T H E SOLUBILITY OF ATMOSPHERIC GASES I N SOLUTIONS OF AMMONIUM CHLORIDE BY J. H. COSTE AND E. R. ANDREWS
The solubility of atmospheric oxygen in solutions of various salts was determined by C. G. MacArthur’. His results for the halides of alkali metals are of the order one would expect, having regard to the solubility of oxygen in sea water but those for ammonium chloride solutions are very remarkable. They are for a temperature of zs0C. Concentration M M M M 4 8 Infinity Solubility of oxygen in cc 0.07 1.16 z .31 5.78 per litre In discussing these results MacArthur says: “Another fact to be noted is the marked effect of ammonium chloride on the solubility of oxygen. It raises with the concentration, but the decrease in solubility is extremely large. At first it was thought to be due to a combination of the ammonium chloride with the manganous chloride,2thus preventing the manganous compound from being oxidized by the oxygen in solution. This was shown not to be true by adding the ammonium chloride to water containing the usual amount of the alkaline iodide and manganous chloride solutions and finishing the determination as usual. This gave an amount of oxygen that distilled water usually contains. Though it is more likely that the ammonium chloride so influences the oxygen that it is not available to oxidize the manganous hydroxide, it is still possible that the ammonium chloride markedly decreases the amount of oxygen in solution.” It appears, therefore, that MacArthur had some doubt as to the real significance of his results although their accuracy appears to have been accepted by other authors. If ammonium chloride in relatively dilute solutions really conferred this property upon water much use could be made of the fact in pas analysis and manipulation. The general similarity in behaviour of ammonium chloride and the chlorides of the alkali metals raises a strong presumption that there is some explanation of MacArthur’s results other than a specific effect on the solubility of oxygen. The use of ammonium chloride in preventing the eo-precipitation of hydroxides of manganese, zinc, nickel and cobalt with those of iron, chromium and aluminum is well known to most students. The experiment cited by MacArthur only shows that ammonium chloride will not re-dissolve and reJ. Phys. Chem. 20,495 (1916). Winkler’s manganous process for the determination of dissolved oxygen
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duce manganic oxide when it has formed, as it does very rapidly in Winkler’s process. It does not show that a manganese ammonium complex is not found. The interaction of manganous and ammonium compounds has been studied by W. Her+ and is referred to in some of the larger text-books of inorganic chemistry. We have experimented with ammonium chloride solutions and find that the salt undoubtedly exerts a considerable retarding effect on the precipitation of the hydroxides of manganese, the solution remaining clear for some time and only yielding a considerable precipitate, apparently all manganic, after some hours. Solutions of ammonium chloride in distilled water through which air had been bubbled for some hours, were examined (a) by Winkler’s manganous process, using 7 0 cc of the solution and 0 . 3 cc of each of the reagents (b) by Winkler’s gasometric process in which the oxygen and nitrogen dissolved in the water are evolved by a current of carbon dioxide generated in the water from calcite by means of hydrochloric acid2. It will be seen from the table below that the content of oxygen and nitrogen in ammonium chloride solutions differs but little from that in sodium or potassium chloride solutions of the same molar concentration. Molar Temperature Concentration of of NH&1 Saturation
2M M
Oxy-gen
22
7.79
I9
10.15
3.93 5.70
20
IO.29
5.04
20
10.65
M 2
Nitrogen and Argon
1
5.48
litre Winkler’s Manganous Process Oxveen I
. 6 (overnight)
2
.6 ( 2 0 hours)
5.46 (18hours)
No separation of hydroxide of manganese was observed in six hours with the 2 M solution, some turbidity but no precipitate was observed in 4 hours with the normal solution whilst the M and M/2 gave some precipitate at once and action was complete in eighteen hours. The ammonium chloride solutione were sought to be saturated with the gases of air by passing a fine stream of bubbles for about 3 hours. It is not suggested that the figures in the above table are of the highest accuracy but they show conclusively that the order of solubility of the gases of air in ammonium chloride is very different from that given by MacArthur and also the inapplicability of the manganous process in presence of large proportions of ammonium salts. Z. anorg. Chem. 21, 243; 22, 279. 2L. W. Winkler: Z. anal. Chem. 40, 5 2 3 (1901). I
