On the Problem of Solubility in Analytical Chemistry
Abstract Generated abstract
This paper examines how external electrolytes affect the solubility, solubility product, and activity coefficient of silver bromate, a problem presented as important for improving analytical precipitation methods. Using iodometric determination of bromate ions and gravimetric determination of silver ions, the study compares numerous nitrate, sulfate, acid, and bromate systems at 25°C, including three-component systems and a four-component system combining a common ion electrolyte with sodium nitrate. The results indicate that electrolyte concentration and chemical nature strongly influence silver bromate behavior, and that effects of electrolytes without common ions show periodic trends related to the position of the cation in the periodic system. The four-component data show that common-ion suppression of solubility and nitrate-induced solubility increase can partially or fully compensate each other depending on concentration.
Full Text
On the Problem of Solubility in Analytical Chemistry
Yu. L. Lel’chuk
(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)
The problem of solubility is among the most urgent problems of modern analytical chemistry. The solution of this problem will make it possible, to a considerable extent, to refine existing chemical methods of analysis and will open broad possibilities for developing new methods of analysis. The essence of this problem lies in developing reliable ways of quantitatively taking into account all factors that jointly affect the solubility and solubility product of precipitates in systems of analytical significance. The need to accumulate experimental data in this area is specially indicated by the Conference on Classical Methods of Chemical Analysis at the GEOKhI of the Academy of Sciences of the USSR [1].
The present communication is a continuation of our work on investigating the influence of electrolytes of various types on the solubility, solubility product, and activity coefficient of silver bromate precipitate [2—6]. In all, we studied 20 three-component systems, in which the following were used as electrolytes: nitric acid; nitrates of lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc, cadmium, aluminum, and thorium; sulfuric acid; sulfates of lithium, sodium, potassium, magnesium, aluminum, and zirconium; and potassium bromate. In addition, we investigated the solubility in the four-component system silver bromate—potassium bromate—sodium nitrate—water, where both an electrolyte with a common ion and an electrolyte without common ions simultaneously affect the solubility and solubility product of the precipitate. The main purpose of the work is to establish general regularities in the influence of electrolytes of various types on the solubility and solubility product of a precipitate in the absence of explicit chemical interaction in the corresponding systems.
As our investigations have shown, the choice of a silver bromate precipitate for this purpose should be considered especially successful. Silver bromate is chemically stable in aqueous and aqueous-alcoholic solutions of many electrolytes over a sufficiently wide range of their concentrations. The existing quanti-
Modern methods for the determination of bromate ions and silver ions make it possible not only to follow the slightest changes in the solubility of the precipitate, but also to record any appreciable chemical changes of this compound in its saturated solutions. In all the systems we studied, the concentration of bromate ions was determined iodometrically by the procedure described by Kolthoff and Sandell [7]; the concentration of silver ions was determined gravimetrically in the form of silver chloride, the precipitation and settling of the precipitate being carried out in beakers protected from light by caps of thick black paper. Filtration and drying of the precipitate were carried out in glass filters with a No. 3 filtering bottom. The relatively high solubility of the silver bromate precipitate in water \((8.25 \cdot 10^{-3}\ \mathrm{mol/l})\) [2] enabled us to make no changes whatsoever in the method for determining solubility even in a three-component system with a common bromate ion, where the solubility of the precipitate in a \(0.3\ \mathrm{M}\) potassium bromate solution decreases by a factor of 11.73. The high specific gravity of silver bromate \((5.206)\) and its crystalline structure [8] account for the rapid settling of the precipitate. Complete clarification of the saturated solution above the precipitate occurs as early as 10–15 min after vigorous stirring with a mechanical stirrer has been stopped.
The principal data for most of the systems studied have been published by us previously [2–6]. In addition to these, some summary data are given below in the tables, making it possible to draw certain conclusions about the general nature of the effect of electrolytes of various types on the solubility, solubility product, and activity coefficient of silver bromate. In these tables we use the following notation: \(I\) — ionic strength of the solution, \(K\) — activity coefficient of silver bromate, \(P\) — solubility of the precipitate in \(\mathrm{mol/l}\), \(PR\) — solubility product of the precipitate.
