Читать книгу Legal Chemistry - Alfred Naquet - Страница 17

Marsh's test is based upon the reduction of arsenious and arsenic acids by nascent hydrogen, and the subsequent transformation of these bodies into water and arsenetted hydrogen, a compound from which the arsenic can be readily isolated. When pure hydrogen is generated in a flask having two openings, one of which is provided with a perforated cork through which a safety-tube passes, the other with a tube bent at a right angle and drawn out to a small point at the free extremity, the evolved gas, if ignited, burns with a pale non-luminous flame. The air should be completely expelled from the apparatus before igniting the gas. Upon bringing a cold porcelain saucer in contact with the point of the flame, only water is formed. If, however, a small quantity of a solution containing arsenious or arsenic acids is introduced into the apparatus by means of the safety-tube, arsenetted hydrogen is produced. This gas burns with a bright flame, yielding fumes of arsenious acid. In case a large amount of the poison is present, it can be recognized by the appearance of the flame, and by inclining a glass tube towards it upon which a portion of the arsenious acid becomes deposited. These indications are, however, not distinguishable in presence of only a small amount of arsenic, and the following distinctive properties of the gas should be verified:

1st. At an elevated temperature it is decomposed into its two constituent elements.