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224 CONVERSION OF THE SULPHATES OF THE ALKALIES.
The carbonates of the alkalies can also be formed by first forming these organic salts from the sulphates, evaporating the solution to dryness and burning the residue; in fact, I frequently find it more convenient to convert the sulphates of the alkalies into their carbonates by this last instead of the first process. And finally I would remark that where magnesia is present with the sulphates this is also separated from the alkalies.
ACTION OF SOME OF THE ALKALINE SALTS
UPON THE SULPHATE OF LEAD.
It has been for some time known that certain neutral salts possess the property of dissolving to some extent the sulphate of lead, which properly belongs neither to the acids nor bases constituting these salts. By referring to Berzelius' Chemistry it will be found that the acetate and nitrate of ammonia are among the number. "One part of the sulphate was dissolved in 47 parts of a solution of the acetate of specific gravity 1.036, and in 172 parts of a solution of the nitrate of specific gravity 1.144." In the Annalen der Chem. und Phar. (vol. xxxiv, 235) will be found the following statement under the head of Reactionen: "Sulphate of lead is easily dissolved, and in a large quantity, by a solution of neutral tartrate of ammonia; a concentrated solution forms after some time a stiff jelly like silica." This last is no doubt a double tartrate of lead and ammonia.
I had also observed some time previously that a solution of the citrate of ammonia, when poured upon the sulphate of lead and allowed to stand, altered the character of the sulphate, and this, with the other fact above stated, led to the examination of what was really the action of these as well as other ammoniacal salts in general upon the sulphate in question, and it was found that in every case it was decomposed.
Citrate of ammonia.—If a solution of citrate of ammonia be poured upon the sulphate of lead and shaken together, the clear solution will be found to contain the sulphate of lead, as shown by hydrosulphuric acid, and a salt of baryta (taking care in testing with the baryta to acidulate first with pure nitric acid, to prevent the formation of the citrate of baryta.) If they be allowed to remain several weeks in contact, the solution will be found to contain more lead, the sulphate having undergone decomposition, sulphate of ammonia and a double citrate being the result; as this latter salt is not very soluble, a large portion of it remains in the form of a precipitate. The rapidity of this
change is in proportion to the concentration of the solution of the citrate. If instead of performing the experiment in the cold we boil a tolerably concentrated solution of the citrate with the sulphate of lead, a very large quantity of the latter will be dissolved, and the solution become perfectly transparent; if it be set aside and allowed to cool, in the course of a few hours an abundant white precipitate will be formed, and upon testing the clear solution sulphuric acid, ammonia, citric acid, and oxide of lead will be found present. The precipitate when washed affords citrate of lead and ammonia. I was at first inclined to think it simply a citrate of lead, attributing the ammonia present to some of the citrate not washed out; but from its possessing certain characters which do not belong to the simple citrate I consider it a double citrate of lead and ammonia. It contains not the slightest trace of sulphuric acid. It was not analyzed, from the difficulty of obtaining it perfectly pure, as the water used to wash it decomposes it, and as yet this difficulty has not been surmounted. So then the result of the action of the citrate of ammonia upon the sulphate of lead is first to dissolve it, and subsequently to decompose it, forming the sulphate of ammonia and citrate of lead and ammonia.
Tartrate of ammonia —If a solution of this salt be added to the sulphate of lead and shaken with it in the cold, the clear solution will be found to contain both lead and sulphuric acid, and if set aside for a few weeks the precipitate will have changed its character, having assumed a crystalline nature; the solution will no longer contain lead, but the quantity of sulphuric acid present will be found to have increased. The precipitate now consists of tartrate instead of sulphate of lead, which is completely soluble in dilute nitric acid, affording no precipitate with a salt of baryta. If the mixture of the tartrate and sulphate be boiled, this change takes place more rapidly and in a manner somewhat different from the case of the citrate; the sulphate will not be dissolved in such large quantities, and moreover, by continuing to boil the solution after the sulphate has been completely dissolved, the tartrate forms during the ebullition and is precipitated in little shining crystals. If the ebullition be continued a sufficient length of time, the whole of the lead previously dissolved will combine with the tartaric
acid. This is different from what takes place with the citrate, which when boiled upon the lead-salt dissolves it, and no length of ebullition will produce a precipitate. The action of the tartrate is first to dissolve the sulphate, decompose it in part. and form a double tartrate of lead and ammonia, which last salt is subsequently decomposed by continued contact with water, or still more rapidly by its solution being boiled.
Acetate of ammonia.-This salt also dissolves to some extent the sulphate of lead, but not so readily as either of the above salts. If the solution be boiled and evaporated to dryness, crystals of sulphate of ammonia are obtained, and an uncrystallizable salt of lead, probably an acetate of lead and ammonia; from the difficulty of separating the sulphate of ammonia from it, it is impossible to pronounce positively whether it is a double salt or simply an acetate of lead. We see in this reaction the existence of a soluble salt of lead and the sulphate of ammonia simultaneously in the same solution, without a precipitate being formed.
Oxalate of ammonia dissolves but slightly the sulphate of lead, owing no doubt to the impossibility of forming a double salt; but it will nevertheless decompose largely, the sulphate furnishing the oxalate of lead.
Muriate of ammonia, if boiled with the sulphate of lead, will decompose it instantly, furnishing the chloride of lead and sulphate of aminonia.
The nitrate of ammonia does the same, forming nitrate of lead and sulphate of ammonia.
The carbonate and succinate of ammonia produce similar effects.
The action of most of the corresponding salts of potash and soda was examined, and with very similar results. The fact is it would appear that those alkaline salts which dissolve the sulphate of lead decompose it without reference to the time occupied in the solution, as in the case of the carbonate of ammonia, which decomposes the sulphate at the very instant of its solution; and it is impossible to detect at any one time other than a trace of lead in solution, whereas the quantity of sulphuric acid is constantly increasing.
The explanation is clear: the sulphate of lead is a salt with a strong acid and feeble base; the alkaline salts used contain
feebler acids and stronger bases; they dissolve the sulphate, thus affording an opportunity for the acids and bases to act upon one another under favorable circumstances, and to follow a natural law in chemistry: the stronger acid combined with the stronger bases, and vice versa.
From the foregoing facts some important hints might be afforded to analytical chemistry, for it will be at once seen that the presence of any of the alkaline salts in a solution from which it might be wished to precipitate lead in the form of a sulphate would affect the accuracy of the result. What is true of the sulphate of lead may be found also true for other insoluble salts. Moreover, this shows the importance in the analysis of mineral waters, for instance-of weighing well the relative strength of the various acids and bases therein found, in order to ascertain what salts are present, and not to be contented with evaporating the water to dryness, and considering such salts as remain to be those existing in the water, for many of them may be formed during the evaporation. It is not at all improbable that before many years the examination of mineral waters will be based as much upon calculation as upon analysis, the former of course being guided by the latter and by certain laws not yet developed.