[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,
NH
Molec. Wt. 17		Carbonic acid Renders lime-water milky.
Dissolved solid matterResidue left on evaporation.
ChloridesAfter acidulating with nitric acid, it gives a precipitate with silver nitrate, which after washing is readily soluble in ammonia and reprecipitated by nitric acid.
SulphatesAfter 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 bottlesBlack precipitate with sulphureted hydrogen.
Nitric acid,
H, NO
Molec. Wt. 63		Traces of sulphuric acidAfter dilution it gives a precipitate with barium nitrate.
ChloridesAfter dilution it gives a precipitate with silver nitrate.
Peroxide of nitrogenThe 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 chlorineLiberates iodine from solution of potassium iodide. See also "Chlorides," nitric acid.
Sulphuric acidAs above for nitric acid.
Perchloride of ironYellow color. Brown precipitate with ammonia added till it smells slightly.
Sulphuric acid,
HSO
Molec. Wt. 98		Bisulphate of potassiumResidue on evaporation.
Sulphate of leadMilkiness 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 CHO
Molec. Wt. 60		WaterDoes not solidify when cooled to 17° C. (53º F.)
Sulphurous and hydrochloric acidsWhite precipitates with silver nitrate.
Aldehyde, or volatile tarry matterBlackens in the light after adding silver nitrate.
Organic sulphuric acidSmell of garlic.
Citric acid,
HC HOHO
Molec. Wt. 210		Tartaric acidStrong solution of potassium. Acetate added to a strong solution of the acid will deposit white crystalline bitartrate.
Pyrogallic acid,
(CH)HO
Molec. Wt. 126		Metagallic acidBlack residue, insoluble in water.
Silver nitrate, AgNO
Molec. Wt. 170		Free nitric acidReddens litmus paper. (Neutral silver nitrate does not affect litmus.)
Potassium carbonate,
KCO
Molec. Wt. 138		Chlorides and sulphatesSame as for ammonia.
Potassium iodide,
KI
Molec. Wt. 166		Potassium carbonateA strong solution is alkaline to test paper.
Sulphates and chloridesSame as for ammonia.
Potassium iodateA 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 iodideSee potassium iodide.
Sodium carbonate,
NaCO
Molec. Wt. 106		Chlorides and sulphatesSame 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 sulphateAs for "sulphates" in ammonia.
Potassium cyanide, KCN
Molec. Wt. 65, and hydrate, KHO
Molec. Wt. 56		Potassium carbonate nearly always presentEffervescence with dilute acids, giving off a gas carbonic anhydride, which renders lime-water turbid.
KaolinChalkEffervescence with dilute acids.
Water, HO
Molec. Wt. 18		Sulphates and chloridesSame as for ammonia.
Calcium carbonate, temporary hardnessDeposited by boiling. Test as for calcium chloride. See sodium chloride.
Ammonia, almost always present in distilled and rain waterBrown coloration, or precipitate with Nessler's reagent.
GelatineAlumAsh, sometimes as much as ten per cent.
Fatty matterSeparated by precipitation with alcohol. Dissolved out by ether or benzine, and left as a residue on evaporation of the solvent.
Ammonium bromide (NH)Br
Molec. Wt. 98		Potassium bromide or other non-volatile bodiesLeaves a residue when heated.
Ammonium chlorideSame as for chlorides in ammonia.
Pyrogallic acidPowdered glassLeft behind on solution.
Potassium iodidePotassium bromideThe crystals of bromide are usually more transparent than those of iodide, but no reliance can be placed on this.
Silver nitratePotassium nitrate, sometimes present in the fused sticks - not in the crystalsWill not yield the full quantity of chloride on precipitation with HCl. Gives a purple color to flame.
Sulphuric acidWhen 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 acidGives a brown color to the acid.
Hydrochloric acidArsenicMarsh's test.
Some yellow samples contain no iron, but an organic salt, and give an alkaline ash on ignition of the residue after evaporationReinsh's test; a small piece of copper foil becomes coated on boiling in dilute acid.
Calcium chlorideCalcium hydrateThe 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 generallyBroken 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.