On this pages are given particulars of just those chemicals which were used in everyday film photography, with those of a few others not so regularly employed. The facts here collected are those which it is useful to know for the proper making-up of solutions: and they also enable photographers unacquainted with chemistry to identify chemicals by their different names. A few of the uses of some of the chemicals are briefly stated, in brackets, at the end of each paragraph.

A

• Acetic acid CH₃COOH.—The glacial acid—sp. gr. 1.055—is usually specified in photography. Soluble in all proportions in water.. alcohol, ether and chloroform. Dissolves gelatine, oils and fats. Solidifies at 50 deg. F. American formula; often specify the dilute acid of 28 to 31 per cent strength. (Acidifying fixing baths, acetate film cements.)
• Albumen.—Typical albumen occurs in white of egg, which may be used for preparation of sensitisers, etc., but prepared dry albumen is more convenient on account of its good keeping properties.
• Alcohol.—Ordinary alcohol is ethyl alcohol (ethanol) C₂H₅OH, which when of sp. gr. 0.794 is "absolute" ( = 100 per cent). Alcohol containing 10 per cent of water is " rectified spirit ". Methylated spirit is rectified spirit plus 10 per cent crude wood spirit, one eighth per cent mineral naphtha, J to 1 per cent pyridine and methyl violet to colour it. Milkiness which occurs when the spirit is mixed with water is due to the naphtha. (Concentrated developers, rapid drying, cleaning, solvent for shellac, mastic, sandarac and dammar.)
• Alum.—General name for double sulphate of aluminium and potassium (or ammonium). Potassium alum K₂SO₄Al₂(SO₄)₃. 24H₂0 is the photographic variety which is sold in large white crystals or in powder form which is more easily dissolved. Solubility 1 in 10 in cold water. An acid substance decomposing hypo with deposition of sulphur. it should be free from iron, a trace of which may cause bluish stains on sulphide toned prints. (Hardening bath.)
• Amidol.—Diaminophenol hydrochloride, C₆H₃OH. (NH₂)₂. 2HCI, Fine white or bluish grey crystals, very soluble in water. Almost insoluble in alcohol. Developers are prepared by dissolving amidol in a solution of sodium sulphite. Both dry and in solution it should be stored in well stoppered amber bottles. Neutral or mildly alkaline solutions soon lose activity, but weak acid solutions keep much better.
• Ammonia.—Strong solution of the gas NH₃ in water. The gas combines with water to form partly or completely ammonium hydroxide thus: NH₂+H₂0 = NH₂OH. Strong solution of ammonia should be of specific gravity 0.880 in which form it gradually loses its strength. 0.880 ammonia solution contains approximately 35 per cent by weight of NH₃.
• Ammonium bichromate (NH₄)₂ Cr₂0₇.—Orange crystals, more soluble than the potassium salt. Solubility 1 in 4 in cold water. Used as a sensitiser in photomechanical process work with albumen. It may replace the potassium salt as a gelatine sensitiser in the carbon, carbro and oil processes.
• Ammonium bromide, NH,Br. White crystalline powder dissolving in 1.5 times its weight in cold water; slightly soluble in alcohol. Absorbs moisture and should be kept well stoppered.
• Ammonium carbonate NH₄HCO₃+NH₂.CO.ONH₄. Also known as Rock Ammonia. Sold in hard opaque pieces or cubes. Solubility 1 in 4 in cold water; should not be dissolved in hot water.
• Ammonium chloride, NH₄Cl. Fine white powdery crystals. Solubility 1 in 3 in cold water, 1.5 in hot water. Should be kept well stoppered. (Rapid fixing baths.)
• Ammonium persulphate, (NH₄)₂S₂0₈. Small white crystals, solul I in 1 J cold water. Decomposed by hot water. The crystals rapic absorb moisture and must be kept well stoppered. (Reducing.)
• Ammonium thiocyanate, NH,CNS, also called ammonium sulpl cyanide. Small white crystals which are very deliquescent. \ crystals cannot be dried and are useless. Very soluble in water a alcohol. (Toning P.O.P.; developers for reversal process.)

B

• Borax, sodium tetraborate, Na₂B₄O₂. 10H₂0. White crystalline powder. Solubility 1 in 121 in cold water, much more soluble in 1 water. Due to presence of unneutralised anhydride borax constitu a natural self-contained buffer maintaining a pH of 9.2 at a wi range of concentrations. (Weak alkali for fine-grain developers.)
• Boric acid, ortho boric acid, H,BO,. Solubility I in 29 in cold water; I in 2.9 in hot water. Both crystals and powder are availat the former being more easily dissolved. As a buffer with borax it used to control the alkalinity of certain fine-grain developers. Bo acid also forms a component of acid fixing baths.

