THERMOCHEMICAL METHOD FOR DETERMINING TOTAL WATER HARDNESS

I. G. MINAEV, Yu. V. TRUKHIN, M. F. TKACHENKO

Feedwater consumed by thermal power plants and enterprises of the chemical industry must not contain hardness salts, since even a small amount of them disrupts the proper conduct of the technological process and the operating regime of heat-exchange apparatus. Therefore, during feedwater treatment, accurate and reliable control of its quality is necessary. In recent years, the photocolorimetric method [1] has become most widespread for determining hardness salts; it is based on the phenomenon of light absorption by intracomplex compounds of calcium and magnesium with an alkaline solution of the indicator Eriochrome Black or Acid Chrome Dark Blue.

However, with this method accurate results can be obtained only when uncontrolled impurities are absent from the water (compounds of manganese, zinc, lead, copper, etc.).

The need for a more accurate analysis calls for the use of selective, sensitive methods for determining hardness salts in feedwater.

A thermochemical method is proposed, which uses the phenomenon of formation of insoluble salt deposits during heating, boiling, and evaporation of water on the surfaces of boiler units, evaporators, and heaters [2]. In the proposed method (Fig. 1), metal foil 2, immersed by means of guiding and current-conducting rollers 4 in vessel 3 containing the water being analyzed, is used as the heated body. The section of foil between two current-conducting rollers is heated by electric current from a step-down power transformer (1–2 kVA). When the foil is heated, primary embryonic crystals of salts having a negative solubility coefficient form on its individual sections, which are centers of vapor formation and zones of supersaturation.

As the current is increased, the temperature of the foil rises, the number of vapor-formation centers on the foil surface increases, and the intensity of heat exchange with the water increases. The growth of vapor-formation centers promotes the formation of primary embryonic crystals over the entire surface of the foil, while the increase in the intensity of heat exchange with the water raises the rate of mixing of the water in the surface layer of the foil, which leads to the growth of the primary crystals. Crystal growth continues until equilibrium is reached between the concentration of hardness-salt ions present in the water and the magnitude of the product of solubili-

...which characterizes the solubility limit of the given substance at the given temperature.

A further increase in the concentration of hardness-salt ions in the water leads to an additional increase in the amount of salt deposit on the surface of the foil. Consequently, from the amount of salt deposit it is possible to determine the content of hardness salts in the water being analyzed.

Initial studies carried out to determine the sensitivity and selectivity of the method showed that, at the boiling temperature of water and at normal pressure, it is possible to record water hardness only from

\[ 100 \frac{\mu\text{g-eq}}{\text{l}}, \]

and the scale contains salts formed by calcium and magnesium cations. Thus, the sensitivity of the method, with its satisfactory selectivity, is insufficient for monitoring high-purity feedwater, whose hardness varies within

\[ 0 \div 5 \frac{\mu\text{g-eq}}{\text{l}}. \]

Fig. 1. Diagram of the device for measuring total water hardness

To achieve such sensitivity it is necessary to create conditions favoring scale formation:

1) simultaneous presence in the water being analyzed of cations and anions of the given sparingly soluble salt that forms scale;

2) presence in the water of scale-forming salts in such amounts that the product of the concentrations of the corresponding ions does not exceed the value of the solubility product at the boiling temperature of water. These conditions can be obtained either by reaching a water temperature of 300°C, or by introducing into the water being analyzed a precipitant selective for calcium and magnesium cations.

Monitoring the salt content at a water temperature of 300°C was associated with overcoming certain technical difficulties; therefore, further studies were directed toward determining a precipitant. The fluoride anion was chosen as the precipitant. Calcium and magnesium fluorides are thermally stable and do not undergo dissociation in boiling water.

For example, their solubility decreases with increasing temperature in the range 20°–100°C, which promotes their deposition on the heated surface. The use of fluoride anions for carrying out the proposed method is the most effective, since fluorides of other metals (Fe; Al; K; Na, etc.), usually present in feedwater, are soluble and have a positive temperature coefficient of solubility in the range 20°–100°C. The exceptions are the salts \(K_2SiF_6\) and \(CuF_2\). However, formation of these compounds is possible only in an acidic medium, whereas the feedwater used in the above-mentioned branches of industry has a neutral...

or slightly alkaline medium. In determining the lower detection limit it was found that the different carrier (salt) of fluoride anions affects the value of the lower detection limit of the total hardness of water in different ways. For example, to record the total hardness of water

\[ Ж_0 = 0.05 \frac{\text{µg-eq}}{\ell} \]

fluoride anions were required in the amount \(F^- = 65 \cdot 10^{-2}\ \text{g}/\ell\) (KF);

\[ F^- = 62 \cdot 10^{-2}\ \text{g}/\ell \ (\mathrm{NaF}); \qquad F^- = 13 \cdot 10^{-2}\ \text{g}/\ell \ (\mathrm{NH_4F}). \]

This is explained by the fact that the ratio of the formation of sparingly soluble and soluble fluorides depends on the nature of the carrier of fluoride anions.

Fig. 2. Dependence of the instrument reading on water hardness

The most complete precipitation of fluorides on the surface of the foil is achieved when \(\mathrm{NH_4F}\) and KF are used. The carrier concentration was selected experimentally, on the basis of the condition that a change in the dosage of anions, in order to achieve maximum precipitation of sparingly soluble salts, should not affect the accuracy of the measurement. Such a dosing range was found, which made it possible not to use a precision dispenser. This is an advantage in comparison with other methods. The degree of precipitation is also affected by the correct selection of the heater material. It is assumed that materials with high specific resistance have the greatest capacity for the formation of salt deposits. The amount of salt deposit on nickel foil was measured with the aid of a photometric device (Fig. 1). The experimental results, reflecting the dependence of the instrument reading on changes in water hardness, are shown by the curve in Fig. 2.

Conclusions

1. The selectivity of the thermochemical method for determining the total hardness of water satisfies the requirements set by production.

2. It is possible to use the thermochemical method for continuous analysis of water hardness.

3. It is necessary to continue research on the thermochemical method for determining water hardness in order to find the optimum conditions for scale formation before proceeding directly to the design of the instrument.

LITERATURE

1. T. M. Zhivilova. Chemical control of non-heat power plants. In: “Water Treatment, Water Regime, and Chemical Control at Steam-Power Installations.” Moscow–Leningrad, Energiya, p. 150, 1966.

2. Water Regime of Thermal Power Plants. Edited by T. Kh. Margulova. Moscow, Energiya, p. 131, 1965.