The sulphate of baryta is always reduced to the state of sulphide of barium, by exposing it to a red heat in intimate admixture with some carbonaceous substance, such as powdered charcoal, rosin, oil or flour. It is exceedingly difficult, however, if not impossible, to effect in this manner a complete decomposition of the sulphate. Indeed, it is probable that in most cases the quantity of sulphide obtained, is not more than half that which is equivalent to the sulphate employed. A modification which promises to be far more economical was proposed by Dr. Wolcott Gibbs. His proposal was to submit the sulphate to the action of a current of common coal gas at a red heat. It is evident that in this way a perfect decomposition {163} may readily be accomplished, especially if the powdered sulphate is stirred during the operation, so as to expose fresh surfaces to the action of the gas.
The mass obtained after the reduction of the sulphate is submitted to the action of boiling water, and a solution obtained, which, according to Professor H. Rose,[12] contains principally hydrate of baryta and sulphohydrate of sulphide of barium BaS. HS. formed by the reaction of equal equivalents of water and proto-sulphide of barium. It almost invariably contains also a quantity of lime, probably in the form of sulpho-hydrate of sulphide of calcium, or of hydrate of lime, proceeding from the almost constant concurrence of sulphate of lime with native sulphate of baryta. From the presence of this lime originates the principal difficulty in preparing pure barium compounds from this substance. Thus when the carbonate is prepared from the solution by precipitation, with carbonate of soda, or a current of carbonic acid gas, it is found contaminated with carbonate of lime, which is fatal to its use as a reagent in analysis. Also in examining many specimens of commercial chloride of barium, which is prepared from this solution by the addition of chloro-hydric acid, boiling to separate sulpho-hydric acid gas which is evolved, filtration to separate the sulphur which is precipitated and crystallization, I have always found it to contain a small quantity of chloride of calcium, which I have found it impossible to separate entirely by repeated recrystallizations. It has been proposed[13] to separate the chloride of calcium from chloride of barium by the use of very strong alcohol, in which the latter when anhydrous, is insoluble. This method is rather expensive and troublesome as it involves the evaporation to dryness of the chloride of barium solution, the reduction of the previously ignited residue to a very fine powder and digestion in strong alcohol. Attempts were made after some previous experimentation, in which it was found that an {164} aqueous solution of oxalate of baryta precipitated chloride of calcium, but not chloride of barium, to separate the lime from a chloride of barium solution by addition of oxalate of baryta, or simply of a little oxalic acid, but it was soon found that oxalate of lime was somewhat soluble in a solution of chloride of barium, so that a solution of oxalate of baryta, gave no precipitate in a mixture of solutions of chloride of barium and chloride of calcium. It was found also that the precipitate formed by a little oxalic acid in a lime solution, could be re-dissolved by addition of chloride of barium. It may also be mentioned, though irrelevant to the subject, that it was found that oxalate of lime was soluble in solutions of chloride of calcium, of ammonia, and of chloro-hydrate of ammonia.
[12] Poggendorff’s Annalen, 55,416.
[13] Gmelin’s Handbuch, 2,158.
The well known property of carbonate of baryta which the recent investigations of Professor H. Rose have rendered so important in the analysis of phosphates, of completely precipitating lime from its solution by a sufficiently long contact therewith, furnishes us, however, with a perfectly easy and cheap method of purifying the chloride of barium solution. In fact a solution of chloride of barium to which chloride of calcium has been added, having been treated with a little carbonate of baryta, and allowed to stand in contact with it for two days, with occasional agitation, was found on filtration to be free from lime. The only objection to this method, is the considerable length of time required; but I must here describe an elegant modification which was communicated to me by Dr. Wolcott Gibbs, and tested by him in his laboratory; that is to add first to the solution of chloride of barium containing lime, a little solution of hydrate of baryta and then to pass through it a current of carbonic acid gas. The precipitate immediately formed contains of course all the lime.
The only impurity which is prevalent in commercial chloride of barium besides lime, is strangely enough, a trace of lead which is almost always present and sometimes in such quantity that the solution is immediately blackened by sulphuric {165} acid.[14] This is, however, very easily removed, either before or after the separation of the lime by the process of Dr. Gibbs, by passing a little sulpho-hydric acid gas into the solution, gently heating for a short time and filtering.
Commercial chloride of barium thus purified is probably the most convenient source of the other compounds of barium when required pure. Thus pure carbonate of baryta may be prepared from it by precipitation with carbonate of ammonia, or with carbonate of soda, which is free from silica, sulphuric acid and phosphoric acid.[15]
[14] It may be that leaden pans are used for the evaporation or crystallization of the commercial chloride of barium, which would sufficiently account for the presence of lead in the product.
[15] New York Journal of Pharmacy, 136.