} precipi-tate at low sulphate ion concentrations, the technique is limited to use with fairly high sulphate ion concentrations, i ,e., greater than 100 ppm.

Another titrimetric procedure is based on precipitation of sulphate and chromate with barium (11). Barium chromate is added in acid solution and the excess Ba remaining after precipitation of BaS04 is precipitated as chromate by the addition of NaOH to pH 8.3. The excess chromate is then recovered in the filtrate and titrated with thiosulphate. The actual sulphate content is a function of the thiosulphate titer, and evaluation of the sulphate concentration can be made either by comparison with a standard curve or by fo r m.ul a , Some sour, es of error in this method that are reported in the literature include:

1. oxygen errors involved in oxidation of iodide to iodine, 2. time of digestion of the barium chromate precipitate, 3. r eduction of dichromate by chloride acid solution, and 4. co-precipitation reactions.

To obtain good results, strict control over such factors as sulphate concen-tration range of sample, strength of chromate reagent, volume of indicator, time of heating and washing is necessary (11).

Sulphate has also been determined by conductance or oscillametric ti-tration with BaC1_2(12, 13). The titration vessel containing the sulphate so-lution, _{seeded with a few mgm of BaS04 and adjusted to contain between 30}

and 40 per cent ethyl alcohol by volume, is placed in the field of a high-frequency oscillator. The changes in total ionic conductance, due to changes in compo-sition of the s o l utiori on addition of BaC1₂, cause an increase in plate current.

Under the conditions of the titration the relationship is almost linear. The chief advantages of this method are its specificity, relative speed and the fact that small amounts of sulphate can be determined. The main disadvantage is

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that large amounts of foreign ions decrease the sensitivity and accuracy. The method requires a titrimeter with sensitivity, stability, speed, and ease of operation.

The determination of sulphate through the turbidity developed on precipitation as BaS04 has been an attractive and much used method (14,

15, 16). The procedure involves the addition of sized BaC1₂ crystals to an aliquot of soil extract. Gum acatia solution is added to prevent too rapid settling of the BaS04 precipitate. Turbidity readings are taken in the period from 5 to 30 minutes after the precipitation has occurred in a photo-electric colorimeter using a blue filter. Sulphate is then determined by reference to a standard sulphate curve. Ma s s ourni and Cornfield (17) in-creased the sensitivity of the method by adding an extremely dilute seed suspension of BaS04 to the test solution.

As BaS04 turbidities are affected by the conditions under which they are produced, the prescribed conditions should be carefully adhered to so that each turbidity is produced under identical conditions (15). Sinc e the rate of solution of the sized BaC1₂ crystals serves to control the rate of reaction, a suitable size range must be selected. Different batches of BaC1₂ crystals give rise to slightly different regression coefficients making it preferable to prepare a standard sulphate curve for each batch of crystals. A suitably buffered m ediurn is necessary to ensure a favourable rate of dis-solution of BaC1₂ as this influences the optical density and reproducibility of the BaS0₄ cloud (16). Concentrations of calcium greater than 15 meq/liter are likely to cause interference (16).

Determination of Soluble Salt Concentrations in Soils

For more than 50 years the soluble salt or electrolyte concentration of the soil solution has been determined by measuring either the electrical resistance of a soil paste or the electrical conductance of the soil solution extract. Electrolyte present in the soil solution may be partially or com-pletely ionized. If a drop in potential exists across two electrodes placed in the soil solution or soil pa ste, a current will flow between the electrodes. The current flow is due to the migration of ions and is influenced by such factors as degree of dissociation of the salts, moisture content of the soil,

type of exchangeable cations, temperature, and salt concentration (18). Although the conductivity method is generally recommended (4), the semiquantitative soil paste method is still used. This procedure is particu-l a r Iv adaptabparticu-le to fieparticu-ld use because the apparatus is simpparticu-le and rugged, and the measurements can be made quickly. It consists of filling a rubber re-sistance cup with a saturated soil paste, placing the cup between the electrical contacts of a portal 1t; salt bridge, and determining the resistance of the

paste (4,5). The soil paste resistance values, corrected to 60° F are in-terpreted in terms of salt content as a percentage by weight of the soil

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by means of a sulphate and chloride column (5), or a standard calibration curve. A saturated soil paste of the correct consistency will glisten due to reflection of light, flows slightly when the container is tipped, but is thick enough so that no free water stands on the surface. An estimate of the soil saturation moisture content is necessary for determination of the salt content.

Two types of resistance cups are available (a) the "Bureau of Soils Cup!' (5, 18), a cylindrical soil conductivity cell of 50 rn l capacity, made of hard rubber, with two massive electrodes of nickel-plated brass exten-ding the full height of the cell; and (b) a soil cup having small platinized platinum disk electrodes as described by Scofield (19). The principles of the salt bridge are described by Jackson (5).

