Investigation of an electrochemical method for its application in measuring the carbonate hardness of water
Abstract Generated abstract
This paper investigates an electrochemical method for continuous determination of carbonate hardness in water, proposed as an alternative to acidimetric titration for use in automatic control of industrial water treatment. The method is based on depositing calcium carbonate, magnesium carbonate, and magnesium hydroxide on a cathode under weak direct current, then relating the amount of precipitate to carbonate hardness, including by photoelectric measurement of reflected light. Experiments examined precipitate composition, selectivity, sensitivity, and the effects of current density, temperature, pH, and water mixing. The results indicate good selectivity, measurable response from about 10 to 500 microgram equivalents per liter, and an optimal current density of 25 to 100 mA per square decimeter for stable precipitate formation.
Full Text
INVESTIGATION OF THE ELECTROCHEMICAL METHOD FOR ITS APPLICATION IN MEASURING THE CARBONATE HARDNESS OF WATER
P. M. Titanov, M. F. Tkachenko, I. G. Minaev, A. Ya. Tarasov
I. Introduction
The problems of water treatment and chemical control of the quality of process waters, including carbonate hardness, are of great practical importance.
As is known, the formation of scale on the walls of various heat-exchange apparatuses causes great damage at thermal power stations and at enterprises of the oil, oil-refining, chemical, and many other branches of industry, where water is widely used for cooling apparatuses and chemical products; this water contains a significant amount of carbonate-hardness salts, which constitute the predominant part of the total hardness of water and are the principal source of scale formation.
For purifying water from impurities, including carbonate hardness, various physicochemical methods and a complex technology for its treatment exist [1].
In order to operate properly water-treatment installations for preliminary softening of water, careful and continuous monitoring by means of automatic instruments of the content of carbonate-hardness salts in the water being treated is necessary. The availability in production of automatic continuous analyzers of carbonate hardness will ensure reliable purification of water from carbonate-hardness salts and will make it possible to transfer water-treatment installations to automated control, where automatic analyzers of the carbonate hardness of water will serve as sensors of the automatic-control system. Automation of monitoring and control will substantially increase the efficiency of utilization of water-treatment equipment.
At present, to determine the carbonate hardness of water, the acidimetric method of analysis is used, according to which the water being analyzed is titrated with hydrochloric acid in the presence of the indicator methyl orange [2]. This method of determining the carbonate hardness of water is of low sensitivity and nonselective. The most substantial shortcoming of this method is that it does not make it possible to create continuous automatic instruments for monitoring carbonate hardness directly at industrial installations.
The proposed method for determining carbonate hardness does not have the indicated disadvantages; it makes it possible, using available technical means, to create a continuously operating instrument of simple design, and is distinguished by the fact that carbonate hardness is determined from the amount of calcium carbonate and magnesium hydroxide hydrate deposited on the cathode of an electrochemical cell as a result of the electrochemical decomposition of calcium and magnesium bicarbonates by hydroxyl ions formed in the near-cathode region when a weak direct current \((25 \div 100\ \mathrm{mA/dm^2})\) is passed through the electrochemical cell [3].
Automatic continuous-action analyzers of the carbonate hardness of water, constructed on the basis of the proposed method, can be installed in industrial plants, where they can be used as sensors in systems for automatic control of water-treatment technological processes.
II. Essence of the Method
The essence of the proposed method for determining the carbonate hardness of water is as follows: if an electrochemical cell consisting of a cathode and an anode is filled with the water being analyzed and a direct current is passed through it, then at the cathode assimilation of electrons by oxygen dissolved in the water takes place, with the formation of hydroxyl ions. The hydroxyl ions formed at the cathode neutralize bicarbonate ions to carbonic-acid anions, which, interacting with calcium and magnesium cations in a weakly alkaline medium, form on the cathode surface a water-insoluble precipitate of calcium and magnesium carbonates. Magnesium hydroxide hydrate is also formed on the cathode surface as a result of the hydrolysis of magnesium carbonate and partly as a result of direct interaction of magnesium cations with hydroxyl ions.
