The preparation of few substances presents such difficulties as that of Pure Magnesia.
It seems, however, at first glance, that the cheapness and general purity of the sulphate which occurs in commerce, would render this an easy task. Unfortunately, however, no simple process has yet been proposed for obtaining pure magnesia from the sulphate. The usual course is to precipitate from the boiling solution with carbonate of soda, and to expel the carbonic acid from the magnesia alba thus obtained, by ignition. On trying this process, however, it was found that the carbonate of magnesia thus precipitated could not be freed from soda by washing. After an enormous quantity of hot distilled water had passed through it on the filter, the slight residue left by evaporation of the washings, still gave the soda tinge to flame.
It is true that the trace of the soda compound thus retained might probably be washed out of the magnesia after its ignition, but the difficulty and tedium of the operation of washing the very voluminous precipitate, together with the expense attendant upon the necessity of using pure carbonate of soda, to avoid the presence of silica, phosphoric acid, and other impurities, which, if present, would inevitably contaminate the magnesia, induced me to reject this method. In fact this method, which was formerly almost used universally by analysts for the determination of magnesia is now rejected by them, except in some unavoidable cases.[16]
[16] H. Ross’s Handbuch, last edition, 2, 33.
The substitution of carbonate of ammonia for carbonate of soda is inadmissible with any regard to economy, on account of the existence of the soluble double sulphates of ammonia and magnesia. A trial was made to decompose sulphate of magnesia by mixing its anhydrous powder with a large quantity of carbonate of ammonia, and igniting, but the only trace of {200} decomposition which appeared was a slight alkaline re-action of the aqueous solution of the mass.
I must here mention an impurity which I have met with in commercial sulphate of magnesia, and this is a double sulphate of magnesia and potash, which occurs in small crystals, apparently rhombohedrons, among the rectangular prisms of the Epsom salt. It may probably be separable by recrystallization, though this, with sulphate of magnesia, is rather a difficult affair.
The method which I adopted for preparing pure magnesia was the ignition of the nitrate prepared from the commercial magnesia alba. The impurities in the commercial carbonate which I made use of were sulphate and chloride, a surprisingly large quantity of silica, a trace of phosphoric acid easily detectable by molybdate of ammonia, oxide of iron, alumina, lime, alkalies and some organic matter. A small excess of this impure article was added to commercial nitric acid and the whole boiled; the silica, oxide of iron, alumina and phosphoric acid were thus separated by the excess of magnesia and the filtered solution contained no trace of either of them; the solution was slightly colored by organic matter.
Either of two methods may now be adopted for separating the lime.
One is to add a late excess of ammonia, then a little oxalic acid, and filter. To this method, besides the expense of so large a quantity of pure ammonia the necessity of the subsequent decomposition and expulsion by heat of the very large quantity of nitrate of ammonia formed is a serious objection.
Unsuccessful attempts were made to separate the lime by adding oxalic acid immediately to the neutral solution of nitrate of magnesia. It was found upon experiment that oxalate of lime is somewhat soluble in a solution of nitrate of magnesia.