C

• Calcium chloride, CaCl2. The commercial fused form is an act desiccant. It can be reactivated by heating in an iron shovel or to Used to protect ferroprussiate papers from deterioration by moistu
• Calgon.—Essentially sodium hexametaphosphate, Na₂(Na,P,O but with a small addition of alkaline phosphate to reduce slit acidity. Solvent of otherwise insoluble calcium and magnesit salts which are precipitated by alkalis in tap water. Used for obtaini clear solutions of alkaline developers.
• Caustic potash, correctly potassium hydroxide, KOH, sometin called potassium hydrate. The purest form used in photograp is sold in sticks or pellets. Rapidly becomes moist when expo! to air and should be stored in well corked, not glass-stoppered bottl Dissolves in half its weight of water with production of much he for which reason solutions should be made in vitreous enamelled stainless steel vessels—not aluminium, which is attacked by caul solutions even in the cold. Caustic solutions should be kept in rubber stoppered or polythene bottles. Used as a powerful energiser for slow reducers such as hydroquinone. Used also for preparing concentrated phenolate developers. See caustic soda.
• Caustic soda, sodium hydroxide, NaOH. Slightly less powerful than caustic potash, but similar in other respects and requires the same precautions. The stick form is generally used in photography. It is cheaper than caustic potash and may replace the latter provided the equivalent weight is used. The two equivalents are: Caustic soda 40, caustic potash 56. For removing gelatine from waste negatives the commercial or scouring grade may be used.
• Chlorquinol, monochlorhydroquinone, adurol (Hauff) C₆H₃Cl (OH)z. A white or slightly tinted crystalline powder readily soluble in water, also in alcohol. More energetic than hydroquinone, and less sensitive to temperature.
• Chrome alum, double sulphate of chromium and potassium (or ammonium), K₂SO₄. Crz(SO,),. 24H₂O. Forms violet crystals, appearing ruby red by transmitted light. Slowly soluble I in 10 in cold water, decomposes in hot water which should not be used. Solution is aided by grinding crystals to a fine powder. Valuable hardener of gelatine which action is increased by adding a few drops of ammonia to give very slight alkalinity. (Stop baths and hardener in fixing baths.)
• Citric acid, C,H₄ (OH) (COOH)x. Small colourless crystals, or sticky caking powder. Forms complex compounds with many basic salts which it dissolves. (Acid fixing baths; preservative for developing agents in sulphite solutions; dish cleaner.)
• Copper sulphate, blue vitriol, CuSO₄. 5H₂0. Blue crystals. Solubility 1 in 2J in cold water. The photographic quality should be pure and free from iron. Must be kept well stoppered. (Bleacher; reducer.)

D

• Dextrine.—British gum. For mountants it should be of the purest quality. See Formulae Section under Mountants.

F

• Ferric ammonium citrate, iron and ammonium citrate. Forms the basic sensitiser in ferroprussiate papers, for which purpose only the green scales are used. Very soluble in water and forms into a sticky mass when exposed to air. Keep well stoppered and shielded from light.
• Ferric chloride, FeCl₃. 6H₂O, iron perchloride. Occurs as a yellow or mustard coloured mass; actually crystalline, but appears as amorphous lumps. Light-sensitive and used for ferroprussiate papers; also for etching on copper in photo-engraving. Keep well stoppered and shielded from bright light.
• Ferric oxalate, Fe₂(C₂04)₃. Occurs in soluble green scales or flakes. More sensitive to light than ferric chloride and oxalic acid combined, and replaces them with the production of better blue in ferroprussiate papers. The double iron oxalates of iron and sodium (also potassium and ammonium) are used for the same purpose.
• Formalin, 40 per cent solution of formic aldehyde H.CHO. Water soluble in all proportions. It is pungent and irritating, and is slightly acid. (Hardener and preservative.)

