Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 68,674   Posts: 1,481,903   Online: 1094
      
Page 3 of 8 FirstFirst 12345678 LastLast
Results 21 to 30 of 75
  1. #21
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    I got the following ideas after reading pertinent passages of "The Theory of the Photographic Process", third edition, Mees and James, editors.

    In the pH range where ascorbic acid is not actively a developer, it behaves much like a sulfite. It has been shown (p. 367 of the reference) to be not significantly different from sulfite in affecting the developing activity of metol at pH 8.7 when the molar concentrations of the sulfite and ascorbic acid are the same. The mechanisms by which they work are different, the sulfite by forming the sulfonate of metol and the ascorbic acid by reducing oxidized metol to its original state. It would seem, then, that either sulfite or ascorbate ought to serve the same theoretical purpose in a superadditive mixture of, say, metol and hydroquinone.

    Another property shared by sulfite and ascorbate is the abiliy to reduce the latency period of hydroquinone. It would seem that by most theories of the superadditivity phenomenon, either sulfite or ascorbate should be able to do the job. The required amount of either is quite small compared to the amount of sulfite commonly used. Thus, the combination of metol, ascorbate and hydroquinone should not be considered as a 3-agent developer when the minimum amount of ascorbate to achieve superadditivity is used, any more than the combination of metol, sulfite and hydroquinone is. There is usually little need to remove the sulfite, but when formulating a developer for staining, the less sullfite, the more intense the stain.

    When we substitute ascorbate for hydroquinone in a PQ or MQ developer, we can leave out the sulfite and still have superadditivity because part of the ascorbate takes its place in restoring the phenidone or the metol. In this case, the developer does not stain the image.

    Pp 289 and 304 of the reference will also be of interest.
    Gadget Gainer

  2. #22
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    22,707
    Images
    65
    I believe that this is correct Patrick, and Ascorbate also loses its ability to cross oxidize. Therefore it cannot work as an ET developer or in a superadditive manner as pH goes down.

    This is why I wondered what the sulfite did to pH in your previous post. If pH went up, likely the activity of ascorbate would increase (IIRC, you used ascorbate. I am sorry but I didn't look at that post to refresh my memory).

    In any event, ascorbic acid is pH sensitive as you describe.

    PE

  3. #23
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Ah Ha! The developer under study was a PQ-TEA solution to which I added either nothing, sodium sulfite or sodium ascorbate. The activity of PQ + TEA + water was very low, about what one might expect from the phenidone alone at 0.02 g/l. The activity took a leap when I added about 8 g/l of sodium sulfite.
    Starting over with just the PQ-TEA in water, the activity took an even greater leap when I added 4 g/l of ascorbic acid as the ascorbate.
    In other words, it seemed to me that ascorbate and sulfite were performing about the same task in a PQ type of developer.
    This information is not world shaking, but would have consequences for those who are trying to get the most image stain possible out of catechol or who want to put a complete developer in a glycol or TEA solvent. We have seen it work in Pyrocat MC. Perhaps it has some bearing on the theory of synergism insofar as the replenishment of the "main" agent can be done by the sulfite or by ascorbate instead of the "secondary" developer. It is not possible, at least by me, to determine which is which. A clue is the variation of the activity of metol when either ascorbate or sulfite is added to the solution, where it appears that either one has the same effect as replenishing the metol.
    Anyway, it gives an old codger something to keep his brain going.
    Gadget Gainer

  4. #24
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    22,707
    Images
    65
    Patrick;

    It may be that the activity is linked to a pH change though. Do you have a way to check?

    PE

  5. #25
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Perhaps my best bet would be to start over with an MQ-borax solution in water, like the basis of d-76 without sulfite. With enough borax, the buffering ought to be strong enough to minimize pH changes from no additive to either sodium sulfite or sodium ascorbate. In either case, I would be using the same molar concentration as was used in the experiments I referred to before. However, if there is a change in pH, it should favor the sulfite as sodium ascorbate solution itself is not likely to be higher than 7.6 according to Merck.

    I do not have a way to check pH. I will search the local swimming pool supply places for pH indicators that might narrow it down, at least for differential readings. I am 50 miles from the nearest moderate sized cities and 100 miles from the next size up. Otherwise it's internet or mail, and I'm too impatient.
    Gadget Gainer

  6. #26
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    If I can figure out how to do it, I will attach the graph and some text from the The Theory of the Photographic Process.
    Attached Thumbnails Attached Thumbnails MCS.jpg  
    Gadget Gainer

  7. #27
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    22,707
    Images
    65
    Patrick;

    Generally, any reaction product in chemistry will retard any further reaction to some extent. So, the graph is perfectly reasonable. That is what happens with oxidized HQ which turns into HQ-mono sulfonate. The quinone vanishes from the equation and therefore the reaction speeds up.

    The conversion of oxidized ascorbic acid back to ascorbic acid is through an electron transfer that oxidizes the sulfite to sulfate. These two types of reaction are not identical and yet do involve oxidation and reduction reactions.

    PE

  8. #28
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Quote Originally Posted by Photo Engineer View Post
    Patrick;

    Generally, any reaction product in chemistry will retard any further reaction to some extent. So, the graph is perfectly reasonable. That is what happens with oxidized HQ which turns into HQ-mono sulfonate. The quinone vanishes from the equation and therefore the reaction speeds up.


    PE
    I think you misread the graph. Hydroquinone, sulfite and ascorbic acid are treated separately. There is no effect of hydroquinone on the development by metol at any concentration of hydroquinone that was tested, as you can see by the fact that the empty triangle symbols fall on a straight horizontal line originating at the 0 concentration point. The development by metol is accelerated by either ascorbate or sulfite.

    The reaction products of hydroquinone accelerate development by hydroquinone, according to the same reference. Ascorbic acid "tames" hydroquinone.

    I did what I promised. I mixed a solution of borax, metol and hydroquinone. A snip test of HP5+ with this solution yielded a density above base of 1.3 after 2 minutes at 70 F. I added 3.5 grams of sodium sulfite (anh) to this solution and got a snip test density of 3.04. I added the molar equivalent of the sulfite, 4.9 g, of ascorbic acid converted to sodium ascorbate by 2.45 g of NaHCO3 in a small amount of water. I let the effervescence subside before adding it to the other ingredients. The snip test from this solution measured 3.03 density. I then prepared a solution of metol, borax and sodium ascorbate in an amount equivalent to 6 g of the acid, with the same amounts of metol and borax as the original solution but no hydroquinone. The snip test measured 3.02 density.

    I developed a strip of pictorial negatives, all taken from the same roll of the same scene, in each of the three solutions for 8 minutes at 70F. There were slight differences in contrast, about as one might expect from the differences in snip test density. The grain in 10X enlargement is quite good.

    I will give a more complete report when I can get around to scanning the prints from each concoction.

    Cheers, Y'all.
    Gadget Gainer

  9. #29
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    I left out the fact that I started the ascorbic acid test with a fresh MQ-borax solution. I did not have sulfite and ascorbate in the same solution, in other words.
    Gadget Gainer

  10. #30
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    22,707
    Images
    65
    Patrick;

    It explicitly states that the oxidation products retard development, and their removal accelerates development rate. Therefore, they state that sulfite addition accelerates HQ and Metol development. That is what I was referring to as well as the reference to the superadditive effect between Metol and HQ. The utility of the Elon Ascorbic acid developer (Metol=Elon), known at Kodak as EAA is well know as well.

    PE

Page 3 of 8 FirstFirst 12345678 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin