JPH0517502B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for measuring hydrochloric acid concentration in a chlorine-containing aqueous hydrochloric acid solution. BACKGROUND ART Chlorine production by hydrochloric acid electrolysis is carried out industrially overseas as a means of reusing by-product hydrochloric acid in the process of chlorinating organic substances. In recent years, it has become possible to supply the required amount of chlorine on-site for small and medium-sized chlorine needs such as manufacturing organic and inorganic chemicals and water and wastewater treatment, without having to transport and stockpile chlorine, which is at risk of leakage accidents. There is a trend toward supplying it through hydrochloric acid electrolyzers. In such hydrochloric acid electrolysis equipment, it is required to maintain the concentration of anolyte solution hydrochloric acid within a range where oxygen generation does not occur, from the viewpoint of improving hydrochloric acid generation efficiency and durability of electrolytic cell materials. A hydrochloric acid concentration meter is required. For example, in batch electrolysis, where the anolyte that has become diluted by electrolysis is drained and concentrated hydrochloric acid is introduced, the anolyte is automatically maintained within a certain range of hydrochloric acid concentration, the anolyte should be drained and introduced. A densitometer is required to detect the concentration and output a signal. A silver-silver chloride electrode is usually used to measure dilute hydrochloric acid concentration. However, silver-silver chloride electrodes cannot be used in concentrated hydrochloric acid because both silver and silver chloride are soluble in hydrochloric acid with a concentration of 10% or more by weight. Further, other chloride ion selective electrodes do not have resistance to chlorine-containing concentrated hydrochloric acid. Conductivity meters are currently the only concentration meters that can be applied to highly corrosive solutions such as chlorine-containing hydrochloric acid, but they have the following drawbacks in terms of performance and economy. That is, since the relationship between the conductivity and concentration of hydrochloric acid is not linear and has a maximum value at about 20% by weight, there are two concentrations of hydrochloric acid corresponding to one conductivity value. or,
It is difficult to accurately determine the concentration from the conductivity because the change in conductivity from 10% by weight to 30% by weight is small and the change in conductivity with temperature is large. In this way, the measurement method using a conductivity meter, in particular,
This method for measuring the concentration of concentrated hydrochloric acid has a major drawback in terms of accuracy, and also has the drawback that the measuring device is very expensive, so there is a need for a new measuring method to replace it. Means for Solving the Problems In view of the current situation as described above, the present inventor has conducted extensive research in order to find a simple method for measuring the concentration of hydrochloric acid with high accuracy in a concentrated aqueous solution of hydrochloric acid containing chlorine. Ta. As a result, when a chlorine-containing aqueous hydrochloric acid solution with an unknown concentration to be measured and a chlorine-containing aqueous hydrochloric acid solution with a known concentration as a reference solution are brought into contact via a liquid junction, a phenomenon occurs between the insoluble electrode inserted into both solutions. It has been found that the electromotive force changes almost linearly with the hydrochloric acid concentration in the liquid to be measured when the liquid temperatures of both solutions are the same. Furthermore, it has been found that even when the temperature of the liquid to be measured and the reference liquid are different, the electromotive force can be converted to the electromotive force when the temperature is the same using a simple conversion formula. As a result, by a simple method of measuring the temperature difference between the measurement target liquid and the reference liquid and the electromotive force between the chlorine electrodes inserted in both liquids, we found that It has now become possible to measure hydrochloric acid concentration with high accuracy. That is, in the present invention, a chlorine-containing aqueous hydrochloric acid solution to be measured and a chlorine-containing aqueous hydrochloric acid solution having a known concentration of hydrochloric acid,
The present invention relates to a method for measuring the concentration of hydrochloric acid in a chlorine-containing aqueous hydrochloric acid solution, which is characterized by measuring the electromotive force generated between insoluble electrodes that are brought into contact via a liquid junction and inserted into both solutions, and the temperature difference between the two solutions. In the present invention, from a chlorine-containing aqueous hydrochloric acid solution with a hydrochloric acid concentration of about 2% by weight (0.5mol/Kg), to 36% by weight
It is possible to accurately measure the hydrochloric acid concentration of chlorine-containing aqueous hydrochloric acid solutions with a wide range of concentrations, including concentrated chlorine-containing aqueous hydrochloric acid solutions with a hydrochloric acid concentration of about (15 mol/Kg). In particular, the method of the present invention is advantageous in that it is possible to accurately measure the concentration of a high concentration aqueous hydrochloric acid solution of about 10% by weight (3 mol/Kg) or more, which was difficult to measure accurately using conventional methods. In order to accurately measure the chlorine concentration in the hydrochloric acid aqueous solution, it is desirable that the solubility of the hydrochloric acid aqueous solution at the temperature and the hydrochloric acid concentration, that is, that the chlorine is saturated. However, even if only a small amount of chlorine is present, when an insoluble electrode is inserted, the chlorine electrode reaction will occur dominantly, so 0.01% by weight
Hydrochloric acid concentration can be measured even at moderate chlorine concentrations. In particular, in the chlorine production process using hydrochloric acid electrolysis, the chlorine concentration becomes saturated immediately after the start of the electrolytic operation, so a stable chlorine electrode reaction occurs and accurate measurements can be made. Regarding the chlorine-containing hydrochloric acid aqueous solution used as the reference solution, the vapor pressure of hydrochloric acid is low, and the concentration range of hydrochloric acid is difficult to change.