Some researchers (2 0, 21) consider the soil paste resistanc e method unreliable as an index of soil solution concentration. The accuracy of the method was found to be influenced by such factors as the lack of a cell con-stant for some soil cups, variations in the saturation percentage, variations in soil salinity, conductivity of soil minerals, and surface conductance. Sur-face conductance is due to current transfer by ions adsorbed on the colloidal particles in the soil (22). The magnitude of surface conductance is considered small and is, in most instances, ignored. It is felt that in some cases, e. g., soils of high clay content and low electrolytic concentration, surface con-ductance may contribute significantly to the total concon-ductance of a soil paste (22). It is generally concluded therefore, that when a reasonable degree of accuracy is desired the solution should be extracted from the saturated soil and the electrical conductivity of the solution measured (20,21).

An "in situ" IY1e th od of measuring soil salinity has been investigated

by Shea and Luthin (23). After observing that soil salinity was the main ob-stacle to the development of accuracy in the four-electrode probe method of m ea su r irig soil moisture content, they felt the method might be useful for measuring soil salinity. The principles of the four-electrode probe are de-scribed in detail elsewhere (24,25). Briefly, the method consists of install-ing four electrodes in a line at uniform depth. A known current (I) is fed through the primary circuit connecting the outer electrodes and the potential drop (E) is measured across the secondary circuit connecting the center electrodes. Ohrrr' s law is then used to calculate the resistance between the c cnt c r electrodes. Shea and Luthin (22) concluded that the four-electrode rn e th od could measure soil salinity accurately at soil moisture contents be-tween saturation and 30 c rn water suction, but beyond this suction range the results were erroneous if soil moisture was not considered. This is a serious limitation to the usefulness of the method for soils of arid and semi-arid re-gions where natural soil moisture contents vary from saturation to suctions as high as 15 atmospheres (> 15, 000 c rn water). As water is necessary for the conduction of electrical current large moisture stresses greatly affect

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the resistivity. In addition to the influence of soil moisture soil textural differences affect the magnitude of surface conductance (23).

Kirkham and Taylor (25) observed that instrumental errors of the four-electrode method were important. Displacement of the inner or outer electrodes, unless occurring in such a way as to keep the distance between adjacent electrodes equal, may result in large errors. The presence of cracks, air pockets or stones between the electrodes also significantly af-fects resistance measurements. These factors seriously limit the useful-ness of this method for measuring soil salinity at depths greater than afew feet below the surface.

A fairly quantitative estimate of the soluble salt concentration is ob-tained by determining the electrical conductivity of the saturation extract (4,5). A linear relationship exists between the specific electrical conductance in a water extract of soils and the concentration of salts found by analysis and expressed as milliequivalents of anions (or cations) per liter of solution. Specific conductance is the electrical conductance of a solution measured at 25°C between electrodes one cm2 in cross-section and placed one ern apart. Cell constants are necessary for conductivity cells of dimensions other than those of the "ideal cell". The specific conductance of a soil solution is generally small and is expressed as millimhos per c rn (1000 times mhos per cm).

Relative to the soil paste resistance technique, the conductivity method is less rapid and less suited for field use. A saturated soil paste is prepared and the saturation moisture content is determined in the same manner as for the soil paste resistance method. After the paste has been allowed to stand for a period of time to attain equilibrium, a suction filter is used to obtain

enough extract to determine the specific electrical conductivity. By deter-mining the conductivity of a water extract, errors due to the presence of the soil mineral fraction and variations in soil moisture content are avoided. As the electrical conductivity of solutions increases approximately 2 per cent per degree centigrade increase it is customary to convert the measurement to a standard-reference temperature which is usually 25 ° C (4).

Because the soil: water ratio influences the amount and composition of salts extracted from a soil (26), it is important to specify the soil: water ratio used in the analysis. Extraction of soil at natural field moisture con-tents gives the most accurate measure of soluble soil salts, but the necessary extraction procedures are time consuming and the extracts are more difficult to obtain. The saturation extract has a special advantage over more dilute soil: water ratio extracts in that it is related to field moisture content (4). Measurements of soils indicate that over a considerable textural range the saturation percentage is approximately equal to twice the field capacity and four times the 15 atmosphere percentage. Higher soil: water extracts (1: 2 and 1: 5) provide a less accurate estimate of the salt content of the soil because

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more salts may be removed than are ever present in the soil solution at field moisture contents. Depending on the kind of salts present, even the ionic species extracted may differ from those present in the soil solution (26). For example, the amount of calcium and sulphate from a gypsum-bearing soil is about 5 times as much in a 1: 5 extract as in a 1: 1 extract. If CaC0₃ is present, HCO; and Nat may increase with dilution, the latter being displaced by Catt dissolved from CaC0_{3 •} The more dilute soil: water ratios are more convenient for rapid salinity determinations because extracts may be obtained by filtering without suction or pressure, or dip cell electrodes may be used directly in the soil suspension to measure electrical conductivity.

Probably the one important disadvantage of using the saturation moisture content in determining soil salinity is the human variable involved in determining the proper saturation endpoint. Although this endpoint is reasonably definite, variability in hand mixing pastes and indetermining the correct soil consistency may result in differences in saturation extract electrical conductivity (4). Special precautions are particularly necessary with fine-textured and very coarse-textured soils. In clay soils the satu-ration percentage can be varied by 10 per cent or more depending on the rate at which water is added and the amount of stirring done. A reproducible capillary procedure for obtaining saturated soil pastes has been developed by Longenecker and Lyerly (27). The soil samples are saturated through capillary action for about 18 hours. When using this method individual

samples require less attention and human variability is eliminated. Standard-ization of sieving practices are recommended to avoid differences in moisture uptake. One problem associated with this procedure may be inadequate

moisture uptake by fine-textured soils.