G

• Gelatine is not a definite chemical compound, but a mixtui colloid substances. It swells in cold water, dissolves when the sw mass is heated and sets to a jelly when cool. Gelatine is dissc in the cold by oxalic, hydrochloric, acetic and nitric acids. N with the last-named it forms a glue, and with acetic acid it is a cell cement. Alum, formalin and tannic acid harden gelatine, rend it insoluble and incapable of swelling in water.
• Glycerine, glycerol, C₃H5(OH₃). A colourless syrupy Ii sp. gr. 1.265. Miscible with water and alcohol in all proport At ordinary temperatures glycerine does not evaporate, but ab: water from the air. A glycerine solution will give supplene: gelatine coated films and papers which are liable to crack di excessive drying. After evaporation of the water a trace of glyc remains in the emulsion coating.
• Glycin, para - hydroxyphenylamino - acetic acid, C6H (NH.CH₂COOH). White or cream-coloured powder. Sli soluble in water, but readily so in alkaline solution; almost insc in alcohol. Not to be confused with " glycine " which is a acetic acid and photographically inert.
• Gold chloride.—The yellow crystals commonly sold in B are a compound of gold chloride and sodium chloride—NaA 2H₂O—there being 7,j grains of metal in 15 grains of the chic Brown crystals—HAuCI,. 3H₂O—contain half their weight in r (Toning.)

H

• Hydrochloric acid, muriatic acid. A solution of HCl gas in v The pure commercial acid of sp. gr. 1.16 dissolves the oxide! carbonates of most metals. " Spirits of salt," a crude form conta iron, etc., is a useful cleaner of glass and porcelain vessels conta mineral deposits, etc. Use with great care.
• Hydrofluoric acid.—Fuming and highly corrosive solution c gas HF in water. Commercially sold at 60 per cent strength. be kept in gutta-percha bottles. It is a solvent of glass and is us a film stripper and solvent etch. Extremely dangerous if allow come in contact with the skin.
• Hydroquinone.—Quinol, hydrokinone, hydrochinon, correctly dihydroxybenzene, C6H,(OH)₂. Fine white needle crystals. SOIL I in 18 in cold water, more so in hot water. Soluble in rectified (Low energy developer.)
• Hypo, sodium thiosulphate, Na₂S₂O₃. 5H₂0, incorrectly sodium hyposulphite. Occurs as pea crystals or larger hexa crystals. Extremely soluble in water to the extent of nearly its weight. Chilling of the solution is compensated by using hot when dissolving hypo. Strong acids and acid salts liberate sulph acid or sulphur dioxide with deposition of sulphur due to di position. Sulphurous acid and its acid salts (potassium metabisu and sodium bisulphite) do not react in this way, and with hyp form a stable acid bath. Hypo is a rapid solvent of silver bromid chloride, and to a less extent of silver iodide. It is also obta, in an anhydrous powder which dissolves more rapidly than cn In fixing baths, three parts of dry hypo are equivalent to abot parts of the crystals.

I

• Iodine, I.—Greyish violet flakes or plates of metallic lustre. It is insoluble in water, but soluble in alcohol and in a solution of potassium iodide. With starch, iodine forms an intensely blue compound (iodide of starch), the colour of which is bleached by a small quantity of hypo. In this way iodide of starch may be used as an indicator of the presence of hypo in washing water. Iodine solution is decolourised by hypo.

K

• Kodalk.—A proprietary alkali recently introduced. It is less alkaline than sodium carbonate, but more so than borax, compared with which it is more soluble. Unlike carbonate it does not liberate CO, gas when passed in to acid baths, consequently there is less possibility of blistering the emulsion in hot weather. It is suitable for tropical developers, and may replace borax for a number of purposes.

L

• Lead acetate, sugar of lead, Pb(CH3000),. 3H₂0. Clear white crystals. Solubility 1 in 14 in cold water. Hot water should not be used. Milkiness in solution is due to formation of lead chloride, carbonate or sulphate caused by tap water. (Toning.)
• Liver of sulphur, potassium sulphide. Contains small quantities of sulphate and carbonate of soda, hypo and polysulphide compounds. Sold in reddish brown pieces. Very soluble in water. (Silver recovery; toning.)

M

• Mercuric iodide, bi-iodide of mercury, Hgl,. Bright red powder insoluble in water; readily soluble in sodium sulphite, hypo and potassium iodide. Intensely poisonous. (Lumi6re intensifier.)
• Mercuric chloride, bichloride or perchloride of mercury, HgCI,. In heavy pieces or crystalline powder. Solubility 1 in 16 in cold water, but more readily in boiling water. Solubility is increased with added hydrochloric acid or ammonium chloride. The solution is intensely poisonous and must not come into contact with broken skin. (Intensifier.)
• Methyl alcohol, CH₂OH, methanol. Sp. gr. 0.81. The chief constituent of " wood spirit " or wood naphtha which contains acetone and other impurities. With care may be used to dry celluloid films. (Solvent.)
• Meritol.—A patented developing agent, understood to be paraphenylenediamine pyrocatecholate, CGH4(NHZ)2. C,H4(OH),, a cream or greyish crystalline substance sparingly soluble in cold water, but more so in a warm solution of sodium sulphite. As 'a fine-grain developer it is claimed to have advantages over paraphenylenediamine.
• Metol, mono-methyl-paraminophenol sulphate, OH.C,H4(NH. CH₃), l/2H2SO4. White crystalline powder. Metol dissolves with some difficulty in sulphite solution. Hence, in making up developers, the metol should be dissolved with only a small portion of the sulphite.