It is preferable that the hydrochloric acid concentration is below the level of
Moreover, those having a hydrochloric acid concentration of about 2% by weight (0.5 mol/Kg) or more are preferably used. Although it is possible to measure even a trace amount of chlorine in the standard solution,
For accurate measurement, it is preferable that the sample be saturated with chlorine. The sample liquid and the reference liquid can be used at a temperature in the range of about 0°C to 80°C, but the temperature of the reference liquid is preferably about 50°C or lower. In the method of the present invention, the above-described sample solution and reference solution are brought into contact via a liquid junction, an insoluble electrode is inserted into both solutions, and the electromotive force generated between the two electrodes is measured. It is necessary to measure the temperature difference between The material of the liquid junction used when bringing the sample solution and reference solution into contact can withstand chlorine-containing hydrochloric acid.
There is no particular limitation as long as it is a porous material, ion conductor, etc. that does not cause liquid movement, and porous materials such as various sintered glasses, ceramics such as alumina and zirconia, and fluororesin-based cation exchangers can be used. Ion conductors such as can be used. In addition, the insoluble electrode inserted into both solutions is not particularly limited as long as it is a conductor that is insoluble in chlorine-containing hydrochloric acid, but for example, it can be made of a metal such as iridium, titanium plated with iridium, or coated with iridium oxide, ruthenium oxide, etc. Electrodes coated with metal oxides such as titanium can be suitably used. The means for determining the temperature difference between the two liquids is not particularly limited, and may be measured using an ordinary thermometer, but a thermocouple such as a copper-constantan thermocouple or a chromel-alumel thermocouple may be used. This is convenient when using these electromotive forces as input signals to the densitometer. In the method of the present invention, the reason why the concentration of hydrochloric acid in the sample solution can be determined by measuring the electromotive force of the chlorine electrode generated between the reference solution and the sample solution and the temperature difference between the two solutions, and The calculation method is shown below. A sample solution having a temperature T _s and a hydrochloric acid molar concentration m _s ;
with a reference solution having a temperature T _r and a molar concentration m _r of hydrochloric acid,
When an insoluble electrode is inserted and a battery based on the chlorine electrode reaction 2C ^- C ₂ +2e (1) is assembled, the electromotive force V is expressed by the following equation. V=E _s,Ts −E _r,Tr −E _J +E _T +E _H (2) Here, E _s,Ts is the equilibrium potential of the chlorine electrode of the sample solution at the hydrochloric acid concentration m _s and the temperature T _s , and E _r,Tr is the chlorine electrode equilibrium potential of the reference solution with hydrochloric acid concentration m _r and temperature T _r , E _J is the liquid junction potential at the liquid junction, E _T is the thermal diffusion electromotive force due to the temperature difference,
E _H is the potential due to temperature change in the standard hydrogen electrode potential. E _s,Ts is calculated using the Nernst formula, E _s,Ts = E° _Ts + (RT _s /2F)n{P _s,Ts / (γ _s,Ts m _s
)
² } (3). Here, E° _Ts is the standard chlorine electrode potential at temperature T _s , R is the gas constant, F is Faraday's constant, P _s,Ts is the chlorine partial pressure, and γ _s,Ts is the average activity coefficient of hydrochloric acid. . When chlorine is dissolved in hydrochloric acid, a C ₃ ^- ion is produced by the reaction C ^- +C ₂ C ₃ ^- (4).
Some of the ^- ions become C ₃ ^- ions, and the C ^- ion concentration in hydrochloric acid decreases slightly. However, C
₃ ^- The contribution of ion generation to the chlorine electrode equilibrium potential of chlorine-containing hydrochloric acid is 1 mV when the chlorine partial pressure is less than 1 atm.
Since it is calculated as follows, the contribution of C ₃ ^- ions to E _s,Ts can be ignored. Using known values for E° _Ts , P _s,Ts , γ _s,Ts, from 25°C to 80°C, from 1 mol/Kg to 11 mol/Kg.
