rti 



390 On a Quantitative Blowpipe Assay of Chro77iium. 



Chromate of lead may be completely decomposed by fusion 

 with nitre or the carbonates ; but when the melted mass is treated 

 with water, the reverse decomposition takes place, chromate of 

 lead being again formed. Tin, bismuth, and probably cadmium 

 behave similarly. This difficulty is best obviated by transform- 

 ing those oxyds into silicates. The ore is first intimately mixed 

 in the agate mortar with an excess (three to six heaped spoon- 

 fuls) of silica obtained in the wet way, A spoonful of nitre is 

 then added and triturated with the mixture, whereby it is loos- 

 ened from the mortar; it is then introduced into the crucible by 

 single spoonfuls, as in the treatment of chrome green before de- 

 scribed; after which the rnortar is "rinsed" by triturating in it 

 five or six successive half-spoonfuls of nitre. The flakes of sili- 

 cate formed adhere to the sides of the crucible, appearing of a 

 white or yellowish tint; if from incomplete mixture of the in- 

 gredients, any metallic chromate has remained nndecomposed, 

 it will appear of a dark red or orange on cooling. In this case 

 no subsequent addition of silica will remedy the evil, and the 

 experiment must be rejected. Most of these fusions are executed 

 in from five to ten minutes- 



The alkaline solution obtained by the treatment of the fused 

 mass with water, may contain, besides the chromate, manganate, 

 vanadate, tungstate, molybdate, titanate, silicate, antimoniate, ar- 

 seniate, phosphate of potash, together with other non-metaUic 

 acids; those just mentioned being the most likely to interfere 

 with the determination of the metal in view. It might be sug- 

 gested to precipitate the chromic acid by HgsO, NO5, when none 

 of the above acids are present. Excellent as this method is when 

 time can be allowed, its results fall seriously short of the truth 

 when, as in the field, no delay is admissible. It would be better 

 even to precipitate the chromic oxyd by ammonia, after reduc- 

 tion with alcohol. Neither of these methods is admissible when 

 any of the acids mentioned are present 



It might be thought that a general method for the separation 

 of the acids mentioned, would be found in the evaporation of the 

 solution to dryness with IICl, and subsequent fusion with cyania 

 of potassium and an alkaline carbonate. But the oxyd of chro- 

 mium thus obtained always contains potash, the amount ot 



which is quite indefinite. 



blc. It is well known that when a solution of chrome alum 

 containin^^- an excess of sulphuric acid is heated to 200° Centi- 

 grade, a light green powder of the above composition ^separat^, 

 which is insoluble and undccomposable by water or acids, and is 

 but slowly resolved into its immediate constituents by boiling 



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