"~Litharge~," PbO.--It can be purchased as a yellow heavy powder. It is used in dry a.s.saying as a flux, as a desulphurising agent, and also as a source of lead. It always contains some silver, the amount of which must be determined.

~Litmus.~--This is an organic colouring matter which is turned red by acids and blue by alkalies. For ordinary purposes it is best used as litmus paper, which may be purchased in small books. A solution is prepared by digesting 15 or 20 grams of the commercial litmus in 100 c.c. of water on the water bath. After being allowed to settle, it is filtered and made just faintly red with acetic acid. Then there is added a drop or two of a solution of soda and 10 c.c. of alcohol. It should be kept in a loosely-covered bottle.

~Magnesia~, MgO.--It may be purchased as "calcined magnesia." It is used for making "magnesia mixture," and should be kept in a corked wide-mouthed bottle.

"~Magnesia Mixture.~"--Dissolve 22 grams of magnesia in about a quarter of a litre of dilute hydrochloric acid, avoiding excess. Add 5 grams of magnesia, boil, and filter. Add 300 grams of ammonic chloride, and 250 c.c. of strong ammonia; and dilute with water to 2 litres. It should be kept in a stoppered winchester.

~Magnesium Sulphate~, MgSO_{4}.7H_{2}O.--It can be purchased very pure, and is occasionally used as a standard salt.

~Manganese Dioxide~, MnO_{2}.--It is used in the preparation of chlorine. The commercial article is not pure, but is sufficiently so for this purpose.

~Marble~, CaCO_{3}.--Fragments of the white crystalline variety only should be used. It is used as a source of lime and of carbon dioxide.

~Mercury~, Hg.--This can be purchased pure. It should have a bright surface, flow without a tail, and leave no residue on ignition. It is used as a standard; for amalgamation; and as a confining liquid in gas a.n.a.lysis.

~Mercuric Chloride~ (HgCl_{2}) may be purchased pure. Make a 5 per cent.

solution in water. It is used for destroying an excess of stannous chloride; for removing sulphuretted hydrogen from solution; and as a test for stannous salts.

~Microcosmic Salt~, HAmNaPO_{4}.8H_{2}O.--When fused NaPO_{3} is formed.

It is used in testing for metallic oxides and silica before the blowpipe. The crystals are sometimes used as a standard for phosphoric acid.

"~Nessler"s Solution.~"--Mode of preparation: Dissolve 35 grams of pota.s.sium iodide in 100 c.c. of water; dissolve 17 grams of mercuric chloride in 300 c.c. of water, and pour this solution into that of the iodide till a permanent precipitate is produced; make up to 1 litre with a 20 per cent. solution of potash; add mercuric chloride till a precipitate is again formed; allow to settle and decant. It is used for detecting ammonia.

~Nitre.~--This is pota.s.sium nitrate.

~Platinum Chloride~, 2HCl.PtCl_{4}. (In the crystallised form it has 6H_{2}O).--It may be made as follows:--Take 5 grams of clean platinum sc.r.a.p and dissolve in a flask at a gentle heat in 50 c.c. of hydrochloric acid with the occasional addition of some nitric acid; evaporate to a paste; and then dissolve in 100 c.c. of water. It is used for separating and determining pota.s.sium.

~Phenolphthalein~ is an organic compound used as an indicator; more especially in determining the weaker acids, it cannot be used in the presence of ammonia. Dissolve half a gram in 100 c.c. of dilute alcohol.

~Pota.s.sium Bicarbonate~, KHCO_{3}.--It may be purchased pure; on ignition it leaves the carbonate, K_{2}CO_{3}, which may be used as a standard.

~Pota.s.sium Cyanide~, KCN.--It is used in the dry a.s.say as a reducing agent. The commercial salt is very impure. Purchase that sold as pota.s.sic cyanide (gold) which contains about 95 per cent. of KCN. It is used for copper a.s.saying and occasionally in separation. Make a 10 per cent. solution when wanted.

~Pota.s.sium Bichromate~, K_{2}Cr_{2}O_{7}. It may be purchased nearly pure. It is used as an oxidising agent, for determining iron; and as a test solution. For this last purpose a 10 per cent. solution is prepared.

~Pota.s.sium Chlorate~ (KClO_{3}) can be purchased pure. It is used with hydrochloric acid as a subst.i.tute for aqua regia.

~Pota.s.sium Ferrocyanide~ (K_{4}Fe(CN)_{6}.3H_{2}O), or "yellow prussiate of potash," is used as a test; as an indicator; and for the determination of zinc. Make a 5 per cent. solution.

~Pota.s.sium Ferricyanide~ (K_{6}Fe_{2}(CN)_{12}), or "red prussiate of potash," is used for testing; and as an indicator. Make a 5 per cent.

solution when wanted, as it decomposes on keeping.

~Pota.s.sium Hydrate~, KHO. Purchase that purified with alcohol. It is an alkali, and is used for absorbing carbonic acid, &c.

~Pota.s.sium Iodide~, KI. It may be purchased nearly pure. It is used as a test and for dissolving iodine. It should be used in a 10 per cent.

solution freshly made. The solution decomposes on exposure to light, with separation of iodine.

~Pota.s.sium Nitrate~ (KNO_{3}) can be purchased pure. It is used in the dry way as an oxidizing agent. It is very fusible. It decomposes at a low temperature into pota.s.sium nitrite (KNO_{2}) and free oxygen; and at a higher temperature leaves potash (K_{2}O). It oxidizes sulphur and carbon with explosive violence. This action may be moderated by mixing the nitre with carbonate of soda, common salt, or some other inert body.