The length of time a saturated soil paste is allowed to equilibrate be-fore the extraction is made has undergone considerable investigation. In

actual practice, the equilibration time may vary from 20 minutes to 24 hours (21). If gypsum is present in the soil samples, a period of at least two hours is

recommended (5).

With vacuum filtration essential for obtaining a saturation extract, special suction equipment must be available if the conductivity method of measuring

soil salinity is to be adopted for use under field conditions. A compact por-table vacuum filter has been designed for this purpose by Richards (28). Por-table conductivity bridges and cells are also available (29). The use of this equipment has proven convenient and the accuracy obtained was better than

±

10 per cent, which was considered adequate for most diagnostic purposes.

MATERIALS AND METHODS

To establish the dominance of the sulphate anion in the soil solution over a wide range of soil salinities, and thus possibly establish the electrical conductivity method as an acceptable measure of the sulphate concentration, a

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large number of soil samples having the necessary chemical data were required. Through the kind co-operation of Mr. J. G. Ellis, Saskatchewan Institute of Pedology, chemical data from over 700 soil samples was made available. The required information included water soluble sulphate, bi-carbonate and chloride concentrations (milliequivalents/liter), specific electrical conductivities (millimhos/cm at 25° C), all of which were de-termined on soil saturation extracts, and soil saturation moisture con-tent percentages.

The soil samples were part of those taken during the South

Saskatchewan River Irrigation Project survey. The area sampled is al-most wholly within the Rosetown map area (sheet 72D of the National Top-ographic Series) and is within the Dark Brown Soil zone. The soils of the area are described in the Ro s e town map area soil survey report (unpublished to date). Samples were taken from a variety of alluvial-lacustrine and glacial till geological deposits and include a wide range of soil textural types and soluble salt concentrations. Depths sampled range from the surface down through 12 feet. Site selection and sampling was performed by members of the Saskatchewan Soil Survey and P. F. R. A. Drainage Division.

Chemical analysis was carried out at the P. F. R. A. Drainage Division Laboratory, Vauxhall, Alberta. Specific electrical conductivity, water soluble bicarbonate and chloride were determined on the saturation extract using the standard procedures described by Jackson (5). The sulphate con-centration of the extract was determined by the conventional gravimetric procedure (6).

RESU LTS AND DISCUSSION

The attempt to establish specific electrical conductance as a measure of the soluble sulphate concentration in a soil saturation extract was considered on the basis of two important factors. First, it has been shown that the specific electrical conductance of a water extract is linearly related to the salt concen-tration as found by analysis and expressed as milliequivalents of anions (or cations) per liter of solution (30). Secondly, the sulphate anion is considered the principle soluble anion present in the soils of the prairies (1,2,3). The main objectives of the study were, therefore, to determine the importance of the sulphate anion relative to other anion species over a wide range of salt concentrations and to establish whether the relationship existing between spe-c if ispe-c elespe-ctrispe-cal spe-conduspe-ctanspe-c e, expressed as rni l l irrih o s / spe-crn , , and sulphate spe- con-centration (milliequivalents per liter) was significant enough for consideration of the conductivity method as a semiquantitative estimate of sulphate concen-trations.

Soluble anion species considered in this investigation were limited to sulphate, bicarbonate and chloride, since they are the most common in prairie subsoils (1,2,3). The nitrate anion may be present but, since it is formed chiefly by oxidation of nitrogenous organic matter, it is most concentrated in

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the surface horizon (31). The carbonate anion is usually not soluble in detectable quantities in soils of pH less than 9.0 (4).

Relative Importance of Sulphate in the Soil Solution

The chemical data was processed initially to show the change in sulphate, bicarbonate and chloride concentrations (meq/ liter) and the change in relative importance of each anion specie with increasing solu-ble salt content as measured by specific electrical conductance (mmhos / cm). The samples were divided into fifteen conductivity groups of 1 mmhos/cm range. The concentration range and the mean concentration for each anion specie were calculated for the conductivity groups (Table I). Soluble sul-phate, bicarbonate and chloride percentages (Figure 1) as well as sulphate: bicarbonate and sulphate: chloride ratios (Table II) were also calculated.

The results indicate that an increase in the soil solution concentration was due primarily to an increase in sulphate concentration (Table I). As the specific electrical conductance increased from less than 1.0 mmhos/cm to above 14 mmhos/cm, the mean sulphate concentration increased markedly from 3.3 meq/liter to a high of 304 meq/liter. Sulphate concentrations

ranged from O. 1 meq/liter up to 442 meq/liter. Chloride concentrations showed a gradual increase from a low mean of 0.3 meq/liter to a high mean of 11.2 meq/liter. Relative to the sulphate concentrations, chloride con-centrations were very low, ranging from trace amounts up to 35 meq/ liter chloride. Concentrations of 30 to 35 meq/liter occurred at only one highly saline site. Bicarbonate concentration means were relatively uniform over the entire salinity range. The highest concentrations of bicarbonate generally occurred in samples of low salt concentration (specific electrical conductivities of less than 2.0 mmhos/cm) taken near the soil surface.