Thus, on the surface of the cathode immersed in the water being analyzed, as a result of electrochemical processes, decomposition of the calcium and magnesium bicarbonates responsible for the carbonate hardness of the water takes place, and a precipitate consisting of calcium and magnesium carbonates and partly magnesium hydroxide hydrate is deposited; moreover, the degree of deposition of this precipitate depends on the carbonate hardness of the water, which makes it possible to determine the carbonate hardness of the water from the amount of precipitate deposited on the cathode.
The amount of calcium and magnesium carbonates and partly magnesium hydroxide hydrate deposited on the cathode can be measured by known methods, for example, photometrically from the amount of light reflected from the cathode, conductometrically from the change in the surface contact resistance at the boundary between the cathode and the electrically conducting medium, by means of activation methods of analysis, etc.
By using metal wire or foil as cathodes and continuously drawing them through an electrochemical cell with the flowing water under analysis, carbonate hardness can be measured automatically and continuously.
III. Physicochemical Foundations of the Method
When a weak direct current \((25 \div 100\ \mathrm{mA/dm^2})\) is passed through an electrochemical cell with the water being analyzed, the following electrochemical and chemical reactions will occur at the electrodes:
| At the cathode \(( - )\) | At the anode \(( + )\) |
|---|---|
| \(2\mathrm{H_2O}+\mathrm{O_2}+4e \to 4\mathrm{OH^-}\) | \(2\mathrm{O^{2-}}-4e \to 2\mathrm{O}\to \mathrm{O_2}\uparrow\) |
\[ \begin{gathered} \mathrm{OH}^{-}+\mathrm{HCO}_{3}^{-}\to \mathrm{CO}_{2}^{2-}+\mathrm{H}_{2}\mathrm{O}\\ \mathrm{Ca}^{2+}+\mathrm{CO}_{3}^{2-}\to \underline{\mathrm{CaCO}_{3}}\\ \downarrow\\ \mathrm{Mg}^{2+}+\mathrm{CO}_{3}^{2-}\to \underline{\mathrm{MgCO}_{3}}\\ \downarrow\\ \mathrm{MgCO}_{3}+\mathrm{H}_{2}\mathrm{O}\to \underline{\mathrm{Mg(OH)}_{2}}+\mathrm{CO}_{2}\\ \downarrow\\ \mathrm{Mg}^{2+}+2\mathrm{OH}^{-}\to \underline{\mathrm{Mg(OH)}_{2}}\\ \downarrow \end{gathered} \qquad \begin{gathered} \mathrm{SO}_{4}^{2-}-2e+2\mathrm{H}_{2}\mathrm{O}\to\\ \to 2\mathrm{H}_{2}\mathrm{SO}_{4}+\mathrm{O}_{2}\uparrow\\[1em] 2\mathrm{Cl}^{-}-2e\to \mathrm{Cl}_{2}\uparrow\\[1em] \mathrm{C}+\mathrm{O}_{2}\to \mathrm{CO}_{2}\uparrow \end{gathered} \]
Thus, as a result of the electrochemical and chemical processes occurring in the cell with the water being analyzed, bicarbonate ions are decomposed by hydroxyl ions and a precipitate of calcium and magnesium carbonates forms on the surface of the cathode. At the anode, oxygen, chlorine, and other gases are evolved.
