Alan Johnson
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- Nov 16, 2004
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I received from Rudeofus (thanks) a Kodak paper, W E Lee & DW Miller "The oxidation of pyrazolidone developing agents", Photo Sci Eng 10 no 4 191-201 1966.
I summarize it here, some things are new to APUG, [remarks due to me in brackets].
For phenidone they conclude that oxidation first produces a free radical (semiquinone) which is relatively stable but [in absence of regenerants] eventually converts to 1-phenyl-3-hydroxy-pyrazole [this is the ionic form of pyrolazone given for this reaction in Mason, Photographic Processing Chemistry 1975, no surprise there.]They show phenidone does nearly all the development of silver and the semiquinone very little.
For metol they did not detect a free radical but found something that disappeared in presence of sulfite and was not stable at high pH=12.Tentatively they suggest this is N-methyl-quinonimine.The formation of the final oxidation product can be prevented by the presence of sulfite in the solution. [Though metol was discovered in 1891 its oxidation product is still something of a mystery].They suggest other work shows metol can be regenerated from its semiquinone or from the completely oxidized form.[ This agrees with Mason's book].
The reaction of phenidone derived semiquinone with ascorbic acid appeared to be a simple reduction reaction which regenerated phenidone from the semiquinone.[Often this is assumed, these workers measured it] The rate increased with increasing pH 7 ->10.
[They barely discuss the mechanism of regeneration of phenidone or metol by hydroquinone,there is explanation in Basic Photo Science by Walls & Attridge p 196. Instead they were interested in the sulfite effect]
With hydroquinone the benzoquinone reacts with phenidone to form the phenidone quinone.Therefore it is necessary to have sulfite in developers containing hydroquinone and phenidone to remove this benzoquinone.
At the time of the paper other pyrazolidone agents were under investigation, they show that superadditivity with hydroquinone increases with the stability of the first oxidation product (free radical ,semiquinone).
[ I believe modern theory has the phenidone and metol adsorbed on the surface of the silver grains and the reactions take place there, in this paper they took place in flowing liquid.]
Thanks for comment.
I summarize it here, some things are new to APUG, [remarks due to me in brackets].
For phenidone they conclude that oxidation first produces a free radical (semiquinone) which is relatively stable but [in absence of regenerants] eventually converts to 1-phenyl-3-hydroxy-pyrazole [this is the ionic form of pyrolazone given for this reaction in Mason, Photographic Processing Chemistry 1975, no surprise there.]They show phenidone does nearly all the development of silver and the semiquinone very little.
For metol they did not detect a free radical but found something that disappeared in presence of sulfite and was not stable at high pH=12.Tentatively they suggest this is N-methyl-quinonimine.The formation of the final oxidation product can be prevented by the presence of sulfite in the solution. [Though metol was discovered in 1891 its oxidation product is still something of a mystery].They suggest other work shows metol can be regenerated from its semiquinone or from the completely oxidized form.[ This agrees with Mason's book].
The reaction of phenidone derived semiquinone with ascorbic acid appeared to be a simple reduction reaction which regenerated phenidone from the semiquinone.[Often this is assumed, these workers measured it] The rate increased with increasing pH 7 ->10.
[They barely discuss the mechanism of regeneration of phenidone or metol by hydroquinone,there is explanation in Basic Photo Science by Walls & Attridge p 196. Instead they were interested in the sulfite effect]
With hydroquinone the benzoquinone reacts with phenidone to form the phenidone quinone.Therefore it is necessary to have sulfite in developers containing hydroquinone and phenidone to remove this benzoquinone.
At the time of the paper other pyrazolidone agents were under investigation, they show that superadditivity with hydroquinone increases with the stability of the first oxidation product (free radical ,semiquinone).
[ I believe modern theory has the phenidone and metol adsorbed on the surface of the silver grains and the reactions take place there, in this paper they took place in flowing liquid.]
Thanks for comment.