At these low conductivity levels, bicarbonate makes up a large per-c entage of the soluble anions present in the soil solution (55 per per-cent for the

o -

1. 0 mmhos/cm group and 25 per cent for the 1. 1 - 2.0 mmhos/cm group (Figure 1 )). With bicarbonate concentrations remaining relatively constant and sulphate concentrations increasing rapidly with increasing specific con-ductivity, sulphate: bicarbonate ratios show a sharp increase of from 0.8 to as high as 127 (Table II). At specific conductivities above 3.0 mmhos/cm soluble bicarbonate percentages drop from 6 per cent to less than 1 per cent.

Although soluble chloride concentrations increased slightly with in-creased salinity, and sulphate: chloride ratios show a decrease at very high salt concentrations (Table II), soluble chloride percentages (Figure 1) seldom exceeded 5 per cent and were generally in the order of 2 to 3 per cent. The high chloride concentrations occurred in soils having very high sulphate con-centrations as well, with the result that soluble chloride percentages re-mained low.

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At specific electrical conductivities greater than 3.0 mmhos/cm sulphate made up 90 per cent or more of the anions in the soil solution (Figure 1). More than 80 per cent of the anions in soil samples having

specific conductivities between 2.1 - 3.0 mmhos/cm inclusive, were sulphate. The change in the relative abundanc e of the three anion species with depth was investigated by determining the mean anion percentage for each species at each depth interval from the surface down to ten feet (Table Ill). Eighty-two sampling sites, covering the entire range of soil types and soluble salt contents, were included. At depths below two feet, mean anion per-centages remained relatively uniform with depth, with sulphate making up 94 to 96 per cent, bicarbonate, 2 to 4 per cent, and chloride 2 per cent. The lowest specific electrical conductivities generally occurred at the shallower depths. Out of 230 samples having conductivities of 1. 0 mmhos/cm or less, 118 were from the surface foot and 41 were from the one to two foot depth interval. A large percentage of the anions at these shallow depths was bi-carbonate. Earlier research (1,2,3,32) has shown that bicarbonate con-centrations are generally highest in the surface layers and decrease sharply with depth. The bicarbonate concentration of the soil solution is related to the partial pressure of CO₂ in the soil atmosphere, usually being highest at the shallower depths because of intense root growth and microbial activity.

The probability of encountering soil samples having sulphate

per-centages 90 per cent or higher within each of the specific electrical conductivity groups was also investigated (Figure 2). 80 per cent of the samples in the

3. 1 to 4.0 mmhos/cm conductivity group and 87 per cent of the samples in the 4.1 to 5.0 mmhos/cm group contained 90 per cent or more soluble sul-phate. The sample percentages continued to increase with increasing soil salinity until the 8. 1 10 9. 0 mrriho s / cm group, in which 100 per cent of the soil samples tested had 90 per cent or more sulphate. Above this conductivity level, all soil samples tested had 90 per cent or more of the soluble anions present as sulphate. At the low specific conductivity levels, sulphate per-centages were low due to the dominance of bicarbonate, and only 10 per cent of the samples in the 1. 1 - 2. 0 mmhos / cm group and 37 per cent of the samples in the 2.1 - 3.0 mmhos/cm group had more than 90 per cent sulphate anion.

It is evident from the results that at specific electrical conductivities greater than 1.0 mmhos/cm, the sulphate anion becomes the principle anion in the soil solution, and at specific electrical conductivities greater than 3 rnrrih osyc rn sulphate usually makes up 90 per cent or more of the soluble anions present. Sulphate was also found, on the average, to constitute over 90 per cent of the soluble anions at depths greater than two feet. Increases in soluble salt concentration, as indicated by specific electrical conductivity, were due primarily to increases in sulphate salts and, to a much lesser degree, chloride salts.

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Specific Electrical Conductance As A Measure of Sulphate Concentrations in Soil Saturation Extracts

Statistical methods (33) were used to establish specific electrical conductance as a method for determining the sulphate concentration of soil solutions. The correlation coefficient "r" provided a measure of the extent

of the co-relationship existing between specific electrical conductance (mmhos/cm) and sulphate concentration (meq/liter) of a soil saturation extract. The

re-gression coefficient "b" and rere-gression equation were also computed. The principle purpose of the latter two statistic s was to establish a standard cali-bration curve, based on the experimental data, to enable the prediction of sul-phate concentrations from measured specific electrical conductivity values.

The average rate of change of sulphate concentration per unit change in spec-ific electrical conductance is given by the regression coefficient. Since a large proportion of the anions in soil solutions having specific electrical conductivities less than 2. 0 mrnhos / cm was bicarbonate, the information was divided into two groups for statistical analysis. One group included 284 samples having spec-ific conductivities covering the range 0 to 1. 9 mmhos/cm. The second group included 558 samples all within 2.0 - 20 mmhos/cm range.