Analysis of the data obtained shows that the solubility, the solubility product of the precipitate, and the activity coefficient of silver bromate depend not only on the valence type of the foreign electrolyte, not only on the ionic strength of the liquid phase in which precipitation is carried out, but also on the chemical nature of the corresponding compounds. The latter is especially important to emphasize for the systems we studied, in which no clearly evident chemical interaction of the components was observed. Comparison of all the data obtained makes it possible to draw quite definite conclusions not only about the nature of the influence of different cations depending on the position occupied by the corresponding metals in D. I. Mendeleev’s periodic system of the elements, but also about the relative influence of sulfate and nitrate anions. As is evident from the data of Table 1, the nitrates of metals of one and the same group of the periodic system exert the greater influence on the solubility, solubility product, and activity coefficient of silver bromate, the greater the atomic weight, the atomic number of the element in the system, and the radius of its cation. The nitrates of metals of one and the same period exert the smaller influence on the properties of the precipitate, the greater the atomic weight and atomic number of the metal. From Table 2 it is seen that this same regularity is repeated in sulfate systems.
Behavior of the silver bromate precipitate in normal nitrate solutions
Table 1
| Electrolyte | Nitric acid | Lithium nitrate | Sodium nitrate | Potassium nitrate | Calcium nitrate | Rubidium nitrate | Strontium nitrate | Aluminum nitrate | Thorium nitrate |
|---|---|---|---|---|---|---|---|---|---|
| \(P \cdot 10^{2}\) | 1.29 | 1.32 | 1.48 | 1.63 | 1.42 | 1.48 | 1.66 | 1.38 | 1.52 |
| \(\Pi P \cdot 10^{4}\) | 1.66 | 1.74 | 2.19 | 2.66 | 2.016 | 2.19 | 2.76 | 1.90 | 2.31 |
| \(\mu\) | 1.0129 | 1.0132 | 1.015 | 1.016 | 1.514 | 1.515 | 1.517 | 1.814 | 2.515 |
| \(K\) | 0.575 | 0.564 | 0.501 | 0.455 | 0.485 | 0.469 | 0.447 | 0.488 | 0.428 |
| \(P : P'\) | 1.56 | 1.60 | 1.80 | 1.98 | 1.72 | 1.79 | 2.01 | 1.67 | 1.84 |
| \(\Pi P : \Pi P\) | 2.44 | 2.56 | 3.22 | 3.91 | 2.96 | 3.22 | 4.06 | 2.80 | 3.40 |
| \(K : K'\) | 0.639 | 0.627 | 0.557 | 0.505 | 0.539 | 0.521 | 0.497 | 0.542 | 0.475 |
\(P\) and \(P'\) are the solubility of silver bromate, respectively, in a one-molar electrolyte solution and in water.
\(\Pi P_i\) and \(\Pi P\) are the solubility product of silver bromate, respectively, in a one-molar electrolyte solution and in water.
\(K_i\) and \(K\) are the activity coefficient of silver bromate, respectively, in a one-molar electrolyte solution and in water.
Table 2
Behavior of the silver bromate precipitate in normal sulfate solutions
| Electrolyte | Sulfuric acid | Lithium sulfate | Sodium sulfate | Potassium sulfate | Magnesium sulfate | Aluminum sulfate | Zirconium sulfate |
|---|---|---|---|---|---|---|---|
| \(P \cdot 10^2\) | 1,15 | 1,60 | 1,79 | 2,00 | 1,496 | 1,35 | 1,22 |
| \(\mathrm{PR} \cdot 10^4\) | 1,32 | 2,56 | 3,20 | 4,00 | 2,24 | 1,82 | 1,49 |
| \(I\) | 1,512 | 1,516 | 1,518 | 1,520 | 2,015 | 2,518 | 3,012 |
| \(K\) | 0,656 | 0,466 | 0,418 | 0,382 | 0,488 | 0,544 | 0,554 |
| \(P' : P\) | 1,39 | 1,94 | 2,17 | 2,42 | 1,81 | 1,64 | 1,48 |
| \(\mathrm{PR}' : \mathrm{PR}\) | 1,94 | 3,76 | 4,71 | 5,88 | 3,30 | 2,68 | 2,20 |
| \(K' : K\) | 0,729 | 0,518 | 0,464 | 0,424 | 0,542 | 0,604 | 0,615 |
Thus, the general character of the change in solubility, in the solubility product, and in the activity coefficient of the precipitate under the action of electrolytes of various types, in the absence of chemical interaction in the systems, is clearly periodic in nature and obeys D. I. Mendeleev’s periodic law. The more pronounced the metallic character of the metal, the more strongly its cation increases the solubility and the solubility product of the precipitate and, correspondingly, decreases its activity coefficient. In this connection one should expect that, of all existing nitrates and sulfates, the nitrates and sulfates of cesium and francium will exert the greatest influence on the properties of the precipitate. As far as we know from the literature, no investigator has hitherto arrived at such a conclusion. Possibly this is connected with the fact that no single precipitate has been investigated in this respect so broadly, in the sense of covering a large number of electrolytes of different types and a wide range of their concentrations.