When a direct current of more than \(100\ \mathrm{mA/dm^{2}}\) is passed through the electrochemical cell with the water being analyzed, electrolysis of the water takes place. In this case the following electrochemical and chemical reactions proceed at the electrodes:
\[ \begin{array}{ll} \text{At the cathode }(-) & \text{At the anode }(+)\\[0.4em] 4\mathrm{H}_{2}\mathrm{O}\to 4\mathrm{H}^{+}+4\mathrm{OH}^{-} & 4\mathrm{H}_{2}\mathrm{O}\to 4\mathrm{H}^{+}+4\mathrm{OH}^{-}\\ 4\mathrm{H}^{+}+4e\to 2\mathrm{H}_{2}\uparrow & 4\mathrm{OH}^{-}-4e\to 2\mathrm{H}_{2}\mathrm{O}+\mathrm{O}_{2}\uparrow\\ 4\mathrm{H}_{2}\mathrm{O}+4e\to 2\mathrm{H}_{2}\uparrow+4\mathrm{OH}^{-} & 4\mathrm{H}_{2}\mathrm{O}-4e\to \mathrm{O}_{2}\uparrow+4\mathrm{H}^{+}\\ 4\mathrm{OH}^{-}+\mathrm{HCO}_{3}^{-}\to \mathrm{CO}_{3}^{2-}+\mathrm{H}_{2}\mathrm{O} & \\ \mathrm{Ca}^{2+}+\mathrm{CO}_{3}^{2-}\to \underline{\mathrm{CaCO}_{3}} & \\ \downarrow & \\ \mathrm{Mg}^{2+}+\mathrm{CO}_{3}^{2-}\to \underline{\mathrm{MgCO}_{3}} & \\ \downarrow & \\ \mathrm{MgCO}_{3}+\mathrm{H}_{2}\mathrm{O}\to \underline{\mathrm{Mg(OH)}_{2}}+\mathrm{CO}_{2}\uparrow & \\ \downarrow & \\ \mathrm{Mg}^{2+}+2\mathrm{OH}^{-}\to \underline{\mathrm{Mg(OH)}_{2}} & \\ \downarrow & \end{array} \]
Thus, as a result of the electrolysis of water, oxygen is evolved and hydrogen ions are concentrated in the anode space, while hydrogen is evolved and hydroxyl ions are concentrated in the cathode space. The hydroxyl ions formed at the cathode neutralize bicarbonate ions to carbonic acid ions, which, entering into chemical interaction with calcium and magnesium cations, give on the cathode surface a precipitate consisting of calcium and magnesium carbonates and, in part, magnesium oxide hydrate.
For the correct application of the electrochemical method for determining the carbonate hardness of water, the question of choosing the optimum current-density value is of great importance. In resolving this question one may use the results of work [3], where data are presented on the investigation of the dependence of the amount of cathode deposit on the magnitude of the current density. These data show that the amount of cathode deposit, with increasing current density, first increases, reaches a maximum value at a current density of about \(100\ \mathrm{mA/dm^{2}}\), and then sharply decreases.
In studying the structure of the cathode deposit as a function of the magnitude of the current density, the authors of the work established that when a current of more than \(100\ \mathrm{mA/dm^{2}}\) is used, a less strong, loose
film of the precipitate, while the chemical composition of the precipitate, as the potential and current density increase, changes toward an increase in the content of magnesium hydroxide and other salts, with a simultaneous decrease in the content of calcium carbonate.
The decrease in the amount of cathodic precipitate and the change in the structure of the precipitate at a current density of more than \(100\ \text{mA}/\text{dm}^2\) are explained by the fact that the gaseous hydrogen that forms covers the cathode surface with bubbles, which, when the precipitate is deposited, loosen its layer. The precipitate is thereby permeated by an enormous number of small pores. The bond between the precipitate and the cathode becomes weak, as a result of which it is detached by hydrogen bubbles formed during the electrolysis of water.
The change in the chemical composition of the precipitate is explained by the fact that, at high current density and electrode potential, hydroxides of other accompanying cations begin to be deposited in the precipitate; as a result, the relative amount of calcium carbonate and magnesium against the background of other salts decreases.
The data presented are of very great importance for the proper application of the electrochemical method. They show that, in order to prevent undesirable phenomena when selecting the optimum current density, the following requirements must be taken into account:
- Homogeneity of the structure of the cathodic precipitate.
- The possibility of obtaining in it the greatest amount of calcium carbonate and magnesium oxide hydrate.
- The smallest change in the pH of the water.
