IMPURITIES IN PHOTOGRAPHIC CHEMICALS, AND TESTS FOR SAME.

[Table referred to in a paper read before the Birmingham Photographic Society by G.M. JONES, M.P.S.]

SUBSTANCE.IMPURITIES POSSIBLY PRESENT.TESTS.

Ammonia, NH₃ Molec. Wt. 17 Carbonic acid Renders lime-water milky.

Dissolved solid matter Residue left on evaporation.

Chlorides After acidulating with nitric acid, it gives a precipitate with silver nitrate, which after washing is readily soluble in ammonia and reprecipitated by nitric acid.

Sulphates After acidulating with nitric acid, it gives a precipitate with barium nitrate.

Lime A white precipitate with oxalate of ammonium.

Lead is often present, derived from the action upon flint glass bottles Black precipitate with sulphureted hydrogen.

Nitric acid, H, NO₃ Molec. Wt. 63 Traces of sulphuric acid After dilution it gives a precipitate with barium nitrate.

Chlorides After dilution it gives a precipitate with silver nitrate.

Peroxide of nitrogen The acid is yellow.

Iodine may be present if the acid be prepared from sodium nitrate. After dilution and cooling it gives a blue color with starch, paste, or mucilage.

Hydrochloric acid, HCl Molec. Wt. 36.5 Free chlorine Liberates iodine from solution of potassium iodide. See also "Chlorides," nitric acid.

Sulphuric acid As above for nitric acid.

Perchloride of iron Yellow color. Brown precipitate with ammonia added till it smells slightly.

Sulphuric acid, H₂SO₄ Molec. Wt. 98 Bisulphate of potassium Residue on evaporation.

Sulphate of lead Milkiness on dilution. May be completely freed from lead by diluting with three or four times as much water, and allowing to settle.

Acetic acid (glacial), H C₂H₃O₂ Molec. Wt. 60 Water Does not solidify when cooled to 17° C. (53º F.)

Sulphurous and hydrochloric acids White precipitates with silver nitrate.

Aldehyde, or volatile tarry matter Blackens in the light after adding silver nitrate.

Organic sulphuric acid Smell of garlic.

Citric acid, H₃C₆ H₅O₇H₂O Molec. Wt. 210 Tartaric acid Strong solution of potassium. Acetate added to a strong solution of the acid will deposit white crystalline bitartrate.

Pyrogallic acid, (C₆H₃)HO₃ Molec. Wt. 126 Metagallic acid Black residue, insoluble in water.

Silver nitrate, AgNO₃ Molec. Wt. 170 Free nitric acid Reddens litmus paper. (Neutral silver nitrate does not affect litmus.)

Potassium carbonate, K₂CO₃ Molec. Wt. 138 Chlorides and sulphates Same as for ammonia.

Potassium iodide, KI Molec. Wt. 166 Potassium carbonate A strong solution is alkaline to test paper.

Sulphates and chlorides Same as for ammonia.

Potassium iodate A pretty strong solution becomes yellow from liberation of iodine on addition of dilute sulphuric acid or, better, a strong solution of citric acid.

Potassium bromide, KBr Molec. Wt. 119 Similar to potassium iodide See potassium iodide.

Sodium carbonate, Na₂CO₃ Molec. Wt. 106 Chlorides and sulphates Same as for ammonia.

Sodium chloride, NaCl Molec. Wt. 58.5 Chloride of calcium Chloride of magnesium Oxalate of ammonium (after addition of a little acetic acid) gives a milkiness, or precipitate, indicating calcium; filter this out and add ammonia, chloride of ammonium, and phosphate of sodium (clear solutions). A precipitate indicates magnesium. Both the above cause dampness in wet weather.

Sodium sulphate As for "sulphates" in ammonia.

Potassium cyanide, KCN Molec. Wt. 65, and hydrate, KHO Molec. Wt. 56 Potassium carbonate nearly always present Effervescence with dilute acids, giving off a gas carbonic anhydride, which renders lime-water turbid.

Kaolin Chalk Effervescence with dilute acids.

Water, H₂O Molec. Wt. 18 Sulphates and chlorides Same as for ammonia.

Calcium carbonate, temporary hardness Deposited by boiling. Test as for calcium chloride. See sodium chloride.

Ammonia, almost always present in distilled and rain water Brown coloration, or precipitate with Nessler's reagent.

Gelatine Alum Ash, sometimes as much as ten per cent.

Fatty matter Separated by precipitation with alcohol. Dissolved out by ether or benzine, and left as a residue on evaporation of the solvent.

Ammonium bromide (NH₄)Br Molec. Wt. 98 Potassium bromide or other non-volatile bodies Leaves a residue when heated.

Ammonium chloride Same as for chlorides in ammonia.

Pyrogallic acid Powdered glass Left behind on solution.

Potassium iodide Potassium bromide The crystals of bromide are usually more transparent than those of iodide, but no reliance can be placed on this.

Silver nitrate Potassium nitrate, sometimes present in the fused sticks—not in the crystals Will not yield the full quantity of chloride on precipitation with HCl. Gives a purple color to flame.

Sulphuric acid When vended as pure, it invariably contains a trace of iron. Common acid is also liable to contain arsenic, selenium, thalium, and many other substances. No easy test can be given, as the substances are so numerous some of them volatile, and most require separation from the acid before detection.

Organic matter, as a piece of straw in a carboy of acid Gives a brown color to the acid.

Hydrochloric acid Arsenic Marsh's test.

Some yellow samples contain no iron, but an organic salt, and give an alkaline ash on ignition of the residue after evaporation Reinsh's test; a small piece of copper foil becomes coated on boiling in dilute acid.

Calcium chloride Calcium hydrate The clear filtered solution made with distilled water is alkaline to test paper, and gives a precipitate on breathing into it through a tube.

Pure (?) chemicals generally Broken glass, bits of straw, wood, paper, etc. These impurities either float or sink on solution, and may easily be seen.

G.M. JONES, M.P.S.