FROM WAY BACK WHEN! (2)

(CRCN No. 4. Jan. 1 977)

In CRCN No.72 (Jan. 1994) we discovered by looking back through the Newsletters we could find information that newer members hadn`t come across, or in the case of "To wash or Not to wash" between the Bleach and Fix baths, that this information already existed. You may find the conclusion of this article equally of interest:-

CHEMICALS.

Sodium & Potassium salts are, generally, freely interchangable provided the right amount is used. Most formulae prescribe the use of sodium carbonate anhydrous. However, the potassium carbonate can be used provided 13 parts of the anhydrous salt is used to replace 10 parts of anhydrous sodium carbonate. This will normally maintain the pH value within the correct range.

Crystalline sodium carbonate usually contains a large amount of water of crystallisation. This fact must be allowed for when using this particular chemical. 106 grams of anhydrous sodium carbonate are equal to 286 grams of the crystals. These are the two varieties of soda normally used in photographic formulae.

BROMIDES.

Potassium and sodium bromide are used as strainers. In most cases they are freely interchangable. No weight changes are needed. There are some text books which state that the sodium bromide is not stable as the potassium bromide. This chemical can be safely stored as a 10 or 20% solution.

ALKALIS.

The most common alkalis used in photography are the Sodium or Potassium Carbonates, Sodium and Potassium Hydroxide, Borax and Sodium Metaborate. Sodium and Potassium Hydroxide are the caustic alkalis and have very poor buffering qualities. They are used in very small amounts since they cause large changes in the pH value. The carbonates are only moderate buffers and are normally used in quite large amounts. The purity of the Sodium Carbonate can vary depending on the source and the storage conditions. Prolonged exposure to the atmosphere will change the carbonate to bicarbonate and this change will affect the pH value of the solution. Baking the affected carbonate for a few hours at a high temperature will drive off the absorbed hydrogen and purify the salt. A saturated solution of Sodium Carbonate has a pH value of 11.60.

The Potassium Carbonate is very hygroscopic and absorbs water from the atmosphere at a rapid rate. It should be kept in a well corked bottle. It is slightly more active than the sodium salt. In the case of substitution there may be a change in the activity of the developer even though the pH value is adjusted. The Potassium Carbonate is more expensive than the Sodium Salt. In some instances the Potassium and Sodium bicarbonate are used as buffer salt in conjunction with the carbonates. They assist in keeping the pH value constant throughout the working life of the developers.

SPECIAL RESTRAINERS.

Sometimes, special additions are made to developers to offset the fogging tendencies of some other chemicals in the developer solution. These are usually Potassium Iodide or the inorganic ant-foggants. The Potassium Iodide is used in very small quantities usually about 1 in 40,000. The best way to add this is in the form of 0.1% solution. It keeps very well. Potassium Iodide is also used as an addition to fresh developers to `season` them so that they act as used developers.

The anti-fogging chemicals normally used are 6-nitrobenzimidazole nitrate or Benzotriazole. These are used in very small quantities. Both these chemicals have the property of acting purely as fog restrainers and have no other effect on the film. They can be kept in the form of a 0.2% solution.

Of the details given for the First (B&W) and, Colour Developers, the present CRC E6 Formulae doesn`t use Metol, Hydroquinone and Phenidone, but Potassium Hydroquinone Mono, and the Colour Developer, CD3.

CD3.

This is the salt used in most Kodak processes. It is marketed under the names C.D. 3 by Kodak or MYDOCHROME (May & Baker); Merk call it Colour Developer No.3. The correct chemical term for it is: 4-amino -N-ethyl-N-(beta-3- methane sulphonamido-ethyl)-m-tolu idinesesquisulphate.

It is quite acidic. and is a slow acting agent and normally requires the addition of `accelerators` such as ethylenediamine or its salts. As it is acidic in nature care must be taken to see that the pH value is correct.