Disadvantages: The ancient Chua's method is also susceptible to other interferences. For example, the sample is sulfide, sulfite, thiosulfate, etc. , can generate hydrogen sulfide or sulfur dioxide in acidic solution, and react with HgBr to generate black HgS or metal Hg, thus interfering with arsenic spots. Nitric acid can be added to oxidize sulfide into sulfate to eliminate interference. The advantages of ancient color method in detecting arsenic salt are that zinc reacts with acid to generate new ecological hydrogen, and the new ecological hydrogen reacts with trace arsenic salt in medicine to generate volatile hydrogen arsenide gas. When it meets with mercury bromide test paper, it produces yellow to brown arsenic spots, which can be compared with arsenic spots produced by a certain amount of standard arsenic solution under the same conditions to determine the content of arsenic salt in drugs.