N

• Nitric acid.—Strongly corrosive liquid. Commercial strong pure acid of sp. gr. 1.42 contains 71 per cent of HNO3. Oxidises alcohol and other organic solvents. Burns skin and clothes.

P

• Paraminophenol (base), NH,. C6H4OH. Yellowish-white crystalline powder. Very slightly soluble in cold water, but more freely in hot water. The hydrochloride is generally used because of its greater solubility and better keeping properties. Forms a pher with caustic alkalis, permitting a high concentration of the ba salt.
• Paraphenylenediamine (base), C6H4(NH₂)₂. Yellowish-whit dark brown crystals. Much less soluble than the hydrochl~ Used either alone or with glycin and/or metol with or without z
• Potassium bichromate, more correctly dichromate, K2( Large orange-red crystals. Solubility 1 in 14 in cold water; s( in own weight in hot water. An acidified solution is a po\ oxidising agent. (Bleacher in reversal and intensifying proc gelatine and albumen sensitiser; cleaner.)
• Potassium bromide, KBr. Small colourless crystals. Soh I in 11 in cold water and in own weight in hot water. (Rest. and re-halogeniser.)
• Potassium carbonate, carbonate of potash, K₂CO₃. Knowr as potash. White anhydrous (dry) deliquescent powder. Dis in less than its own weight of water. The so-called crystalline v is of uncertain composition. (Developers; rapid drying of nega
• Potassium chloroplatinite, proto-choride of platinum and potas KZPtCl,. Small red crystals. Solubility 1 in 6 in cold water; ins in alcohol. The salt should contain 46 per cent of platinum Distilled water should be used for solution to which is added one of hydrochloric acid. Must be kept in well stoppered bottles. num toning.)
• Potassium cyanide, cyanide of potash, KCN. The comn variety is sold as hard white pieces (fused) containing 30 to cent KCN. The pure substance, however, contains at least cent KCN. A double salt which contains 98 to 100 per cent is available which may replace the potassium salt, but the strength must be taken into consideration when making up the N hoven intensifier. Very soluble in water and alcohol, potassium c is extremely poisonous and rapidly absorbed in skin abrasions etc. Dangerous prussic acid vapour results from the action acid on solid cyanide or its solution. Cyanide is a powerful of the silver halides, and has perceptible solvent action on the dev image. (Local reducer.)
• Potassium ferricyanide, red prussiate of potash, K,Fe Deep ruby red crystals, often with a yellow powdery coating may be removed by rinsing. Solubility 1 in 21 in cold water. cyanide solution keeps better combined with some commo (Reducer; bleacher; ingredient in ferroprussiate sensitisers.)
• Potassium iodide, iodide of potash, KI. Small white dissolving in less than their weight of water. Slightly soh alcohol. Colour changes slightly to yellow in light, but is no del to its use as a solvent of iodine in intensifiers.
• Potassium metabisulphite, K₂S₂05. White transparent usually slightly incrusted, rendering them opaque. Should t well sealed to prevent deterioration. Fairly soluble in cold forming sulphite and sulphurous acid. Partly decomposed water which should not be used. (Acidifying fixing baths; press and buffer in developers.)
• Potassium permanganate, permanganate of potash, KMnO₄. Small black crystals with a bronze lustre. Solubility 1 in 16 in cold water; readily soluble in hot water. Heavy stain caused by its solution can be discharged by a solution of sodium or potassium metabisulphite or oxalic acid. Powerful oxidiser, but useless when reduction turns it brown. (Stain remover; bleacher; disinfectant; deodorant.)
• Potassium thiocyanate. potassium sulphocyanide or sulphocyanate, KCNS. Small white deliquescent crystals, very soluble in water. More stable than the ammonium salt, especially in the presence of alkalis. Used as a silver halide solvent in fine-grain and reversal developers.
• Pyrocatechin, catechol, CBH,(OH),, correctly ortho-dihydroxybenzene. White crystalline, very soluble substance, less energetic than hydroquinone. Has many applications. In conjunction with a caustic alkali and highly diluted it forms a fine-grain developer. With sodium carbonate it gives sepia to red tones to chlorobromide papers. Used highly diluted, also with carbonate or a caustic alkali it ranks with metol and paraminophenol as a surface-acting developer suitable for under-exposed negatives. Pyrocatechin combined with sodium carbonate, but with little or no sulphite constitutes a tanning or hardening developer. A negative so developed and treated with hot water will yield a relief image capable of being selectively dyed.
• Pyrogallic Acid, pyro, pyrogallol, C6H₃(OH)₃, correctly 1-2-3trihydroxy benzene. Obtainable as white feathery sublimed crystals or as normal crystals of about 1/10th the sublimed bulk. The two forms are identical in composition and are extremely soluble in water and alcohol. Pyro oxidises rapidly in solution unless preserved in normal or acid sulphites. Pyro should always be added after the preservatives have been dissolved. Valuable as a fog inhibitor in conjunction with other developers.