A direct linear relationship was found to exist between specific electrical conductance of a saturation extract (mmhos / cm at 25 ° C) and water soluble

sulphate concentration, as determined by analysis and expressed as meq S04 per liter of saturation extract (Figure 3). Both the regression coefficient and the correlation coefficient were significant at the 0.01 level of probability. For the specific electrical conductance range 2.0 to 20 mmhos/cm, the linear

regression equation was Y = 17. 5X - 15.9, where Y is the sulphate concen-tration (meqfliter) and X is the specific electrical conductance (mmhos/cm at 25°C). The correlation coefficient "r" was 0.981 and the standard error of estimate ± 13.4 meq S04 per liter. For the range 0 to 1. 9 mrnho s / c rn, the linear regression equation was Y = 10.3X - 3.0, with r = 0.830 and the standard error of estimate ± 2. 6 meq S04 per liter of saturation extract.

Recommendations for the use of sulphate resistant cement as protec-tion against sulphate attack of concrete are generally based on the evaluaprotec-tion of sulphate concentrations present in the soils prior to construction. For this reason the sulphate concentrations are usually expressed as a per cent of soil weight (grams S04 per 100 grams of soil) or some other equivalent units (34). A more useful form of the calibration curve presented in Figure 3, therefore, would be one in which specific electrical conductance is shown as a function of sulphate concentration expressed as a per cent of soil weight (Figure 4). In order to convert sulphate concentrations, expressed as milli-equivalents S04 per liter of saturation extract, to a soil weight per cent basis, the volume or weig ht of water required to prepare the saturated soil paste must be known. This factor is given by the saturation moisture content or the "saturation percentage". The calibration curve provided in Figure 4 was con-structed through conversion of data supplied from Figure 3. Sulphate

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concentrations, expressed as meq S04 per liter of saturation extract, corresponding to various specific electrical conductance values, were initially converted to grams of sulphate per milliliter of saturation ex-tract by the following expression: grams S04/ml = meq S04 /liter x . 001 x milliequivalent wt of S04' The milliequivalent weight of sulphate is

O. 048 gm. Grams of sulphate per 100 grams of soil (per cent S04) were determined by multiplying by the number of milliliters of water required to saturate 100 grams of soil (saturation percentage). Therefore, per cent S04 = meq S04/ liter x . 001 x rn e q wt S04 x saturation per cent. Saturation percentages, as indicated by the diagonal lines (Figure 4), help correlate the specific electrical conductance of the saturation extract with the per cent salt content for various soil textures. The amount of water required to saturate a soil varies with soil texture; increasing as the per-centage of clay size particles present in the soil sample increase (Table IV). By employing the calibration curve provided by Figure 4, therefore, sul-phate concentrations expressed as per cent of soil weight can be obtained directly from specific electrical conductance measurements made on saturation extracts, provided the correct saturation percentage curve is selected.

CONCLUSION

Before selecting an analytical procedure for estimating the sulphate status of the soils of a proposed construction site, two important factors

should be considered. First, the extreme variability of sulphate concentrations that may occur in soils demands a thorough sampling program, both over

the entire construction site and throughout the foundation depth.

Secondly, precautionary measures against sulphate attack of concrete, such as the use of sulphate resistant cement, are usually recommended if sulphate concentrations are found to be higher than a minimum "critical" sulphate concentration (34). A high degree of accuracy in the determination of sulphate for this purpose is not necessary. It seems feasible to suggest that a less accurate procedure than those conventionally used, but one which is more rapid and simple to perform, is desirable in order to accommodate the large number of samples which should be taken if a true estimate of the potential sulphate hazard of the site is to be realized.

The dominance of the sulphate anion over the bicarbonate and chloride anions in the soil solution was established for a wide range of soil salinities using a variety of soils from the Dark Brown soil zone. The results sub-stantiate the work of others (1,2,3) who suggested that the soluble salts of the prairie soils wer e predominantly sulphate salts. A highly significant co-relationship was found to exist between specific electrical conductance and sulphate concentration of a saturation extract. It was concluded therefore that the specific electrical conductance procedure provided asufficiently ac-curate indirect method for determining the sulphate status of soils. This method cannot be recommended for soils in which sulphate is not present

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as the dominant anion however.

The standard calibration curve (Figure 4), relating sulphate con-centration, expressed as per cent of soil weight, to specific electrical conductance (mmhos / cm at 250

C) of the saturation extract, permits di-rect conversion from electrical conductance units to per cent sulphate, which is the unit upon which recommendations for the use of sulphate resistant cement are usually based. The influence of the soil mineral fraction on electrical conductance is avoided by extracting the soil

solution, and the single calibration curve, prepared from extracts of a variety of soil materials, was considered adequate for all soils.

