[pure-silver] Re: Chemical subsitution

DEAR RICHARD,

      Well, as we say in Barbados, now I con-FUSED!  In your e-mail below
(was that your info or Doug Nishimura's info?) where it states that Hypo
Clear is mostly sulfite and metabisulfite, it also states that, "Kodak has
added a small amount of sodium citrate and tetra-sodium EDTA to bind-up
unwanted metal ions effectively".  Hmmmmmmmmm, is Doug wrong in his
assessment that there is sodium citrate in hypo clear instead of the citric
acid that you cited from the patent info?  Please advise.

            CHEERS!

                  BOB

 

-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Richard Knoppow
Sent: Friday, April 27, 2007 6:21 AM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: Chemical subsitution

 

 

----- Original Message ----- 

From: "Eric Nelson" <emanmb@xxxxxxxxx>

To: <pure-silver@xxxxxxxxxxxxx>

Sent: Thursday, April 26, 2007 6:25 PM

Subject: [pure-silver] Chemical subsitution

 

 

>I have discovered that I am out of sodium bisulfite

> and need to make some HCA tonight for prints I just

> finished.

> I have tons of sodium metabisulfite and have looked

> through the archives to see if this compound is

> specifically addressed as an AKA or substitute

> compound for sodium bisulfite.

> I've included some quotes below of Richard's that seem

> to answer this question, so I'm wondering if there is

> a definitive answer out there as to whether I can use

> metabisulfite in place of bisulfite gram for gram in

> the HCA formula or at what ratio it would need to be

> at.

> 

> I'm a cook not a chef so molar aspects of these

> compounds and the Avogadro's number will go over my

> head. ;-)

> And what the heck is bisulfite lye?  Found that

> mentioned in one of my search results.

> Eric

 

     I am not sure where I got this originally but I found 

this searching rec.photo.darkroom using Google groups 

search.

Hypo clearing agent is mainly a mixture of sodium sulfite 

and sodium

metabisulfite. The metabisulfite behaves very much like 

sodium bisulfite (as the

Merck index says, "The bisulfite of commerce consists 

chiefly of sodium

metabisulfite, Na2S2O5, and for all practical purposes 

possesses the same

properties as the  true bisulfite.") (When sodium 

metabisulfite is hydrolyzed

(reacts with water) it changes into bisulfite.)

 

 

Anyway, sodium sulfite and bisulfite are related compounds 

with different

characteristic pHs (add acid to sodium sulfite and you 

create the equivalent of

a solution of sodium bisulfite.) They're put together to 

produce a buffered

solution. (A buffer is a chemical that resists changes in pH 

when acids or

alkalines are added. In the case of a sulfite/bisulfite 

buffer, adding acid just

creates more bisulfite while adding alkaline creates more 

sulfite without

appreciably changing the pH.) Sulfites, as you probably know 

from their use in

developers as preservatives (not as silver solvents), tend 

to oxidize fairly

easily to form sulfates. (This is also why they were used on 

lettuce in salad

bars to prevent the lettuce from turning brown due to air 

oxidation.) So left

exposed to air, the hypo clearing agent slowly changes from 

a solution of

sulfites and bisulfites to a solution of sulfates and 

bisulfates.

 

 

I should add a note in regard to tap water versus distilled 

water for making

prepared chemicals. Prepared chemicals are intended to 

behave more or less the

same way whether you mix them with New York City tap water 

or Tulsa water. They

do this by adding pH buffers (of course hypo clearing agent 

is by nature a

buffered solution anyway), ion strength buffers (when this 

is important), and

chelating agents to bind-up unwanted metal ions such as 

calcium, magnesium (both

found in hard water), iron, etc. In the case of Hypo 

Clearing Agent, Kodak has

added a small amount of sodium citrate and tetra-sodium EDTA 

to bind-up unwanted

metal ions effectively removing them from participating in 

the mixed solution.

The original formulas for HCA used sodium hexametaphosphate 

instead (commonly

called "Calgon"), but of course phosphates are a serious 

pollutant causing the

death of bodies of water by promoting algae growth. You'll 

find similar

differences between D-72 and Dektol. D-72 mixed from scratch 

needs to be mixed

with "pure" water (in the research labs it was DI or 

distilled) wheras Dektol

(same basic formulation with additions to compensate for tap 

water mixing.) F-5

fixer and Kodak Fixer are a similar pair.

 

 

-Doug

Douglas Nishimura

Research Scientist

Image Permanence Institute

 

 

 

---

Richard Knoppow

Los Angeles, CA, USA

dickburk@xxxxxxxxxxxxx

 

 

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