On the basis of the data of work [3], as well as our own experimental studies, it is possible to recommend, for the electrochemical method of determining the carbonate hardness of water, a current density in the range of \(25 \div 100\ \text{mA}/\text{dm}^2\). At the recommended current density, the precipitate obtained on the cathode best satisfies the requirements listed above, and accompanying cations under these conditions practically do not pass into the precipitate.
IV. Apparatus and Methodology of the Experimental Investigation of the Electrochemical Method
To obtain a precipitate on the cathode of the electrochemical cell, an experimental setup was assembled, the diagram of which is shown in Fig. 1. Fluoroplastic-4 and glass were chosen as materials for the manufacture of auxiliary parts meeting the requirements of high chemical resistance. In addition to chemical resistance, the following additional requirements are imposed on electrode materials. The electrodes must not decompose as a result of electrochemical processes, and their surface must not change in the process. These requirements are best met by platinum, graphite, nickel, and copper. The use of platinum electrodes is limited by their high cost, while copper electrodes are oxidized relatively easily by atmospheric oxygen, which reduces the accuracy of measurement. Therefore, we chose graphite and nickel as electrode materials. The output ends of the graphite electrodes were coated, for better contact, with copper deposited electrolytically.
Spectral analysis of the cathodic precipitate was performed on an ISP-67 spectrograph. The phase composition of the cathodic precipitate was determined by the X-ray structural method on a URS-50IM type apparatus; the acidity of the water was measured with an LP-58 type pH meter.
To measure the amount of carbonate-hardness salts that had passed from the water into the precipitate, by the photoelectric method based on measuring the in-
...the intensity of the light reflected from the surface of the foil, which served as the cathode, an instrument was used whose circuit is shown in Fig. 2.
The titrated solutions and auxiliary reagents were prepared by gravimetric and volumetric methods from reagents of the chemically pure and analytical-grade classifications. Samples were weighed on AD-200 analytical balances and dissolved in bidistilled water, the hardness of which was equal to zero. To prepare alcoholic solutions, rectified spirit was used.
Fig. 1. Diagram of the experimental setup for obtaining a precipitate of salts of carbonate hardness on the cathode of an electrochemical cell: B—dc source; \(R\)—rheostat; \(k_1\)—switch; \(K\)—cathode; \(A\)—anode; \(MA\)—milliammeter.
Fig. 2. Circuit for measuring, by the photoelectric method, the amount of deposited salts of carbonate hardness on the cathode surface.
The sensitivity of the electrochemical method was determined on standard samples prepared by diluting raw tap water with bidistillate. The hardness of the original raw water was determined by the acidimetric method.
V. Experimental data and their discussion
The experimental study of the electrochemical method for determining the carbonate hardness of water was carried out in order to solve the following questions:
-
Establishing the phase composition of the cathodic precipitate and determining the quantitative content of calcium carbonate with magnesium hydroxide and secondary salts.
-
Determining the sensitivity and selectivity of the electrochemical method.
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Studying the influence of various factors on the intensity of formation of the cathodic precipitate: temperature, acidity, and intensity of water mixing.
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Clarifying the possibility of recording the amount of cathodic precipitate by the photoelectric method.
As is known, technical waters, in addition to calcium and magnesium cations, contain cations of other metals. These accompanying, uncontrolled impurities can also produce precipitates of the corresponding hydroxides at the cathode and thereby introduce an error into the measurement results. To clarify this question, spectral and phase analyses were performed on the cathodic precipitate obtained from raw tap water at a current density of \(100\ \mathrm{mA/dm^2}\). The results of the spectral analysis showed that the cathodic precipitate contains 97% calcium and magnesium and 3% impurities, the largest portion of which, in decreasing order of their amounts, consists of chromium, iron, manganese, and aluminum. The results...
phase-analysis data showed that the cathodic precipitate consists mainly of calcium carbonate and magnesium hydroxide. The amount of other salts present in the cathodic precipitate proved to be beyond the sensitivity limit of the setup, equal to 3%, which indicates the good selectivity of the electrochemical method. At the same time, the selectivity of the electrochemical method was investigated on standard solutions of various cations prepared with bidistilled water.