~Pota.s.sium Nitrite~, KNO_{2}.--The commercial article is not pure, but is sufficiently so for the purpose required. A saturated solution is used in the separation of cobalt; the solution is made when wanted.

~Pota.s.sium Permanganate~, KMnO_{4}.--This salt can be purchased sufficiently pure. It is much used as an oxidizing agent.

~Pota.s.sium Bisulphate~ (KHSO_{4}) is used as a dry reagent for opening up minerals. It fuses; and at a much higher temperature is converted into pota.s.sium sulphate with loss of sulphuric acid.

~Pota.s.sium Sulphocyanate~ (KCNS) is used for the detection and determination of traces of ferric iron; as also in the separation of silver and copper from some of the other metals. Make a 10 per cent.

solution. It should show no colour on the addition of hydrochloric acid.

"~Red Lead~" (Pb_{3}O_{4}) is used in the dry a.s.say as a flux instead of litharge, from which it differs in containing a little more oxygen. When acted on by nitric acid a brown residue of lead dioxide is left, nitrate of lead going into solution. Like litharge it always carries silver; about 2 milligrams in 100 grams.

~Silver~, Ag.--Pure silver in foil is required as a standard. It may be prepared as follows:--Dissolve sc.r.a.p silver in dilute nitric acid and decant off from any residue; dilute the solution with hot water and add hydrochloric acid until there is no further precipitate, stir; allow the precipitate to settle; decant and wash; dry the precipitate, mix it with twice its bulk of carbonate of soda and fuse the mixture in a crucible until tranquil; clean the b.u.t.ton and roll or hammer it into foil.

~Sodium Acetate~, NaC_{2}H_{3}O_{2}.3H_{2}O.--The crystals may be purchased sufficiently pure. Make a 20 per cent. solution in water. It is used for replacing mineral acids by acetic acid.[7]

~Sodium Acetate and Acetic Acid.~--A solution is used in the determination of phosphates and a.r.s.enates; 100 grams of the salt is dissolved in 500 c.c. of acetic acid, and diluted with water to one litre.

~Sodium Bicarbonate~ (NaHCO_{3})is used as a flux in dry methods. On ignition it leaves the carbonate (Na_{2}CO_{3}), which is used as a standard reagent. Make a 20 per cent. solution of the carbonate for use.

It should be free from chlorides or sulphates, or if impure the amount of impurities must be determined.

~Sodium Hydrate~, NaHO. It may be purchased in sticks, which should be kept in a well-corked bottle. It is sometimes called "caustic soda." It is a strong alkali. It is used for neutralizing acid solutions and for separations where ammonia is unsuitable. Make a 5 per cent. solution for use.

~Sodium Hyposulphite~, Na_{2}S_{2}O_{8}.5H_{2}O.--It may be purchased pure. It is generally known as "hypo." It is used as a standard.

~Sodium Sulphite~ (Na_{2}SO_{3}.7H_{2}O) is used as a reducing agent.

~Sodium Phosphate~, Na_{2}HPO_{4}.12H_{2}O. The crystals may be purchased pure, but they effloresce in dry air with loss of water. It is used as a standard and for precipitating magnesia, &c. Make a 10 per cent. solution.

~Stannous Chloride~, SnCl_{2}.2H_{2}O.--The crystals are best purchased.

If kept dry and free from air they are fairly permanent. A solution is made by dissolving 20 grams in 10 c.c. of hydrochloric acid and diluting to 1 litre. The solution is not permanent. It is a strong reducing agent, and is chiefly used in solution for this purpose.

~Tin~, Sn.--Grain tin should be purchased. It is not pure, but contains 99.5 per cent. of the metal. The chief impurity is copper. It can be used as a standard. When acted on with hot hydrochloric acid it slowly dissolves (more rapidly in contact with platinum) and forms stannous chloride.

~Uranium Acetate~, UO_{2}(C_{2}H_{3}O_{2})_{2}.H_{2}O.--It is best purchased in crystals. The solution is used for the determination of phosphates and a.r.s.enates. A solution of 3 per cent. strength is occasionally used as an indicator.

~Uranium Nitrate~, UO_{2}(NO_{3})_{2}.6H_{2}O.--This salt is very soluble in water and is sometimes used instead of the acetate, which is somewhat difficult to dissolve.

"~Water~," H_{2}O.--Spring or well water is sufficiently pure for most purposes, 100 c.c. will leave a residue of from 10 to 30 milligrams, so that where a salt has to be dissolved out, evaporated, and weighed it should be replaced by distilled water. Rain water, melted snow, &c., always leave less residue than spring water; but in other respects they are often dirtier. Distilled water is best prepared in the office, a gla.s.s or tin condenser being used.

~Zinc~, Zn.--It is sold in a granulated form or in sticks. It generally contains over 1 per cent. of lead, with a little iron and a.r.s.enic. It is used for separating metals from their solutions, and generally as a reducing agent. For the preparation of hydrogen, and in most other cases, sc.r.a.p sheet zinc may be used.

~Zinc Oxide~, ZnO.--The commercial oxide sometimes contains carbonate.

~Zinc Sulphate~, ZnSO_{4}.7H_{2}O.--It is occasionally used as a standard, and can be purchased nearly pure.

FOOTNOTES:

[6] 3HCl + HNO_{3} = Cl_{2} + NOCl + 2H_{2}O.