99 MIL. ADIE OW VOLTAIC COUPLES. X.-On the Quantitative Separation of Magnesia and of the oxides of Nickel Cobalt and Zinc,from Potash and Soda. By HENI~Y WATTS,%.A. F.C.S. Assistant in the Birkbeck Laboratory UrLiversity College London. The separation of magnesia from the fixed alkalis is well known to be attended with considerable difficulty. The method given in Rose’s Analytical Chemistry,* consists in converting the mixed salts * Dr. Normandy’s Tran&tion vol. IT. p. 43. H2 100 MR. HENRY WATTS ON into sulphates adding acetate of baryta in excess to remove the sulphuric acid filtering ; converting the acetates of baryta magnesia and the alkalis in the filtrate into carbonates by ignition-then digesting in water and filtering again to separate the alkaline carbonates from the insoluble carbonates of baryta and magnesia The magnesia and baryta are then separated by means of sulphuric acid.This method is very troublesome and complicated; and after all does not give very accurate results. An easier method is to make use of baryta-water which preci- pitates magnesia leaving the alkalis in solution and then to remove the excess of baryta either by sulphuric acid or by carbonate of ammonia. This method is unobjectionable when only the alkalis are to be quantitatively determined; but if the quantity of magnesia is likewise to be estimated a great deal of trouble is occasioned by the formation of carbonate of baryta which always takes place more or less during the filtration and washing whatever pains we may take to exclude the air.The magnesia and baryta have then to be separated by sulphuric acid; and this involves the necessity of expelling a considerable excess of sulphuric acid by heat which is a troublesome process. To obviate these difficulties I have devised the following process which is nothing more than a particular application of a method in very general use. It consists in precipitating the magnesia by a known weight of carbonate of soda using a considerable excess ;then boiling and filtering; treating the filtrate with a slight excess of acid; evaporating to dryness and igniting the residue to render it neutral ; weighing the neutral salt thus obtained; and making the proper correction for the quantity of soda-salt introduced.The mode of conducting the process will be best understood by the following examples Twenty-five grains of sulphate of magnesia and potash in well- defined crystals were dissolved in water and the liquid was mixed with solution of 10 grains of perfectly anhydrous carbonate of soda. The mixture which was strongly alkaline was then boiled for half-an- hour the water being renewed as it evaporated. This continued boiling is essential to the complete separation of the magnesia. The carbonate of magnesia was collected on a filter washed with boiling water then dried and ignited. The quantity of magnesia thus obtained was 2.555 grains; by calculation it should be 2.557. The filtrate which contained sulphate of potash sulphate of soda and excess of carbonate of soda was then slightly acidulated with sulphuric acid to convert the carbonate into sulphate so that THE SEPARATION OF MAGNESIA FROM THE ALKALIS.101 sukhuric acid should be the only acidpresent. The liquid was lastly evaporated to dryness in a porcelain crucible and the residue strongly ignited to render the sulphates neutral; carbonate of ammonia being added to remove the last portions of the excess of acid. The residue gave Sulphate of potash + sulphate of soda = 24.015 gr. Now it must be remembered that 10 grains of dry carbonate of soda were introduced at the beginning of the process; and 10 grains of carbonate yield 13-4 grains of anhydrous sulphate. Deducting this from the weight of the mixed sulphates we have 10.615 grains of sulphate of potash which corresponds to 5.73 of potash.The calculated quantity is 5.84. In a second experiment conducted in the same manner the quantity of magnesia was 2.56 and that of potash 5.77. The following table contains the results of the two experiments calculated to 100 parts and likewise the mean of the two. The fourth colizmn gives the theoretical quantities according to the formula KO SO + MgO SO + 6 HO; and the last coluinn gives the differences between the third and fourth. The quantities of sulphuric acid and water of crystallization were likewise deter- mined by the usual methods in order to prove that the salt was of definite constitution I. 11. Mean. Calculation. Difference. Magnesia .10.24 10-22 10.23 10.23 0.00 Potash . . 23.08 22.92 23-00 23.36 0.36-Sdphuric acid 39.40 39-76 39-58 39.68 0.10-Water . . 