E _{s and Ts} can be calculated for any T _s and m _s up to Kg. When m _s is constant, it is recognized that E _s,Ts tends to decrease linearly as T _s increases, and for E _s,Tp at a certain reference temperature T _p , using temperature coefficient A, E _s, An approximate expression of _Ts = E _s,Tp + A (T _s - T _p ) (5) can be given. According to the present inventors, if the reference temperature T _p is 25°C (298K) and the value of A = -1.35mV/K is used at a temperature of about 80°C or less and a hydrochloric acid concentration of 1 to 11 mol/Kg, (5 ) E _s,Ts obtained from equation (3) agreed with E _s,Ts obtained from equation (3) within ±2 mV. In other words, it was confirmed that the temperature change in E _s,Ts can be evaluated using equation (5). Therefore, by replacing the reference temperature T _p with the reference liquid temperature T _r , E _s,Ts −E _r,Tr ≒ (RT _r /2F)n{P _s,Tr / (γ _s,Tr
m _s ) ² }-(RT _r /2F)n{P _r,Tr /γ _r,Tr m _r ) ² }+A
An approximate expression of (T _s −T _r ) (6) can be given. Here, P _s,Tr and γ _s,Tr are the equilibrium activity coefficients of chlorine partial pressure and hydrochloric acid under the conditions of _Tr in m _s . Using equation (6), the difference in chlorine electrode equilibrium potential between two hydrochloric acid solutions with different temperatures and hydrochloric acid concentrations can be related to the temperature difference and hydrochloric acid concentration between the two hydrochloric acid solutions. Regarding the liquid junction potential E _J , according to the diffusion potential equation, E _J ≒ (RT _r /F)∫ ^ms _nr (2T _H −1) dn (γm)
We can obtain the relational expression (7). Here, T _H is the transport number of hydrogen ions. The thermal diffusion electromotive force E _T and the temperature change E _H in the standard hydrogen electrode potential are quantities that cannot be determined individually in principle. Therefore, the present inventors created a battery consisting of two hydrogen electrodes with equal concentrations but different temperatures, and measured the electromotive force.
The temperature and hydrochloric acid concentration dependence of the sum of E _T and E _H was actually measured. E _T and E _H tend to increase slightly as the hydrochloric acid concentration increases, but we obtained measured values that can be regarded as almost constant against changes in concentration. Furthermore, the temperature change increased linearly as the temperature increased. That is, the following relationship was obtained: E _T +E _H ≈B (T _s - T _r ) (8), and the temperature coefficient B was 0.6 mV/K. From the above consideration of E _{s, Ts} , E _J , E _T and E _H ,
Substituting equations (6), (7), and (8) into equation (2), the approximate equation for V is V≒(RT _r /2F)n{P _s,Tr /(γ _s,Tr m _s ) ² }−
(RT _r /2F)n{P _r,Tr /(γ _r,Tr m _r ) ² }−(RT _r /
F)∫ ^ms _nr (2T _H −1)dn(γm)+(A+B)・(
T _s
−T _r ) (9) can be obtained. On the right side of the above equation, the third term is the electromotive force of the battery consisting of two chlorine electrodes at temperature T _r , and the fourth term can be regarded as the temperature correction term. By rearranging equation (9), the electromotive force V _Tr between a hydrochloric acid solution at temperature T _r and concentration m _r and a hydrochloric acid solution at temperature T _r and concentration m _s is defined by the following equation. (RT _r /2F)n{P _s,Tr /(γ _s,Tr m _s ) ² }−
(RT _r /2F)n{P _r,Tr /(γ _r,Tr m _r ) ² }−(RT _r /
F)∫ ^ms _nr (2T _H −1)dn(γm)≒V−(A+B)
(T _s − T _r )≡V _Tr (10) As can be seen from the right side of equation (10) above, the relationship between the sample solution at temperature T _s and hydrochloric acid concentration m _s and the reference solution at temperature T _r and hydrochloric acid concentration m _r The electromotive force V between can be converted into an electromotive force V _Tr when the sample liquid and the reference liquid are at the same temperature. On the other hand, the transfer number of hydrogen ions (T _H ) depends not only on the temperature and the concentration of hydrochloric acid but also on the material used for the liquid junction, so it is difficult to calculate the left-hand side of the above equation (10). Therefore, depending on the individual circumstances of the material used for the liquid junction,
It is necessary to experimentally determine in advance the electromotive force V _Tr between the reference solution at temperature T r and hydrochloric acid concentration m _r and the sample solution at temperature T _{r and} hydrochloric acid concentration _{m s} as a function of m _s . . In a chlorine-containing hydrochloric acid aqueous solution, as can be seen from FIGS. 1 and 2 in Examples 1 and 2 below,
An almost linear relationship was observed between V _Tr and m _s . In this way, find the relationship between V _Tr and m _s , and measure the temperature difference T _s − T _r between the reference solution and sample solution and the electromotive force V between the chlorine electrode inserted in both solutions. , if we calculate V Tr based on the right side of equation (10) above, that is, V _Tr = V−(A+B)(T _{s −Tr} ) (10)′, we can obtain the _value obtained in advance.