Although simple equipment has been designed for extracting the soil solution from samples in the field, the use of the soil resistivity cup, for measurement of the specific resistance or conductance of a saturated soil paste or the use of dip cell electrodes with 1: 1 soil to water suspen-sions, are more convenient for field use and are also more rapid. These methods are considered less accurate as a result of the influence of the soil minerals on the resistance or conductance of the soil paste but this decrease in accuracy is not sufficient to discount these methods as being useful for this particular application. 'In this case, individual calibration curves for each soil type could improve the accuracy of the measurements.

The moisture content of the soil has a significant effect on the re-sistivity of that soil. This one particular factor limits the use of "in situ" resistivity measurements as an indicator of sulphate concentration. Un-less the moisture content of the soil is known at the time of the resistance measurement, values obtained cannot be interpreted correctly in terms of sulphate concentration.

Because of the variability of sulphate concentrations in most soils, the usual soil sampling procedures followed by engineers investigating the sulphate status of a construction site are inadequate. As a rule, samples are selected at the foundation footing depth at one or two locations within the site. This method of sampling provides little information on the vari-ability of sulphate with depth, with different soil strata, or over the con-struction site. The number of s a rnp l e s selected is probably small because of the tim ec c on s urn irig nature of the conventional gravimetric procedure used

hy m o st engineers to rn e a s u r e sulphate concentration. Too much time would Ln;ocyuired to analyze the number of soil samples necessary for a good es-timate of the sulphate status of the site using this analytical procedure.

Because it is simple, sufficiently accurate, and enables analysis of a relatively large number of samples in a short period of time, measurement of the specific ell' :trical conductance of a saturation extract is recommended as a means of evaluating potential sulphate hazard. As an added advantage, measurements can be made on the construction site. A saturated soil paste

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r

i

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- 14

-is prepared using d-istilled water. A portion of the paste is used for evaluation of the saturation percentage, and the remainder is used for extraction of the soil solution. An immediate indication of the sulphate concentration is obtained from the specific electrical conductance measure-ments made on the soil extract. After the saturation percentage has been determined, the sulphate percentage may be estimated using the standard calibration curve (Figure 4).

REFERENCES

1. Mitchell, J. l'Alkali" Soils in Saskatchewan. Sci. Ag r . 18: 120-125. 1937. 2. Ehrlich, W. A. and R. E. Smith. Halomorphism of Some Clay Soils

in Manitoba. Can. Jour. Soil. Sci. 38: 103-113. 1958.

3. Bowser, W. E. l Milne, R. A. and R. R. Cairns. Characteristics of the Major Soils Groups in an Area Dominated by Solonetzic Soils. Can. Jour. Soil Sci. 42: 165 - 179. 1962.

4. Diagnosis and Improvement of Saline and Alkali Soils. USDA Agriculture Handbook No. 60, 1954.

5. Jackson, M. L. Soil Chemical Analysis. Prentice-Hall In c . , Englewood Cliffs, N. J. 1958.

6.

Kalthoff, 1. M and E. B. Sandell. Textbook of Quantitative Inorganic Analysis 1952. MacMillan Company, New York.

7. Munger, J. R., R. W. Nipp l e r and R. S. Ingols. Volumetric Determination of Sulfate Ion U sing Barium and a Standard Disodiurn Dihydrogen

Ethylendiamine Tetraacetate Solution. Anal. Chern. 22: 1455 -1457. 8. Maghe, V. B., N. R. Talati and C. M. Mathur. A Modified EDTA Method

for Determination of Soluble Sulfates in Soils and Waters. Curro Sci. 33: 242. 1964.

9. Standard Methods for Examination of Water and Sewage. Am. Pub. Health Assoc. New York. 11th Ed. 1960.

10. Siegfriedt, R. K., J. S. Wiberley and R. W. Moore. Determination of Sulfur After Cornbu stion in a Srnall Oxygen Bomb. Anal. Chern. 23:

1008-1011. 1951.

11. Cantin o, E. C. Titrimetric Determination of Sulfate in Natural Waters and Soil Extracts. Soil Sci. 61: 361-368. 1946.

12. Milner, 0.1. Titration of Sulfates with Aid of a High-Frequency Oscillator. Anal. Ch ern , 24: 1247-1249. 1952.

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13. Blaedel, W. J. and H. V. Malmstadt. High-Frequency Titrations. A Study of Instruments. Anal. Chern. 22: 734. 1950.

14. Sheen, R. T., H. L. Kahler, and E. M. Ross, W. H. Turbidimetric Determination of Sulfate in Water. Method. Anal. Chern. 7: 262-265. 1935.

and L. D. Betz. Betz-Hellige

15. Chesnin, L. and C. H. Yi en , Turbidimetric Determination of Avail-able Sulfates. Soil Sci. Soc. Ame r , Proc. 15: 149-151. 1951. 16. Butters, B. and E.M. Chenery. A Rapid Method for the Determination

of Total Sulfur in Soils and Plants. Analyst 84: 239-245. 1959. 17. Massoumi, A. and A. H. Cornfield. A Rapid Method for Determining

Sulfate in Water Extracts of Soils. Analyst 88: 321-324. 1963.

18. Soil Survey Staff. Soil Survey Manual. U. S. D. A. Agriculture Handbook No. 18. 1951.

19. Scofield, C. S. Measuring the Salinity of Irrigation Waters and of Soil Solutions with the Wheatstone Bridge. U. S. Dept. Ag r , Ci r , 232.