Fig. 3. Dependence of the change in the readings of a photoelectric device on the concentration of salts of carbonate hardness of water at different cathode holding times in the analyzed water: 1—5 min; 2—10 min; 3—15 min.
In this case it was established that precipitation of uncontrolled cations in the form of their hydroxides on the cathode of the electrochemical cell is observed at concentrations exceeding their content in industrial waters.
Taking the results of the studies into account, it may be stated that the electrochemical method for determining the carbonate hardness of water fully meets the requirements of selectivity. Salts that do not determine the carbonate hardness of water are deposited on the cathode at a current density of 100 mA/dm² in negligible amounts; i.e., this method makes it possible to determine mainly only the salts of carbonate hardness.
The results of studying the sensitivity of the electrochemical method on standard samples showed that noticeable deposition of carbonate-hardness salts on the cathode of the electrochemical cell is observed when their content in water is 10 μg-equiv/L and higher; moreover, the sensitivity of the method depends on the time for which the cathode is held in the water being analyzed.
When the water is heated, the amount of cathodic precipitate increases; at 100°C, a change in its structure is observed: from even and continuous it becomes island-like.
Mixing (circulation) of the water reduces the amount of cathodic precipitate, which can be explained by mixing of the alkaline and acidic media from the near-cathode and near-anode spaces. To avoid mixing of the water, the anodic and cathodic spaces should be separated from each other by a porous partition (for example, porous glass).
A change in the acidity of the water in the alkaline region (pH > 7) has little effect on the intensity of formation of the cathodic precipitate; in an acidic medium, however, deposition of the cathodic precipitate worsens, which is explained by the dissolution of calcium and magnesium carbonates.
To determine the possibility of recording the amount of cathodic precipitate, the applicability of the photoelectric method was investigated. Figure 3 shows the dependence of the readings of the photoelectric device on the carbonate hardness of water for cathode holding times in the analyzed water of 5, 10, and 15 minutes. It is evident from the figure that there is an unambiguous relationship between the degree of deposition on the cathode of calcium carbonate and magnesium hydroxide precipitate, which depends on the carbonate hardness of the water, and the intensity of light reflected from the surface
the light cathode makes it possible to determine carbonate hardness from 10 to 500 μg-eq/l; moreover, the sensitivity of the method is determined by the duration of exposure of the cathode in the water being analyzed.
When the photoelectric method is used to measure the amount of cathodic precipitate, special attention must be paid to the appearance and structure of the precipitate, which depend on a number of factors characterizing the deposition regime: current density, temperature, and water circulation.
As the studies showed, when the current density is increased above 100 mA/dm², the precipitate becomes loose and nonuniform. The same is observed when the water temperature is increased. When the water is stirred, the amount of precipitate decreases. All these factors must be taken into account in developing the design of an instrument for continuous automatic measurement of the carbonate hardness of water by the electrochemical method.
Conclusions
- An electrochemical method for determining the carbonate hardness of water has been proposed and investigated.
- The essence and physicochemical foundations of the method have been set forth.
- The principal issues of the method have been studied: sensitivity, selectivity, and inertia.
- The influence of various factors on the intensity of formation of the cathodic precipitate has been clarified: temperature, acidity, and intensity of stirring of the water.
- The possibility has been demonstrated of recording the amount of cathodic precipitate of carbonate-hardness salts by a photoelectric method, according to the intensity of light reflected from the surface of the foil.
References
- A. I. Baulina et al. Water Treatment at Thermal Power Stations. Moscow—Leningrad, “Energiya,” 1966.
- A. K. Babko, I. V. Pyatnitskii. Quantitative Analysis. Moscow, “Vysshaya shkola,” 1962.
- A. A. Farkhadov, S. A. Ragimova. An Electromechanical Method for Combating Scale. Baku, Azerbaijan State Publishing House, 1964.