The concentration of hydrochloric acid in the sample solution can be determined from the relationship between V _Tr and m _s . In the above formula (10)′, A is
-1.35mV/K may be used, and 0.6mV/K may be used as B. Note that the relationship between E _s,Ts and m _s in concentrated hydrochloric acid of about 1 mol/Kg or more is logarithmic, which holds true for dilute hydrochloric acid of about 0.1 mol/Kg or less, because the average activity coefficient increases significantly with increasing concentration. It's not a relationship, it's almost a linear relationship. Such a relationship is a desirable relationship for measuring the concentration of medium to high concentration hydrochloric acid. Effects of the Invention In the method of the present invention, for a chlorine-containing hydrochloric acid aqueous solution at any temperature and concentration, by measuring the temperature difference with the reference solution and the electromotive force between the reference solution and an insoluble electrode inserted in the sample solution, The concentration of hydrochloric acid in a sample solution can be easily determined with high accuracy. Furthermore, in the method of the present invention, the detection elements used are only two insoluble electrodes and a temperature detection element such as a thermocouple, making it possible to measure the hydrochloric acid concentration with a very simple device. In this manner, the present invention provides a highly accurate method for measuring hydrochloric acid concentration using an inexpensive and simple device for aqueous chlorine-containing hydrochloric acid solutions having a wide range of hydrochloric acid concentrations. Examples Hereinafter, the present invention will be explained in more detail with reference to Examples. Example 1 A chlorine-containing aqueous hydrochloric acid solution at 25°C with a saturated chlorine concentration of 1 mol/Kg and a total pressure of 1 atm was used as a reference solution, and a 6 mmφ glass tube sealed at one end with a silica-alumina porous material was used. This sintered body constitutes a liquid junction between the sample liquid and the reference liquid in the glass tube, and a 0.5 mmφ iridium wire electrode is used as the insoluble electrode. The electromotive force between the chlorine electrodes of the sample solution and reference solution V _Tr
The relationship between this and the hydrochloric acid concentration m _s in the sample solution was determined.
The results are shown in Figure 1. From Figure 1, T _Tr and m _s are:
It can be seen that there is an almost linear relationship. Also, hydrochloric acid concentration
3mol/Kg chlorine-containing hydrochloric acid aqueous solution and hydrochloric acid concentration
When similar measurements were performed using a 5 mol/Kg chlorine-containing hydrochloric acid aqueous solution as the standard solution, each m _s
V _Tr =0 at =3mol/Kg and m _s =5mol/Kg
Other than that, m _s and V _Tr had a linear relationship with almost the same slope as in FIG. Hydrochloric acid concentration 2, 4, 6 and 8 mol/sample solution
Using a chlorine-containing aqueous solution of Kg, the chlorine electrode potential was measured at each liquid temperature of 25°C and 50°C with respect to the reference solution. When the liquid temperature is 50℃, from the measured value of electromotive force V
When calculating V _Tr , in V _Tr = V - (A + B) × (T _s - T _r ), A
= -1.35 mV/K and B = 0.6 mV/K were used. When the hydrochloric acid concentration of the sample solution was calculated from the obtained V _Tr using FIG. 1, a result was obtained that almost matched the hydrochloric acid concentration of the sample solution used. Example 2 As a reference solution, a total pressure of 1 with a hydrochloric acid concentration of 1 mol/Kg
A chlorine-containing hydrochloric acid aqueous solution with a saturated chlorine concentration at atmospheric pressure is used, and a fluororesin-based ion exchange membrane (trademark: Nafion), which is a cation conductor, is used as the material for the liquid junction.
117 (manufactured by Dupont), using a titanium wire plated with iridium as an insoluble electrode, the electromotive force V _Tr was measured at each temperature of 25°C and 50°C when the reference solution and sample solution were at the same temperature. is shown in Figure 2.
From Figure 2, in both cases of 25℃ and 50℃
It can be seen that m _s and V _Tr have an almost linear relationship.