S

• Silver nitrate, lunar caustic, AgNO,. Solubility 2 in 1 in water. Chlorides in tap water cause milkiness, hence always use distilled water. Black or brown stains on fingers or clothes can be removed by tincture of iodine followed by strong hypo.
• Sodium bicarbonate, bicarbonate of soda, NaHCO3. Fine white powder. Solubility 1 in 11.3 in cold water. Feebly alkaline, and decomposed by hot water.
• Sodium bisulphite, NaHSO₃. Obtained as a fine white powder, but the lye or liquid of average density 36 deg. Baume (= 1.33 sp. gr.) is more stable. This colourless or pale yellow liquor can be used instead of sodium sulphite, although its acidity must be taken into consideration. Sodium bisulp!;ite or metabisulphite may replace potassium metabisulphite in th..= same proportions.
• Sodium bromide, NaBr. White crystals or granular powder. Slightly deliquescent. Solubility I in 1.5 in cold water. (Restrainer.)
• Sodium carbonate, carbonate of soda. Two forms are commonly available, crystalline, Na₂CO,. 10H₂0, and anhydrous, Na2CO2. The latter is known also as "dry" and in pharmacy as "exsiccated", the American equivalent being " desiccated ". To convert crystals to the anhydrous equivalent multiply by 0.371. To convert anhydrous to the crystal equivalent multiply by 2.7. Crysth carbonate is contaminated by air and should be kept well se solubility I in 1.5 in cold water. Anhydrous carbonate absorbs moi from air and should also be well sealed; solubility I in 6 in water.
• Sodium metabisulphite, Na₂S₂O6. Similar in most respect the potassium salt which it may replace. The crystal form is suitable than the powder.
• Sodium sulphide, Na₂S. 9H₂O. When pure is in small cc less deliquescent crystals. Dilute sulphide solution rapidly ox and for this reason a 20 per cent stock solution should be ava for dilution to working strength just prior to use. The fumes h; deleterious effect on unexposed sensitive materials, and solo should be quickly washed down the sink after use. The comma quality is used to precipitate silver residues in solution.
• Sodium sulphate, sulphate of soda, Glauber's salt, Na₂SO₄. 10 Large transparent efflorescent crystals, 2J parts of which are to one part of the anhydrous salt. Prevents swelling of the ge in alkaline solution, hence its use in tropical developers. Solu 1 in 3 in cold water, more so in hot water up to 90 deg. F., beyond solubility becomes less.
• Sodium sulphite, sulphite of soda, Na₂SO₃. 7H₂0. Large efflorescent crystals. Solubility 1 in 2 in cold water; maximum bility at 100 deg. F. The crystals oxidise in air to sulphate indi by powdery coating, which should be removed by rinsing before Two parts of the crystals are equal to one part of the anhydrou which is more easily soluble. Sulphite solution rapidly oxidises in doing so loses its power as a preservative. In developers t retarded by preserving the reducer with an acidified sulphite sol in which case the alkali is kept as a separate solution. Sodium su is a solvent of the silver halides which explains its high concent: (100 grams or more of the dry salt per litre) in certain fine developers.
• Sodium tribasic phosphate, tri-sodium phosphate, Na₃PO₄. 1L White crystals, very soluble in water. More active than sc carbonate and used in conjunction with paraphenylenedi and glycin. It is also obtainable in a dry form, containing one me of water.
• Sulphuric acid, oil of vitriol, HISO₄. Syrupy, highly cor heavy liquid, of sp. gr. 1.84, containing 98 per cent of the rea In preparing solutions, the acid must always be added slowly water. If water is added to the acid the great heat developer crack the vessel and throw out the contents with explosive vic (Reversal baths.)

T

• Tannic acid.—A yellowish or light brown powder, very s in hot water. It is of uncertain composition, but the pharmacy quality is most suitable for photographic use. Tannic acid sc is miscible with chrome alum solution and formalin separatel incompatible with both together. (Hardener.)