Phenidone is Ilford's trademark for l-Phenyl-3-Pyrazolidone which was first prepared in 1890. It was not until 1940 that Dr. J. D. Kendall, in the laboratories of Ilford Limited, discovered the photographic reducing properties of this chemical. However, large scale manufacturing of the compound did not become feasible until 1951.

One of the most novel properties of Phenidone is the efficiency effected because it is re-activated by Hydroquinone. In other words, as the Phenidone is oxidized the Hydroquinone reacts with those oxidized products to reform useable Phenidone.

Phenidone is more expensive than Metol but because the Phenidone is used in much smaller amounts than Metol, the cost is actually less.

Like most organic developing agents, Phenidone's developing properties are dependent on the pH of the solution. Developer activity increases with an increase in pH.

When Phenidone is used alone in, for example, a carbonate-sulfite solution, it gives very fast but extremely soft working developers. A comparison of a developer of such a type with one containing Metol, instead of Phenidone, shows that a silver image can be developed more rapidly with the Phenidone developer, but only a very low density is obtained with the latter no matter HOW LONG DEVELOPMENT IS CONTINUED. This is due apparently to rapid inhibition of the process of development by the primary oxidation products of the developer.

With the addition of Hydroquinone to a Phenidone developer, which also contains Carbonate and Sodium Sulfite, the Phenidone not only acts the same as Metol, that is, the combination of the two are super-additive, but the Phenidone possesses the property of being regenerated by the Hydroquinone. This results in a developing solution that is not only active from the start, but one that retains its activity longer.

Phenidone is slightly soluble in cold water, but as the temperature of the solution increases Phenidone becomes more soluble, It is readily soluble in both acid and alkaline solutions.

PHENIDONE HAS A NUMBER OF ADVANTAGES:

1. Phenidone is used in amounts which is only a fraction of that of the Metol.
2. Pharmacological tests on Phenidone have indicated that it has a very low oral toxicity, and that with normal users it is unlikely to cause dermatitis. A number of years ago Dignan Photographic paid for a series of toxicity tests on this chemical. These tests passed the FDA requirements so that we could safely state that Phenidone is non-toxic. This does not mean that no one could be allergic to it. It does mean that to date, we have not heard of cases where Dermatitis has been caused by this chemical. We would certainly appreciate receiving any information to the contrary.
3. Small quantities of Phenidone enables active developers to be produced at pH's which are lower than those necessary with comparable Metol- Hydroquinone developers (D-76, etc. ) This results in better keeping properties in dish and tank use.
4. At a given pH it is possible to achieve a higher degree of CONTRAST than with a HQ developer at the same pH. This is important in the preparation of high contrast X-ray developers.
5. The final oxidation product of a Phenidone -Hydroquinone developer is COLORLESS, which reduces, but does not eliminate, the staining of fingers and clothes. This is due in part to the Hydroquinone.
6. Phenidone is NOT as sensitive to BROMIDE and to changes in bromide concentration during development as buffered Metol- Hydroquinone developers. (One of the byproducts of development is BROMIDE--which acts as a retainer. The lack of Bromide sensitivity means that the working solution using Phenidone will REMAIN CONSTANT OVER A LONGER WORKING PERIOD OF THE SOLUTION).
7. Ilford claims "That a Phenidone -Hydroquinone developer is MORE EFFECTIVE THAN ANY OTHER AVAILABLE DEVELOPER FOR FORCED DEVELOPMENT WHERE LAST-DITCH EXPOSURES WITH MINIMAL HIGHLIGHT BLOCKING IS DESIRED."
8. Phenidone gives no fog with LOW-SPEED emulsion papers. However, with high speed emulsions it may produce fog. This can be eliminated by addition to the solution of an anti-fogging chemical, such as BENZOTRIAZOLE High-speed negative material tends to fog, so that a formula compound with Phenidone and Benzotriazole is advantageous. Metol and Elon are trade names for the same chemical.
9. BENZOTRLAZOLE added to a paper developer compound with Phenidone, not only prevents fog, but also has the effect of modifying the TONE OF THE SILVER IMAGE so that COLD BLUE-BLACK TONES (which are generally considered desirable) are produced.

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