The flask, a Bunsen’s wash-bottle of 200 c.c. capacity, is provided with a small separating funnel of 65 c.c. capacity, with glass stop-cock. This is a very material aid to the obtaining of a slow and even evolution of gas, an important desideratum when all loss is to be avoided; for with only a funnel tube, every time a small portion of fluid is added, a sudden rush of gas takes place, with probably a small, but still more or less appreciable, loss. But the separating funnel, filled with the acid mixture, can be so arranged as to give a constant and regular supply of fluid at the rate of two or three drops per minute, more or less. The gas generated is dried by a calcic chloride tube, and then passes through a tube of hard glass, heated to a red heat by a miniature furnace of three Bunsen lamps with spread burners, so that a continuous flame of 6 inches is obtained, and with a proper length of cooled tube not a trace of arsenic passes by. The glass tube where heated is wound with a strip of wire gauze, both ends being supported upon the edges of the lamp frame, so that the tube does not sink down when heated. The small furnace is provided with two appropriate side pieces of sheet metal, so that a steady flame is always obtained. When the quantity of arsenic is very small, the tube is naturally so placed that the mirror is deposited in the narrow portion; but when the arsenic is present to the extent of 0·005 grm., the tube should be 6 mm. in inner diameter, and so arranged that fully 2 inches of this large tube are between the flame and the narrow portion. When the quantity of arsenic is less, the tube can naturally be smaller.
Acids of different strengths are made as follows:—
| Acid No. 1. | |
| 545 | c.c. pure conc. H2SO4. |
| 5000 | c.c. H2O. |
| Acid No. 2. | |
| 109 | c.c. pure conc. H2SO4. |
| 1640 | c.c. Acid No. 1. |
| Acid No. 3. | |
| 218 | c.c. pure conc. H2SO4. |
| 1640 | c.c. Acid No. 1. |
| Acid No. 4. | |
| 530 | c.c. pure conc. H2SO4. |
| 1248 | c.c. H2O. |
25 to 35 grms. of granulated zinc, previously alloyed with a small quantity of platinum, are placed in the generator, and everything being in position, the apparatus is filled with hydrogen by the use of a small quantity of acid No. 2. After a sufficient time has elapsed, the gas is lighted at the jet, and the glass tube heated to a bright redness.
The arsenical solution in concentrated form is mixed with 45 c.c. of acid No. 2, and the mixture passed into the separating funnel, from which it is allowed to flow into the generator at such a rate that the entire fluid is introduced in one hour or one and a half; 40 c.c. of acid No. 3 are then added and allowed to flow slowly into the generator, and, lastly, 45 c.c. of acid No. 4. The amount of time required will vary with the amount of arsenic: 2 to 3 mgrms. of arsenic will require about two to three hours for the entire decomposition, while 4 to 5 mgrms. will need perhaps three to four hours. Where the amount of arsenic is small, only 25 grms. of zinc are needed, and but 45 c.c. of acid No. 2, 30 c.c. of acid No. 3, and 30 c.c. of acid No. 4; but when 4 to 5 mgrms. of arsenic are present, it is better to take the first mentioned quantities of zinc and acids.
The arsenic being thus collected as a large or small mirror of metal, the tube is cut at a safe distance from the mirror, so that a tube of perhaps 2 to 6 grms. weight is obtained. This is carefully weighed, and then the arsenic removed by simple heating; or, if the arsenic is to be saved (as in a toxicological case), dissolved out with strong nitric acid. The tube is then cleaned, dried, and again weighed, the difference giving the weight of metallic arsenic, from which, by a simple calculation, the amount of arsenious oxide can be obtained. Some test results are given as follows; they were obtained by introducing definite quantities of arsenious oxide in the form of a solution mixed with 45 c.c. of No. 2 acid, &c.:—
| Quantity of Arsenic introduced. | Wt. of Metallic Arsenic found. | Theoretical Wt. of Metallic Arsenic. | ||
|---|---|---|---|---|
| 0·005 | grm. | As2O3 | 0·00373 | 0·00378 |
| 0·005 | „ | „ | 0·00370 | 0·00378 |
| 0·004 | „ | „ | 0·00300 | 0·00303 |
| 0·002 | „ | „ | 0·00151 | 0·00151 |
Sanger estimates and tests for minute quantities of arsenic by the Marsh-Berzelius process, and uses a generator of hydrogen; that is to say, the hydrogen is evolved in the ordinary way from zinc and sulphuric acid, and the issuing gas dried by calcic chloride; but into this flask is also delivered from another flask, charged with sulphuric acid and zinc, pure hydrogen, so that into the second flask, little by little, may be added the solution to be tested; and, owing to the generating flask, the gas may be made to give a uniform current, and at the end of the operation all arsine swept out. To estimate the quantities of arsenic in the gas, the reduction tube is heated, and a mirror or mirrors obtained, and compared with a set of standard mirrors. The standard mirrors are made as follows:—One grm. of arsenious oxide, purified by repeated sublimation, is dissolved with the aid of a little sodic bicarbonate, and, after acidification with dilute sulphuric acid, made up to 1 litre. This standard solution contains 1 mgrm. of As2O3 in every c.c., and is used to make a second standard solution, containing 0·01 mgrm., to every c.c., by diluting 10 c.c. to a litre. Of this last solution, 1 c.c., 2 c.c., 3 c.c., and so on, are measured and introduced into the reduction flask, and the standard mirrors obtained. It is recommended, for obvious reasons, to make more than one standard for each quantity, for the appearance of the mirrors from the same amount of arsenic varies. The tubes are hermetically sealed, and, when not in use, kept in the dark.
This process is convenient for small amounts of arsenic; but, as stated before, the results are given as metallic arsenic, whereas the films appear never to be composed of pure metallic arsenic, but a mixture of hydride and suboxide. Test experiments give, however, fair results.[785]