40. How to identify whether acetic acid contains sulfuric acid or formic acid?
Answer: If acetic acid contains sulfuric acid or formic acid, it is generally difficult to distinguish them using quantitative methods. Therefore, qualitative methods can be used to detect whether the acetic acid contains sulfuric acid and formic acid. The method to identify sulfuric acid is: take 100mL of sample, add 5g of barium chloride, stir it to dissolve, let it stand for about 15 minutes, and observe whether there is white precipitate in the sample solution. If there is white precipitate in the sample, it is sulfuric acid. Barium, confirmed to contain sulfuric acid. The identification of formic acid can be qualitatively tested using the mercury chloride test. The principle is: formic acid or alkali metal formate is heated together with mercury chloride in an acetic acid-acetate buffer solution to precipitate white crystalline mercury chloride (calomel): 
Under the following conditions, there is no reaction even with a large amount of the following acids: acetic acid, glycolic acid, lactic acid, oxalic acid, tartaric acid , citric acid, malic acid. Therefore, this test method is recommended for the identification of formic acid (formate) from a mixture of carboxylic acid and sulfonic acid (or their alkali metal salts). Operation method: Take one drop of the acidic, neutral or weakly alkaline dilute solution to be tested, place it in a micro crucible, and add one drop each of 10% mercuric chloride solution and buffer solution. Place the mixture in the oven at 100. C evaporate to dryness (drain). Dissolve the evaporation residue with a drop of water, and add a drop of c(NH3)-0.1mol/L ammonia solution. Depending on the content of formic acid (salt), dark or light black to gray will appear. Each 100mL buffer solution contains 1mL of glacial acetic acid and 1g of sodium acetate. The identification limit of this method is: 5μg formic acid.
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