Recommended equipment and techniques

At present, a wide range of analytical equipment is commercially available. Table 8 presents a list of equipment and well-established analytical techniques available for determination and analysis of the various parameters and constituents of water. These techniques are summarized in this section.

Several techniques must be applied to obtain a complete analysis of the water sample. There is no such thing as ‘the’ single technique that provides a complete set of water quality information. The final choice of the equipment to be used is largely dictated by the availability of the chemical-analytical infrastructure. Yet, some remarks and recommendations with regard to the selection of the techniques can be made.

5.3.1. pH

The pH is probably one of the most frequently measured parameters in water chemistry. Most popular pH sensors are similar to the one shown in Fig. 25, which uses a glass membrane as the pH sensitive medium. The

TABLE 8. ANALYTICAL TECHNIQUES AND EQUIPMENT FOR DETERMINATION AND ANALYSIS OF WATER PARAMETER AND CONSTITUENTS

Parameter/constituent Equipment/technique

pH pH metre

Conductivity Conductometer

Temperature Thermocouple or RTD

Fe²⁺ ICP-MS, ICP-AES, AAS

Cu⁺ ICP-MS, ICP-AES, AAS

Al³⁺ ICP-MS, ICP-AES, AAS

Ag⁺ ICP-MS, ICP-AES, AAS

Cl^– IC, ISE, UV-VIS

NO₂^– IC, colourimetry

TSS Analytical balance

137Cs Gamma spectrometry

Na⁺ ICP-MS, ICP-AES, AAS

K⁺ ICP-MS, ICP-AES, AAS

Ca²⁺ ICP-MS, ICP-AES, AAS

Mg²⁺ ICP-MS, ICP-AES, AAS

Hg+ ICP-MS, AAS

NO₃^– IC, colourimetry

NH₄⁺ IC, colourimetry

CO₃^2– IC

H₂O₂ Fluorimetry

O₂ (dissolved) Membrane electrode

3H LSC

60Co Gamma spectrometry

131I/¹³³I Gamma spectrometry

potential of the glass membrane is picked up at the inner side of the membrane, and a reference electrode is separated from the water sample by means of a diaphragm or a conductive gel.

Despite numerous technical advances, pH determination is still not straightforward. The measurement is strongly influenced by the composition of the water, its temperature and its conductivity [67, 68]. Care must be taken when using conventional equipment. The ion-poor or low conductivity character of cooling water, in combination with the high electrical resistance of the sensing glass membrane of conventional glass electrodes, significantly hampers accurate determination of the pH resulting incorrect results. The use of pressurized electrodes allows measurements under pressure.

5.3.2. Conductivity

A wide range of conductometers are available; a meter with an analysis domain between 0 and 1000 S/cm is recommended. It is common for the various instruments available that measurements of a sample with a very high

— or in contrast — a very low conductivity may be problematic.

The conductivity equipment is more robust than a pH meter and is less sensitive to upsets in the system, making the conductivity a more reliably measured parameter for assessment of the water quality.

For laboratory measurements, Pt black surface type electrodes, as shown in Fig. 26, or goldplated stainless steel electrodes are recommended. It is also recommended that the measurement of conductivity should be performed using alternating current (AC) in order to prevent polarization of the electrodes.

5.3.3. Temperature

There are a series of devices that can be used for temperature measurements. The most common are thermocouples resistance temperature detectors (RTDs). Thermocouples use the principle discovered by Thomas Seebeck in 1821, that an electromotive force (emf) is created whenever the two extremes of a metal wire are subject to different temperatures. Joining one extremity of two dissimilar metals, the emf measured on the other extremity is proportional to the temperature difference between the two extremities. Because they have been widely used, a series of standards have been established for thermocouples, including specific pair of wires, connectors, geometry and types of junctions. Standard tables have been also developed correlating the emf, as a function of the measured temperature, for each type of thermocouple. The most common types of thermocouples are type J, type K, type E and type T. Table 9 shows the main characteristics of each of them [9].

FIG. 25. Combined glass electrode.

RTDs use the principle that for some materials the electrical resistance is highly dependent of the temperature.

Because of its high precision, Platinum is the most common type of RTD, and is sometimes referred to as platinum resistance thermometer (PRTs). Other elements used for RTDs are nickel and copper.

Once the specific thermocouple or RTD is properly calibrated, any one of them can be routinely used for measuring the temperature of the water in the cooling system. However the user must consider that Platinum RTDs present better precision and lower drift, i.e. better stability. Therefore, for high precision measurements of temperature, requiring deviations smaller than 0.5^oC, Platinum RTDs are recommended. If higher deviations are accepted, then thermocouples can be used.

However, the signal from RTDs may be affected by neutrons and gamma radiation, therefore, for measurements close to the reactor core, thermocouples are recommended.

5.3.4. Analysis of metals

Inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectrometry (ICP–AES) are fully accepted techniques for metal analysis. ICP-MS is recommended by leading institutes such as the American Public Health Association and Water Environment Federation [69]. Both techniques have low detection limits for metals and offer the advantage of simultaneous (within one run) analysis of all metals in a sample. Up to the 1980s, atomic absorption spectrometry (AAS) was the leading technique for metal analysis.

In particular, when equipped with a graphite furnace, this technique still is capable of producing high quality data.

The main disadvantage of this technique is that it is time-consuming. In contrast to the ICP techniques, AAS is only capable of measuring one element at a time. In addition, for the analysis of Hg, the AAS must be equipped with a cold-vapour assembly.

TABLE 9. MAIN CHARACTERISTICS OF MOST COMMON THERMOCOUPLES

Type of thermocouple Wires Temperature range (^oC)

J Fe versus (Cu + 43% Ni) –210 to +1200

K (Ni + 10% Cr) vs (Ni + 2% Al + 2% Mn + 1% Si) –270 to +1372

E (Ni + 10% Cr) vs (Cu + 43% Ni) –270 to +1000

T Cu vs (Cu + 43% Ni) –270 to +400

FIG. 26. Conductivity electrode.

5.3.5. Analysis of anions

Over the last decade, ion chromatography (IC) has become the leading technique for the analysis of anions, and in some special cases for metals as well. For most IC instruments, several chromatography columns for various sets of anions and cations are available. During an IC measurement run, all anions in a sample are sequentially measured depending on the selected column. Other relevant, well established techniques are ion selective electrode (ISE), ultraviolet spectrophotometry (UV-VIS) and colourimetry. ISE is still widely applied but more laborious than IC. Like AAS, only one element at a time is measured with ISE.

5.3.6. Other constituents

Dissolved oxygen: the use of an O₂ sensitive membrane electrode is recommended. This technique overcomes many of the limitations and restrictions of the classic iodometric method.

Tritium: Liquid scintillation (LSC) is a well-established technique for the determination of tritium activity (concentration) in aqueous solutions. Most radiological laboratories have access to LSC equipment.

Other radioactive nuclides (¹³⁷Cs, ⁶⁰Co, ¹³¹I, etc.): Gamma spectrometry is the preferred technique to determine radioactive isotopes and their activity levels in aqueous samples, assuming that the activity is high enough. For lower level samples, larger volumes must be collected, which then must be dried/evaporated to increase the concentration of the radio isotopes before it is gamma counted. In some cases, such as ¹³⁷Cs, a carrier can be added before evaporation and the precipitate is then gamma counted.