1932.

20. Re i.t ern.e.i e r , R. F. and L. V. Wilcox. A Critique of Estimating Soil Solution Concentration from Electrical Conductivity of Saturated Soils. Soil Sci. 61: 281-293. 1946.

21. Wilcox, J. C. Determination of Electrical Conductivity of Soil Solution. Soil Sci. 63: 107-117. 1947.

22. van Olphen, H. and M. H. Waxman. Surface Conductance of Sodium Bentonite in VTater. Proc. Fifth National Con£. on Clay and Clay Minerals: 61-80. 1956.

23. Shea, P. F. and J. N. Luthin. An Investigation of the Use of the Four-Electrode Probe for Measuring Soil Salinity In Situ. Soil Sci. 92: 331-339. 1961.

24. Edelfsen, N. E. and A. B. C. Anderson. Method for Measuring Soil-Moisture Soil Sci. 51: 367-376. 1941.

The Four-Electrode Resistance Content Under Field Conditions.

25. Kirkham, D. and G. S. Taylor. Some Tests of a Four-Electrode Probe for Soil Moisture Measurement. Soil Sci. Soc. Amer. Proc. 14: 42 -46. 1949.

26. Reitemeier, R. F. Effect of Moisture Content on the Dissolved and Ex-changeable Ions of Soils of Arid Regions. Soil Sci. 61: 195-214. 1946.

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27. Longenecker, D. E. and P.J. Lyerly. Making Soil Pastes for Salinity Analysis: A Reproducible Capillary Procedure. Soil Sci. 97: 268-275. 1964.

28. Richards, L. A. A Portable Vacuum Filter. Soil Sci. 79: 423-425. 1955.

29. Richards, L. A. Portable Conductivity Bridge and Cells for Salinity Appraisal. Soil Sci. 80: 55-59. 1955.

30. Fireman, Milton and R. C. Reeve. Some Characteristics of Saline and Alkali Soils in Gem County, Idaho. Soil Sci. Soc. Am e r , Pr oc .

13: 494-498. 1949.

31. Kelley, W. P. Alkali Soils, Their Formation, Properties and Reclamation. 1951. Rheinhold Publishing Co r p , , New York.

II

32. Lueken, H. Saline Soils Under Dryland Agriculture in South Eastern Saskatchewan (Canada) and Possibilities for Their Improvement. Part I: Distribution and Composition of Water-Soluble Salts in Soils in Relation to Physiographic Features and Plant Growth. Plant and Soil 17: 1-25. 1962.

33. Le Clerg, E. L., W. H. Leonard and A. G. Clark. Field Plot Technique. 1962. Burgess Publishing Company. Minneapolis 23, Minn.

34. Concrete Manual, United States Dept. of the Interior, Bureau of Rec-lamation , 1955.

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RANGE AND MEAN CONCENTRATIONS OF ANIONS FOR THE VARIOUS SPECIFIC ELECTRICAL CONDUCTIVITY GROUPS

Specific Electrical

Conductivity Total Anions Sulphate Bicarbonate Chloride

Group (rneq/liter) (rneq/liter) (rneq/liter) (rneq/ liter)

rnrnhos / crn Range Mean Range Mean Range Mean Range Mean

o -

1.0 2.5 - 25.4 7.5 O. 1 - 21.1 3.3 0.4 - 12.3 3.9

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3.9 0.3 1. 1 - 2.0 8.4 - 24.6 16.4 4.8 - 22.7 11.9 1.0 - 10.2 3.9

I

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3.5 O. 6 2. 1 - 3.0 21. 2 - 63.4 34.5 14. 1 - 56.8 29.8 0.8 - 5.7 3.5

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3. 1 - 4.0 _{36.2 -} _{61. 2} _50.5 26.9 - 60. 1 46.7 0.8 - 7.6 2.7 _0-5.51 1.1 4. 1 - 5.0 48.8 - 86.7 71. 8 42.4 - 85.0 68. 1 0.8 - 6.4 2.2

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6.3 1.5 5. 1 - 6. 0 67. 0 - 122 88.9 58.2 - 113 85.0 0.4 - 4.5 2.0 0.4 -10.9 1.9 6. 1 - 7.0 82. 5 - 133 107 77.2 - 127 103 O. 7 - 6.4 2.4

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8.9 2.0 7. 1 - 8.0 64. 0 - 178 121 61.0 -174 117 0.4 - 4.9 2.4 0.3 - 5.0 2.3 8. 1 - 9.0 122 - 174 138 113 - 168 ₁₃₄ 0.8 - 5.3 2. 1 0.2-7.9 2. 1 9.1 - 10.0 125 - 188 157 ₁₂₀ - 188 151 1. 1 - 4. 1 2.3 0.2-25.8 3.0 10.1-11.0 161 - 208 180 ₁₄₂ _{- 203} ₁₇₃ 0.8 - 6.8 2.3 1. 3 - 18. 1 4.4 11.1-12.0 160 - 231 187 ₁₅₇ _{- 223} ₁₈₂ 1. 1 - 3.0 2.0 0.2 - 6. 6 2.8 12.1 - 13.0 177 - 253 211 ₁₆₁ _{- 247} ₂₀₁ 1.5- 4.5 2.6 O. 7 - 29. 4 6.8 13.1 - 14.0 194 - 239 217 ₁₈₈ - 231 ₂₀₄ 0.8 - 2.7 1.8 3.1 -30.6 11. 2 14.1

