Very informative experiment. I do not think that effect is due to acidity. I would rather put out a hypothesis that the effect is due to the vitamin C (ascorbic acid) present in lemon juice. Vitamin C is a strong reducing agent that would react quickly with dichromate reducing it to Cr (III) and making gum insoluble. Kind of like a dark reaction in gum, no light needed. It appears as a stain, but it really is not. It is really a chemical fog.
ANyways to test acidity one would use solutions of pure acids.
> Date: Sat, 6 Sep 2008 15:49:18 -0600
> From: firstname.lastname@example.org
> Subject: Re: gum preservatves
> To: email@example.com
> Here it is:
> Scroll down to the very bottom past the color charts.
> I'm not saying it proves anything, but acidity may account for some problems
> that people say are issues with gum, and it is only really meaningful in
> relation to the water control strips done at the same time with the same
> amounts of dilution. Otherwise a lot more tests would have to be done to be
> conclusive. If, as Ryuji and Demachy said, there is a reduction to chromic
> acid with the addition of lemon juice, which I don't know because I am not a
> chemist, I don't know if that is speedier or less speedy than dichromate.
a Z. Anderson
> ----- Original Message -----
> From: "Loris Medici" <firstname.lastname@example.org>
> To: <email@example.com>
> Sent: Saturday, September 06, 2008 3:13 PM
> Subject: Re: gum preservatves
> Thank you!
> Now that you say that, I remember the section about it in the "Learn" part
> of your former website. (Adding lemon juice and staining... I'm not making
> it up right?)
> Judy's note on sizes was interesting and real food for thought, BTW.
> Ryuji's notes also were interesting -> I mean the probability of citric
> acid (or any other organic acids) interacting with dichromate in an
> unwanted manner. That powered my original position which was increasing
> the acidity by not adding alien compounds... (Still don't know if that can
> work or not -&
gt; I may do some tests in the future if the exposure times
> become unbearable to me and/or I can't do nice casein prints...)
> 5 Eylül 2008, Cuma, 1:12 am tarihinde, firstname.lastname@example.org yazmış:
> > ...
> > Have at it, Loris. I found that the more lemon juice drops
> > I added to the mix (with drops of water added to the control
> > group in the same proportion) that I got lots of staining of
> > the highlights and lower contrast, but with paper negs this
> > might be helpful to you--I mean, the lower contrast part.
> > Step wedge steps were not too differentiated.
> > Chris
> > ----- Original Message Follows -----
> > From: Loris Medici <email@example.com>
> > To: firstname.lastname@example.org
> > Subject: Re: gum preservatves
> > Date: Thu,
04 Sep 2008 21:37:54 +0300 (EEST)
> >>David & Chris, that also arrived to my mind just after had
> >>sent my last message... Even if I refrain to introduce
> >>another alien compound into consideration - as a first
> >>impression -, it sounds interesting / promising. You can
> >>bet I will try this (with citric acid)as soon as possible!
> >>Of course there's also the staining issue... I'll see.
> >>Chris, do you know how much lemon juice was Demachy adding
> >>to his gum?
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