(4) Nitrous compounds, nitric acid, hydrochloric acid, chlorides, are all more or less prejudicial.

(5) The gas should come off regularly in not too strong a stream, nor out of too small an opening.

(6) The gas should pass through the red-hot tube at least one hour, if no stain is at once detected.

(7) Towards the end of the operation, a solution of stannous chloride in hydrochloric acid is to be added to the contents of the flask. This addition precipitates any arsenic present in a finely divided state, in which it is readily attacked by nascent hydrogen.[762]

[762] F. W. Schmidt, Zeit. anorg. Chem., i. 353-359.

The characteristics of the metallic stains which may occur either on glass or porcelain in the use of Marsh’s test, may be noted as under:—

Mirror or Crust of ArsenicMirror or Crust of Antimony
Is deposited at a little distance from the flame.Is deposited close to the flame, and on both sides of it, and is therefore notched.
An arsenical stain is in two portions, the one brownish, the other a glittering black.The stain is tolerably homogeneous, and usually has a tin-like lustre.
On heating, it is rapidly volatilised as arsenious acid.Volatilisation very slow; no crystalline sublimate obtainable.
On transmission of a stream of SH2, whilst immediately behind the stain a gentle heat is applied, the arsenic is changed to yellow sulphide;[763] if dry ClH is now transmitted, the arsenical sulphide is unchanged.The same process applied in the case of antimony produces the orange or black sulphide; and on passing dry ClH, chloride of antimony volatilises without the application of heat.
Chloride of lime dissolves the arsenic completely.Antimony not affected.
Protochloride of tin has no action on metallic arsenic.Dissolves slowly but completely the antimony stain.
The arsenic stain, dissolved in aqua regia, or ClH and chlorate of potash, and then treated with tartaric acid, ammonia, and magnesia mixture, gives a precipitate of ammonia magnesian arseniate.[764]No precipitate with antimony.