+

235 - 450 ₃₁₆ ₂₀₃ _{- 442} ₃₀₄ 1.5- 4.9 2.4 3.3 -34.8 9.4 ,

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SOLUBLE SU LPHATE PERCENTAGE AND RATIO OF SULPHATE CONCENTRATIONS TO BICARBONATE AND CHLORIDE CONCENTRATIONS

FOR THE VARIOUS SPECIFIC ELECTRICAL CONDUCTIVITY GROUPS

Specific Electrical

.onductivity Soluble Ratio Ratio Ratio

Group Sulphate S04: H C 03 S04: Cl C1: HC0₃

rnrnho s / on) Percentage (meq/liter) (meq/liter) [rn eqvlitef

--

1 -0 - 1. -0 41 0.8 11 O. 1 1.1 - 2.0 72 3. 1 20 0.2 2. 1 - 3. 0 86 8.5 25 0.3 3. 1 - 4.0 92 17 43 0.4 4. 1 - 5. 0 95 31 45 0.7 5. 1 - 6.0 96 43 45 1.0 6. 1 - 7.0 96 43 51 0.8 7. 1 - 8.0 96 49 51 1.0 8. 1 - 9.0 <)7 64 64 1.0 9. 1 -10. 0 97 66 50 1.3 10. 1 -11. 0 96 75 39 1.9 11. 1 - 12. 0 97 91 65 1.4 12. 1 - 13.0 95 77 30 2.6 13. 1 - 14. 0 94 113 ₁₈ _6.2 14.1

+

96 127 32 3.9

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ANION CON CENTRA TIONS WITH DEPTH*

Specific Electrical Per Cent

Depth Conduc tivity** Sulpha te Bicarbonate Chloride Anions Per Cent

ft Range lvlean Range Mean ｉｾ｡ｮｧ･ lvlean Range Mean Range Mean S04 HC03 Cl

0"- 6" 0.4 - 11 1.0 0.1-145 7. 1 0.4-10.2 4.6 o - 9.9 0.6 o.4 - 1 62 12. 3 58 37 5 I 6"- 12" 0.3 - 21 1.4 0.1-332 15.4 O. 1 - 7.6 I 3. 6 0-13.4 0.8 3.7-349 19.8 79 18 4 1 - 2 0.3 - 20 3. 6 O. 1 - 359 53.3 0.8-12.3 3.7 o - 25.8 1.6 4.0-373 58. 6 91 6 3 I 2 - 3 0.4 - 21 5.2 O. 1 - 381 79.7 O. 1 - 7.6 3.2 o - 18. 1 1.8 4.5 - 393 84.7 94 4 2 3 - 4 0.3 - 19 6.2 0.1-392 95.7 1. 1 - 7.. 6 3.2 o - 24. 1 2.2 3.5-402 101 95 3 2 4 - 5 0.3 - 19 6.6 0.1 - 392 103 0.8 - 7.6 2.8 o - 30.6 2.3 3.5-402 108 95 3 2 5 - 6 0.4-1'1 6.8 O.1 - 442 107 0.8 - 7.6 2.7 o - 30.6 2.4 4. 1 - 450 1 12 96 2 2 6 - 7 0.4 - 1'" 7.0 3. 0 - 442 111 0.7 - 7.6 2.3 o - 30.9 2.3 5.8-450 1 15 96 2 2 7 - 8 I 0.4 - 19 6.8 1.9-329 104 0.4 - 7.6 2. 1 o - 30.9 2.2 5.7-338 109 96 2 2 8 - 9 0.4 - 19 6.5 0.2-356 98.7 0.4 - 7.6 2.0 o - 34. 8 2. 1 4.6 - 366 103 96 2 2 9 - 10 0.4 - 20 6.5 0.2 - 343 99.4 0.4 - 7.6 1.9 0-29.4 2.0 4.6-355 103 96 2 2

* Data are frorn 82 individual sampling sites covering a wide range of salt concentrations and soil type.

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EFFECT OF CLAY CONTENT ON SOIL SATURATION PERCENTAGE FOR SOILS INCLUDED IN STUDY

Soil Clay Content Saturation

Association Per Cent Percentage

Asquith 10 35 Bradwell 19 46 Elstow 30 55 Tuxford 45 66 Regina 54 75 Weyburn 23 44

Rapid semi-quantitative method of determining water soluble sulphate in some Saskatchewan soils

Publisher’s version / Version de l'éditeur:

Internal Report (National Research Council of Canada. Division of Building

Research), 1969-06-01

Archives des publications du CNRC

Rapid semi-quantitative method of determining water soluble sulphate

in some Saskatchewan soils

by

ANAL VZED

1969

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