26.96 26.92 26-94 26-73 0.21+ 99.68 99.82 99.75 100.00 It will be seen from this that the process is capable of affording very good results. To ensure accuracy however it is absolutely necessary that the solution after the carbonate of soda has been added to it be well boiled for at least half-an-hour. The object of this continued boiling is to decompose a difficultly soluble double carbonate of soda and magnesia which is formed on the first addition of the alkaline carbonate.* The carbonate of soda must likewise be added in considerable excess ; otherwise the precipitation will not be complete. The precipitate of carbonate of magnesia must be washed * Vide Rose Normandy’s Translation II.35. MR. HENRY WATTS ON with boiling water and the washing not too long continued; for the carbonate is not completely insoluble. The washing should be dis- continued as soon as the wash-water ceases to give a distinct alkaline re-action; when this takes place the water begins to dissolve the carbonate of magnesia. When as in the above examples the quantity of magnesia can be approximately estimated beforehand it is easy to calculate the quantity of carbonate of soda required to precipitate it ;considerably more than that quantity should however be used. But if no such estimate can be formed-and this will generally be the case-a certain quantity of the carbonate of soda may be weighed out and then added in small portions at a time till the liquid becomes strongly alkaline.The residue may then be weighed and the difference of the two weighings will give the quantity used for the precipitation When a quick approximation is desired rather than a very accurate result a solution of known strength may be used and the quantity determined by a graduated measure. But where great accuracy is an object the method of weighing is much to be preferred. Great care should of course be taken that the carbonate of soda is absolutely pure and anhydrous. The best mode of preparing it is to ignite the bicarbonate or sesquicarbonate. In the above examples the magnesia and alkali were in the form of sulphates. If they are in the form of chlorides the determination of the alkali will be easier; because the excess of hydrochloric acid is more easily driven off than that of sulphuric acid.If they are in the form of nitrates or if two or more acids are present it is best after separating the magnesia to add sufficient sulphuric acid to convert the whole into sulphates. When both potash and soda are present the best plan will be after precipitating the magnesia to convert the alkalis into chlorides which may always be done; then determine the total weight of the alkaline chlorides ;deduct the weight of chloride of sodium equivalent to the carbonate of soda used; and lastly estimate the quantity of chloride of potassium in the usual manner by precipitation with bichloride of platinum.The same method is applicable to the separation of nickel cobalt and zinc from the fixed alkalis. The usual mode of effecting this separation is by means of sulphide of ammonium. But this method though practicable is attended with very great difficulties; for the sulphides of these metals arc to a certain extent soluble in excess of sulphidc of ammonium; and if an excess of this reagent be not used THE SEPARATION OF MAGNESIA FROM THE ALKALIS. 103 and the precipitate not washed with water containing it the precipi- tate oxidizes and is converted into a soluble sulphate which runs through the filter. The following examples will show that the method of precipitation with a known weight of carbonate of soda gives results as accurate as those obtained with the magnesia-salt.I omit the details of the process as they are exactly similar to those above described. The quantities of sulphuric acid and water are likewise given for the same reason as in the former case. Sulphate of zinc and potash ZnO SO + KO SO + 6 HO Exp. Calculation. Difference. Oxide of zinc . . 18.08 18.44 0.36-Potash . . 21-08 21.22 0.14-Sulphuric acid. . 35.88 36.06 0.18-Water . . 2473 24.28 0°43+ 99.77 100*00 Sulphate of nickel and potash Ni 0 SO + KO SO + 6 HO Exp. Calculation. Difference. Protoxide of nickel . 17-12 17-16 0.04-Potash . . 21-52 21.55 0.03-Sulphuric acid. . 36.35 36.62 0.27-Water. . . 24.96 24.67 0.29+ 99-95 10*OOQ The same precautions are necessary as in the case of magnesia viz to use a considerable excess of carbonate of soda boil for a long time wash with boiling water and not too long.The examples above given are sufficient to illustrate the method. I have caused it to be tried in a great number of instances by pupils working under my direction in the laboratory of University College. The results are always satisfactory when due attention is paid to the precautions above specified. I now lay it before the Chemical Society in the hope that it may contribute something towards the removal of an acknowledged difficulty in analysis