By John Pearle
Some General Chemical Notes.
Read (and keep) the data sheets for your chemicals. Although they are written for large scale users they contain useful first aid information.
Some general safety notes are attached, read these, too.
Code 5. Ammonium Thiosulphate Solution, 60%.
1000mL of solution contains 792g ammonium thiosulphate.
Code 6. Benzotriazole.
Benzotriazole can be difficult to dissolve. It is only used in small amounts so the best method is to make up a 1% stock solution in 10% sodium carbonate. To do this add 10g anhydrous sodium carbonate to 75 mL hot water (55-60°C). Make up to 100 mL with more hot water and add 1g benzotriazole. Solution can be speeded up by first adding the benzotriazole to a few drops of isopropyl alcohol. This solution keeps well.
Code 7. Benzyl Alcohol.
This is difficult to dissolve in water. Unless your formula specifies otherwise, make a 50% solution in diethylene glycol. Use a glass, polyethylene or stainless steel mixing vessel - NOT polystyrene. Mix well. Add this to the (warm) water first, then add the other ingredients in the order given.
Codes 10, 12, 21, 23, 53, 75 and 82
Most of these are developing agents and sensitive to air so the package will contain a photographically inert gas. Normally “Tetenal Protectan” which is inflammable, so take care when opening the package.
Code 12. Dimezone ‘S’. (Phenidone)
Phenidone - an Ilford name - is difficult to dissolve in water and may need water as hot as 80ºC to dissolve. The CRC supplies Dimezone “S” - a Kodak version - which may be used as a direct equivalent. It dissolves without problems and has better keeping properties.
Code 23. Metol.
This will oxidise quickly in solution so add any sulphite as soon as the metol has dissolved. You can add a small pinch (1 or 2g) of the sulphite before the metol to reduce this but don’t add too much or the metol
won’t dissolve.
Codes 36, 48 and 78. Thiocyanates.
These will absorb moisture from the air very readily, so are best kept as any cases it is also possible to interchange sodium and potassium compounds (and some others, such as lithium, too) but this needs to be done with care and may have effects that are specific to the formula involved. Unless specifically allowed by your formula, treat such substitutions as a basis for experiment.
August 2001. Iss 4.solution, e.g. 20%. If they are supplied as a solid the package will contain a photographically inert gas - such as “Tetenal Protectan” - some of these are inflammable so take care when opening the
package.
Code 48. Sodium Thiocyanate.
Some formulæ specify sodium thiocyanate liquid. This is normally a 51%w/w solution (Kodak), 100mL of this contains 66.8g sodium
thiocyanate.
Code 54. EDTA FeNH Solution, 50%.
100mL of solution contains 65.5g ferric ammonium EDTA.
132mL EDTA FeNH 50% is equivalent to 100g EDTA FeNa.
Code 77. Sodium Sulphide, 60%.
This grade is a hydrate and contains 40% water by weight. It is a solid so 100g of sodium sulphide 60% contains 60g sodium
sulphide.
Code 81. Potassium Sulphite Solution, 45%.
100mL of solution contains 65.2g potassium sulphite.
Chemical Substitutions 13.0
Some chemicals are available in “anhydrous or dessicated”, “crystal”, “monohydrate” or similar forms. For photographic purposes these can be interchanged provided that the amounts used are adjusted. In effect, these different forms contain different amounts of water. Anhydrous contains no water, monohydrate contains one molecule of water for each molecule of wanted chemical and so on.
Crystalline (crystal) forms of different chemicals may contain different amounts depending upon the compound. Conversion factors are often given in photographic books or as part of the formula itself. Remember that the greater the amount of water included in the chemical, the greater the amount you will need to use to get the same amount of the required compound.
Substitutions
for some common chemicals are:-
The
Formula calls for:-
You have:-
Multiply the formula amount by:-
Sodium
sulphite crystalline.
Anhydrous.
0.5
Sodium carbonate, monohydrate.
Anhydrous.
0.8
Sodium carbonate, crystalline.
Anhydrous.
0.4
Sodium bisulphite.
Sodium metabisulphite.
1.0
Sodium acetate, anhydrous.
Trihydrate (3H2O)
1.7
Sodium acetate, trihydrate (3H2O)
Anhydrous.
0.6
Sodium thiosulphate, anhydrous.
Crystalline (Hypo, 5H2O)
1.6
Disodium phosphate,
(12H2O)
Anhydrous
0.4
Disodium phosphate, anhydrous.
Dodecahydrate (12H2O)
2.5
In
many cases it is also possible to interchange sodium and potassium
compounds (and some others, such as lithium, too) but this needs to
be done with care and may have effects that are specific to the
formula involved. Unless specifically allowed by your formula, treat
such substitutions as a basis for experiment.
August 2001. Iss 4.