The following Table gives the differences between the
Yellow Precipitates formed with Sulphuretted Hydrogen
and Arsenic, Cadmium, and Per-Salts of Tin:
| Arsenic. | Cadmium. | Per-Salts of Tin. | |
|---|---|---|---|
| Colour. | Yellow. | Yellow. | Dirty yellow. |
| Action of ammonia. | Soluble. | Insoluble. | Insoluble. |
| Action of hydrochloric acid. | Insoluble. | Soluble. | |
| With cyanide flux. | Sublimes as metallic arsenic. | Sublimes as brown oxide. | No sublimate. |
Marsh‘s Process.—This method for the detection of arsenic is founded on the fact that the several compounds of arsenic, except the sulphide and metallic arsenic itself, form a gaseous compound with nascent hydrogen, from which it may be readily separated by appropriate treatment. The solution to be tested should, therefore, be prepared as proposed by Brande and Taylor, given on a preceding page.
Precautions.—(1) Absolute purity of reagents. (2) The sulphuric acid should be diluted with five times its weight of water, and allowed to cool. (3) The suspected fluid should be added gradually. (4) Generate the gas regularly. (5) If no stain be at once produced, keep a portion of the exit tube red-hot for at least one hour.
The usual form of the apparatus is that of a U-shaped glass tube, about one inch in diameter and eight inches high, supported in a vertical position on a wooden stand. One end of the tube is fitted with a tap, and terminates in a glass tube drawn to a fine point; the other end is closed with a cork.
The apparatus is used as follows: A piece of pure zinc is dropped into the tube, and shaken into such a position that it occupies the bottom of that limb of the tube which is furnished with the tap. Water is then added, and subsequently sufficient pure sulphuric acid to cause a moderately brisk evolution of hydrogen. The production of hydrogen gas from pure zinc and pure sulphuric acid is sometimes slow, and may be facilitated by adding a few drops of platinic chloride solution to the contents of the flask previous to the addition of the sulphuric acid. The gas being allowed to accumulate for a short time, the tap is then partially turned on, and the gas ignited; if, on depressing a piece of white porcelain momentarily in the flame, no deposit or discoloration occur, the reagents used may be taken as pure. By the use of Thorpe‘s apparatus for Marsh‘s test, in which the hydrogen is obtained by the electrolysis of water, the absence of arsenic in the reagents and apparatus is ensured. The tap is now connected with a tube of thin, hard glass, drawn out to a fine point at the end and having a constriction in the middle. The liquid to be tested being now placed in the apparatus, the gas is again ignited, and a piece of white porcelain momentarily depressed in the flame, when, if arsenic be present, a black, circular, metallic-looking stain will appear, which has the following composition. In the centre is the unoxidised metal, round this is a mixed deposit, and outside this the zone of arsenious acid. While the gas is passing, the exit tube should be heated to redness a little behind the constricted part, when a dark ring will appear if arsenic be present. The black deposit on the porcelain may be either arsenic or antimony, but may be distinguished as follows:
| Arsenic. | Antimony. | |
|---|---|---|
| Nature of the stain. | Metallic brilliancy | Absence of metallic lustre. |
| Effect of heat. | Volatile. | Non-volatile. |
| Heated with a little | Dissolves. | Oxidises to a white |
| nitric acid. | insoluble powder. | |
| Warmed with a strong | Dissolves immediately. | Slowly dissolved. |
| solution of chloride | ||
| of lime. | ||
| Treated with | Detached but not dissolved, | Soluble: on evaporation, |
| bisulphide of | but if heated to drive off | orange-yellow sulphide |
| ammonium. | ammonia yellow sulphide | formed. |
| formed. | ||
| The nitric acid | A brick-red precipitate | No reaction, but if |
| solution evaporated | soluble in ammonia | ammonia and potash |
| to dryness gives | are added, a black | |
| with nitrate of silver. | precipitate is ultimately | |
| formed. |
The portion of the tube on which the dark ring has been deposited is now cut off, broken into fragments, and heated in a small, hard glass tube—when, if arsenic be present, a white sublimate will be obtained of well-defined octahedral crystals. If the sublimate be treated with sulphide of ammonium, it is detached but not perfectly dissolved, and on evaporation of the solution to dryness, a residue of the yellow sulphide of arsenic will remain, which, if heated with strong nitric acid, and evaporated again to dryness, will give a brick-red precipitate with nitrate of silver solution, soluble in ammonia. The process of Marsh may be used quantitatively by passing the issuing gas through a glass tube, dipping into a strong solution of argentic nitrate. A portion of the tube is kept at a red heat, when, if arsenic be present, it is deposited in the metallic form in the cool portion. The glass tube containing the stain is cut with a file and weighed. The stain is then removed by strong nitric acid, the tube dried and weighed: the difference in weight equals the amount of metallic arsenic. The nitrate of silver solution is now treated with pure hydrochloric acid, filtered, and the filtrate neutralised with sodium carbonate, titrated with standard solution of iodine. By dipping the end of the issuing tube into a fresh solution of argentic nitrate, the absence of colour will show that all the arsenic has been obtained.
Instead of the U-shaped tube a Wolff‘s bottle or Erlenmeyer‘s flask may be used, and the exit tube carrying off the gas bent twice upon itself and connected with a glass bulb containing calcium chloride. From this bulb the long, hard glass tube proceeds, pointed at the end to form a gas jet; the gas is lighted at the end, and if a Bunsen flame be applied at a short distance from the end, a deposit of the arsenic, if present, will form on the distal side of the point at which the flame is applied.
Reinsch‘s Process.—First obtain a clear solution by filtration or otherwise, and then proceed as follows: Strongly acidify the liquid with hydrochloric acid, introduce some pieces of copper foil, and heat to near the boiling-point of the liquid. Both the acid and metal must be previously tested to ensure their freedom from arsenic. Any arsenic present will then be deposited on the copper in the metallic state, either in the form of a black lustrous deposit when the arsenic is present in any quantity, or else as a steel-grey coating when a minute quantity only is present. In either case, the copper foil, after remaining for some time in the suspected fluid, is taken out, cut into small pieces, introduced into the bottom of a hard glass tube, and heated to low redness, when the arsenic will sublime as arsenious acid in octahedral crystals, forming a ring in the cooler portion of the tube. The deposit is identified as arsenious acid by the form of the crystals, and by its deportment with the various reagents, as in the treatment of similar sublimates mentioned under Marsh‘s Process. Two precautions have to be taken in applying this test: do not use too large a portion of copper foil at first, and do not remove the copper too quickly from the boiling fluid. A solution containing arsenic acid or an alkaline arsenite, mixed with sulphuric acid, does not produce any deposit on metallic copper even after long boiling, unless the quantity of the arsenic present be considerable; the deposition may, however, be ensured by adding sulphurous acid or a sulphite, whereby the arsenic is reduced to arsenious acid (G. Werther, J. Pr. Chem., lxxxii. 286; Jahresb., 1861, p. 851).