Page images
PDF
EPUB

3. Zn produces the same reaction as with stannous salts. (See above.)

4. The blow-pipe reaction for Stannic is the same as for Stannous Salts. (See above.)

74. ANTIMONY. Sb, c.w. 122.

1. HQS produces in acid solutions an orange precipitate of Sb2S3, soluble in alkaline sulphides, in KHO, in boiling concentrated HCl, but insoluble in (NH4)2CO3.

2. KHO produces a precipitate of Sb2O3, soluble in excess of the reagent.

3. (NH4)HO produces a precipitate of Sb2O3, insoluble in excess of the reagent.

4. H,0 produces in solutions of SbCl; a white precipitate of Sboci, soluble in tartaric acid : compare corresponding reaction with bismuth (69, 3).

5. Zn, in presence of HCl and platinum, precipitates Sb as a black powder, which adheres to the platinum. This is best done either by placing a strip of Zn and Pt (in contact) in a solution of Sb made acid with HCl, or by placing a similar solution in a platinum capsule and dropping in a piece of Zn. The black stain on the platinum is not dissolved by cold HCI, but is immediately dissolved by warm HNO3.

6 (Marsh's Test).-If a solution of Sb be placed in a flask along with Zn and dilute H2SO4, SbH, is given off as a gas, which is decomposed by heat, Sb being deposited. The apparatus is arranged as in the figure. a is the evolution flask; b, a tube containing

[ocr errors]

CaCl, to absorb moisture; and c, a tube of hard glass attached to the drying tube, which is drawn out to a point at the extreme end, so as to form a jet. When the dilute acid and zinc have been in contact some time, and when the air has been expelled from the flask, the hydrogen evolved is lighted at the jet and a cold porcelain crucible lid is held against the flame; if a black stain be produced on it, the materials employed

с

FIG. 16.

are not pure, and must be rejected. Having ascertained that the Zn and H,SO4 are pure, add now by the funnel tube a few drops of a solution of antimony, and observe that the flame now burns with a bluish green colour, and gives off white fumes (Sb,03), and that on placing a cold porcelain lid against the flame a dull black stain of metallic antimony is deposited on it. Obtain several of these stains in order to compare them with the corresponding arsenic ones, and then heat the tube c with the lamp flame. Observe the deposition of Sb close to the flame, and the simultaneous decrease of the green tint of the flame at the jet. Obtain several of these mirrors, observe their silvery lustre, and keep some for comparison with arsenic mirrors.

Further reactions.—(a) Add to the stain on porcelain a drop of NaC10 : the stain will remain undissolved.

(6) Cut off with a file the portion of tube containing the metallic mirror, and heat it in a dry test tube. The mirror will be oxidized to Sb2O3, which will deposit as a sublimate on the test tube. Examine this with a lens, and ascertain that it is amorphous.

(c) Attach another tube containing an antimony mirror to an apparatus evolving dry H,S, and warm the mirror gently (heating first the part of the mirror farthest from the evolution flask) : observe the change in colour from the formation of orange Sb Sz. Now detach the tube and pass through it (without heating) a current of dry HCl gas; the Sb Sz will be converted into SbCl3, which is volatile, and may be collected by dipping the end of the tube under water. On adding HQS to this liquid, orange Sb Sz will be re-precipitated.

7. Allow a current of SbH, to pass through a solution of silver nitrate : SbAgz will be precipitated (black) and nitric acid left in solution. Filter and dissolve the residue in a hot solution of tartaric acid, add a few drops of HCl, and pass HQS through the solution : an orange precipitate of Sb Sz will be obtained.

8. Heated with NaHCO, on charcoal in the reducing blow-pipe flame, yields brittle globules of the metal,

3

1

and a white incrustation of Sb,0on the charcoal. Fumes of the oxide are also given off after the metal has been removed from the flame, and they occasionally condense round the bead in a crystalline mass.

75. ARSENIC. As, C.W. 75.

1. H2S produces in acid solutions a yellow precipitate of As,S3, soluble in alkaline sulphides, in KHO, in HNO3, and in (NH4)2CO3, but nearly insoluble in boiling concentrated HCl. (Compare reactions for Sb, 74, 1.)

2. AgNO3 produces in neutral solutions a pale yellow precipitate of AgzAsOz. This is best obtained by adding first the AgNO3 solution, and then drop by drop a very dilute solution of (NH4)HO prepared by adding one or two drops of ordinary (NH4)HO to a test-tube full of H,O. The precipitate is readily soluble in excess of (NH4)HO, hence the necessity for using a very dilute solution of that reagent.

3. CuSO4 added under the same conditions as the AgNO3, produces a pale green precipitate of CuHASO, (Scheele's green), soluble in (NH)HO.

4. Acetic acid, added to solutions of As2O, and then KHO in slight excess, yields (after evaporation to dryness), on ignition in a small tube, oxide of cacodyl 2(As(CH3)2), readily recognized by its powerful and characteristic odour. If SnCl, be added to the contents of the tube after ignition, the equally charac. teristic smell of cacodyl chloride, As(CH3)2C1, is observed. These experiments (and also Marsh's test (5))

[ocr errors]

must be done with an exceedingly small quantity of substance, owing to the poisonous properties of the products.

5. Proceed exactly as in Marsh's test for Sb (74,6), substituting a solution of As for one of Sb, and observe the bluish flame with which the mixture of H and Ash, burns, and also the production of white fumes of As2O3. Obtain, as in the case of Sb, stains on porcelain lids, and mirrors by heating the hard glass tube. Compare these with the antimony stains and mirrors, and observe that the latter in the case of As are deposited at a greater distance from the heated part of the tube, owing to the greater volatility of As. Observe also the distinction in colour of the stains : dark brown or almost black in the case of Sb, and, when seen in thin films, pale brown and lustrous in the case of As.

Further reactions.—(a) Add to one of the stains on porcelain a drop of Nacio: it will be rapidly dissolved.

(6) Cut off the portion of tube containing a metallic mirror, and heat it in a dry test tube. The mirror of As will be oxidized to As,O3, which will be deposited in crystals on the cool part of the tube. Examine these with a lens, and observe the octahedral form of the crystals. Take out the piece of tubing which contained the mirror, and dissolve the crystals left in the test-tube in warm H,0; add to this solution AgNO3 and very dilute (NH)HO, and observe the yellow precipitate of AgzAsOz. Or, to the aqueous solution of

« PreviousContinue »