As regards the influence of anions, of the two anions investigated by us, as is seen from the data of Tables 1 and 2, for the majority of systems the sulfate anion exerts a relatively greater influence.
In the future we propose to continue the investigations in order to verify to what extent our conclusions for the silver bromate precipitate will be valid for other precipitates under the same physicochemical conditions.
Of definite interest also are the data obtained by us for the systems silver bromate—potassium bromate—water and silver bromate—potassium bromate—sodium nitrate—water, where changes in the solubility and in the solubility product of the precipitate in the first system occur under the influence of the common bromate ion, and in the second under the combined influence of an electrolyte with a common ion and an electrolyte without a common ion. Owing to the limited solubility of potassium bromate in water, its highest concentration in our experiments was 0.3 mol/l. At this concentration the solubility of silver bromate in the system silver bromate—potassium bromate—water decreases by a factor of 11.73, while the solubility product increases by a factor of 3.1. As is seen from the data of Table 3, the decrease in the solubility of the precipitate in this system takes place regularly as the concentration of potassium bromate increases, whereas the change in the solubility product is of a more complex character. At low concentrations of potassium bromate there is a decrease in the solubility product from \(6{,}80 \cdot 10^{-5}\) in water to \(4{,}38 \cdot 10^{-5}\) in a 0.003 M solution, i.e. it decreases by
1.55 times. With further increase in the concentration of potassium bromide, there is a uniform increase in the solubility product, reaching, for 0.3 M potassium bromide, the value \(2.11 \cdot 10^{-4}\).
Table 3
Behavior of the silver bromide precipitate in potassium bromide solutions
| Potassium bromide, mol/l | \(10^{-3}\) | \(3 \cdot 10^{-3}\) | \(10^{-2}\) | \(3 \cdot 10^{-2}\) | \(10^{-1}\) | \(3 \cdot 10^{-1}\) |
|---|---|---|---|---|---|---|
| \(P \cdot 10^3\) | 7,32 | 5,29 | 3,99 | 2,49 | 1,06 | 0,703 |
| \(SP \cdot 10^5\) | 6,09 | 4,38 | 5,58 | 8,09 | 10,70 | 21,10 |
| \(P' : P\) | 1,13 | 1,56 | 2,07 | 3,31 | 7,78 | 11,73 |
| \(SP' : SP\) | 0,89 | 0,64 | 0,82 | 1,19 | 1,57 | 3,10 |
\(P\) and \(P'\) are the solubility of the precipitate, respectively, in water and at the given concentration of potassium bromide.
\(SP\) and \(SP'\) are the solubility product of the precipitate, respectively, at the given concentration of potassium bromide and in water.