Also, compared to Example 1, V _Tr at the same m _s
The value was larger in Example 2, but this is because the material of the liquid junction used in Example 2 was a cation exchange membrane, so the hydrogen ion transfer number T _H had a large value. be. Using the 25℃ or 50℃ standard solution mentioned above, measure the electromotive force V between the chlorine electrodes of the sample solution and the reference solution and the temperature difference between the two solutions using chlorine-containing hydrochloric acid aqueous solutions with various hydrochloric acid concentrations as sample solutions. Then, the concentration of hydrochloric acid in the sample solution was determined from FIG. When converting the measured value V of electromotive force to the electromotive force V _Tr between isothermal liquids, in the conversion formula V _Tr = V - (A + B) (T _s - T _r ), A = -1.35 mV/K, B=
0.6mV/K was adopted. The results are shown in Table 1 below.

[Table] As is clear from Table 1, the chemically analyzed value of the hydrochloric acid concentration in the sample solution and the value measured by the method of the present invention are almost in agreement, indicating that the method of the present invention is effective. Example 3 A concentration meter to which the method of the present invention is applied was installed in a hydrochloric acid electrolysis system for the purpose of detecting the concentration of hydrochloric acid in the anolyte and managing the anolyte in the hydrochloric acid electrolysis system. A schematic diagram of the hydrochloric acid electrolysis system is shown in Figure 3. In this system, an anolyte storage tank/gas-liquid separator 2 with an internal volume of approximately 5 is installed above an electrolytic cell 1 with an electrode area of 50 cm ² , and a cathode-side gas-liquid separator 3 is installed on the cathode side of the electrolytic cell. Established. Anolyte storage tank 2 and electrolytic cell 1
A liquid reservoir 4 of approximately 10 cm ³ was provided in the anolyte circulation line between the two, and an iridium wire electrode was inserted therein to serve as a chlorine electrode 5 for the sample liquid. On the other hand, a 1 mol/Kg aqueous hydrochloric acid solution 7 saturated with chlorine as a reference solution is brought into contact with the anolyte in the anolyte storage tank 2 through a liquid junction 6 made of porous sintered silica-alumina ceramics. An iridium wire electrode was inserted into a chlorine-saturated hydrochloric acid aqueous solution 7 to form a reference liquid chlorine electrode 8. A sample liquid chlorine electrode 5 and a reference liquid chlorine electrode 8 are attached to the concentration controller (not shown).
A thermocouple electromotive force V _T due to the temperature difference between the two liquids was introduced using a copper-constantan thermocouple 9. The concentration controller calculates V _Tr based on the formula V _Tr = V + K・V _T (11), outputs V _Tr to the recorder as an indicator of anolyte hydrochloric acid concentration, and also calculates V Tr for automatic introduction and discharge of anolyte. It was used as a voltage signal. In the above equation (11), K is the temperature correction coefficient, and
Constantan thermocouple temperature coefficient 23.3K/mv,
A=-1.35mv/K, B= in the above equation (10)
Based on 0.6mv/K, a value of K=17.5 was used. The electrolytic test of an aqueous hydrochloric acid solution was conducted continuously for three months or more, during which time hydrochloric acid was introduced through the supply hydrochloric acid inlet 10 and hydrochloric acid was discharged through the waste hydrochloric acid outlet 11. During the electrolytic test, the concentration of hydrochloric acid in the anolyte was monitored by a densitometer using the method of the present invention, and its discharge and introduction were performed automatically, ensuring that the anolyte was automatically managed without any abnormality. It was.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between V _Tr and m _s in Example 1, and FIG. 2 is a graph showing the relationship between V Tr and m _s in Example 2.
A graph showing the relationship between m _s and FIG. 3 is a schematic diagram of a hydrochloric acid electrolysis system equipped with a concentration meter based on the method of the present invention. 1... Electrolytic cell, 2... Anolyte storage tank, 3...
Cathode side gas-liquid separator, 4...Sample liquid, 5...Sample liquid chlorine electrode, 6...Liquid junction, 7...Reference liquid, 8...
Reference liquid chlorine electrode, 9... thermocouple, 10... replenishment hydrochloric acid inlet, 11... waste hydrochloric acid outlet.

Claims (1)

[Claims] 1 The chlorine-containing aqueous hydrochloric acid solution to be measured and the chlorine-containing aqueous hydrochloric acid solution with a known hydrochloric acid concentration are brought into contact via a liquid junction, and the electromotive force generated between the insoluble electrodes inserted into both solutions and the temperature difference between the two solutions are measured. A method for measuring the concentration of hydrochloric acid in a chlorine-containing hydrochloric acid aqueous solution.