Table 4
Behavior of silver bromide in the system silver bromide–potassium bromide–sodium nitrate–water
| Potassium bromide, mol/l | Sodium nitrate, mol/l | \(P \cdot 10^3\) | \(SP \cdot 10^5\) | \(K\) | \(P : P'\) | \(SP' : SP\) | \(K' : K\) |
|---|---|---|---|---|---|---|---|
| 0,001 | — | 7,32 | 6,09 | 0,951 | 0,887 | 0,90 | 1,06 |
| 0,001 | 0,001 | 7,54 | 6,44 | 0,924 | 0,914 | 0,95 | 1,03 |
| 0,001 | 0,010 | 8,22 | 7,58 | 0,852 | 0,996 | 1,11 | 0,95 |
| 0,001 | 0,100 | 9,67 | 10,30 | 0,731 | 1,172 | 1,51 | 0,81 |
| 0,001 | 1,000 | 13,50 | 19,60 | 0,530 | 1,636 | 2,88 | 0,59 |
| 0,010 | — | 3,99 | 5,58 | — | 0,484 | 0,82 | — |
| 0,010 | 0,001 | 4,82 | 7,14 | 0,878 | 0,584 | 1,05 | 0,98 |
| 0,010 | 0,010 | 5,08 | 7,66 | 0,846 | 0,616 | 1,13 | 0,94 |
| 0,010 | 0,100 | 6,37 | 10,40 | 0,727 | 0,772 | 1,53 | 0,81 |
| 0,010 | 1,000 | 11,20 | 23,70 | 0,482 | 1,358 | 3,49 | 0,54 |
| 0,100 | — | 1,06 | 10,70 | — | 0,128 | 1,57 | — |
| 0,100 | 0,001 | 1,07 | 10,80 | 0,714 | 0,131 | 1,59 | 0,79 |
| 0,100 | 0,010 | 1,20 | 12,10 | 0,674 | 0,145 | 1,78 | 0,75 |
| 0,100 | 0,100 | 1,34 | 13,60 | 0,636 | 0,162 | 2,00 | 0,71 |
| 0,100 | 1,000 | 2,69 | 27,60 | 0,446 | 0,326 | 4,06 | 0,50 |
Table 4 gives the most interesting data for the four-component system silver bromide—potassium bromide—sodium nitrate—water. In this system, the change in the solubility of the precipitate is the result of the summation of two mutually superposed effects acting in opposite directions. At a potassium bromide concentration of \(10^{-3}\) mol/l, the decrease in the solubility of the silver bromide precipitate due to the common ion is fully compensated by the corresponding increase in the solubility of the precipitate due to sodium nitrate already at a concentration of the latter of \(10^{-2}\) mol/l. At a potassium bromide concentration of \(10^{-2}\) mol/l, the decrease in the solubility of the precipitate due to potassium bromide is fully compensated by the corresponding increase in its solubility due to sodium nitrate only at a concentration of the latter of \(5 \cdot 10^{-1}\) mol/l. At a potassium bromide concentration of \(10^{-1}\) mol/l in a one-molar solution of sodium nitrate, the solubility of silver bromide still remains approximately 3 times lower than in water.
The solubility product of the precipitate in this system increases under the action of sodium nitrate by 2.88 times at a potassium bromate concentration of \(10^{-3}\), by 3.49 times at a potassium bromate concentration of \(10^{-2}\), and by 4.06 times at a potassium bromate concentration of \(10^{-1}\) mole/l. As was to be expected, the activity coefficient of silver bromate has the smallest value at the highest concentration of potassium bromate.
In a molar solution of sodium nitrate and 0.1 molar potassium bromate the activity coefficient reaches the value 0.446, i.e., becomes 2 times smaller than in water.
Conclusions
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The solubility of a silver bromate precipitate was investigated in 23 three-component systems of the composition silver bromate—external electrolyte—water at \(25^\circ\)C in the concentration range from 0.001 to 1 mole/l. The external electrolytes used were: nitric acid; nitrates of lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc, cadmium, aluminum, and thorium; sulfuric acid; sulfates of lithium, sodium, potassium, magnesium, aluminum, and zirconium; and potassium bromate.
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The activity coefficients of silver bromate and the solubility products of the precipitate were determined in all the above-mentioned systems.
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It was established that the solubility, the solubility product, and the activity coefficient of silver bromate vary over wide limits depending on the concentration and chemical nature of the external electrolyte.
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It was shown that changes in the solubility, solubility product, and activity coefficient of silver bromate under the action of electrolytes that do not have ions in common with the precipitate, in the absence of any obvious chemical interaction between the components in the systems, are periodic in character and obey D. I. Mendeleev’s periodic law. At equal normalities of the solutions, among all the nitrates considered the greatest increase in solubility and solubility product is produced by potassium nitrate and strontium nitrate, and among the sulfates by potassium sulfate.
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In order to elucidate the combined influence of electrolytes with common ions and of electrolytes without ions in common with the precipitate, when they are simultaneously present, the solubility was studied in the four-component system silver bromate—potassium bromate—sodium nitrate—water. The change in the solubility of the precipitate in this system is the result of the summation of two mutually superimposed effects acting in opposite directions.
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