Purity of raw chemicals

[AgX]

In the most popular german-language book on photochemistry (written in the 70’s by Junge and Hübner, both from Orwo) it is stated:

“Very cautious persons demand that photo-chemicals be of the `for analyzing´ quality [the highest german commercial grade]. This however is over-the-top, in some cases even wrong. Newer reports have shown that some developing agents of highest purity turned out in a photographic system to be useless because a tiny trace of photographically aktive impurity was missing. The otherwise common designations for chemical purity (raw-technical-pure-for analyzing) apply for photographic baths only to a limited extend.”

I always wondered what those necessary impurities were. But I never found out.

 

[Photo Engineer]

AgX;

Can you give a reference to the quoted paragraph? I have heard of this, but not with developing agents.

PE

 

[AgX]

There is no quotation in that book, just that passage I translated.

 

[Lowell Huff]

While this issue about impurities may have a modicum of truth, the reason that we manufacturers buy the "purest" or highest quality raw materials is to provide the HIGHEST QUALITY product for our customers' money. The result of quality is always evident; the lack of it as well.

 

[Ian Grant]

Günter Hübner was an Industrial Chemist at Agfa, VEB Filmfabrik, Wolfen, for quite a number of years and there is/was a set of International Standards for the purity of Photographic chemicals established well before the 70's, many are listed in LFA Mason, Photographic Processing Chemistry.

It is possible for a chemical to be too pure for a particular developer, but it's quite unusual. One example was a developer that required Sodium Carbonate (Crystalline) - which contains some Bicarbonate and the Anhydrous form sold in Europe is too pure.

Agfa, Wolfen, were manufacturing developing agents at that time so Hübner would have been fully aware of the issues of Developing agent purity. I have 2 books he co-authored by Hübner but unfortunately can't read German, but can translate Formulae. (I have both English & German Editions of Windisch, who kindly lists all the chemicals).

Ian

 

[Kirk Keyes]

Newer reports have shown that some developing agents of highest purity turned out in a photographic system to be useless because a tiny trace of photographically aktive impurity was missing.

I'd have to say that something really can't be too pure, other than for economic reasons (Patrick, where are you!?). It really seems like what is going on with that statement it that if there is an active ingredient that is a minor constituent/impurity of another chemical and it removed when purifying that particular chemical, then you've simply misidentified the active ingredient.

 

[nworth]

The American equivalent to what Hubner is talking about is an analytical reagent. While these are almost always safe to use in photography, he is right that it is overkill. There are two critical things for a chemical that is to be used in a developer: that it does not contain a contaminant that will do photographic harm, and that it is pure enough so that the amount you weigh out contains enough of the actual chemical to do the job properly. In most cases, practical grades of chemicals (the purity for general laboratory and synthetic uses) is good enough, and sometimes technical grades (the next lower purity, used for some commercial synthesis and treatments) will work OK. The purity issue can be funny though, because some contaminants that do harm photographically are OK for just about anything else.

 

[sanking]

I'd have to say that something really can't be too pure, other than for economic reasons (Patrick, where are you!?). It really seems like what is going on with that statement it that if there is an active ingredient that is a minor constituent/impurity of another chemical and it removed when purifying that particular chemical, then you've simply misidentified the active ingredient.

Kirk,

Patrick is still in the hospital, and still pretty weak.

Sandy

 

[Alexander Ghaffari]

I'd have to say that something really can't be too pure, other than for economic reasons [...]

Oh yes, anyone who has used double distilled water to clean anything made of stainless steel can attest that things can be too pure.

 

[Photo Engineer]

Well, after thinkiing it over, I agree with Kirk. Going back to my first post in this thread, I remember cases of something like that happening and we had mis-identified the real active ingredient. I forget the case offhand, but if I think of it, I'll post it.

PE

 

[AgX]

On request of PE here is my source:

"Fotografische Chemie - Aus Theorie und Praxis"
by
Dr. Karl-Wilhelm Junge
Ing.chem. Günther Hübner

3.ed, 1979
p.113

VEB Fotokinoverlag Leibzig

 

[Kirk Keyes]

Patrick is still in the hospital, and still pretty weak.

Thanks for the update.

I only invoked his name as I know he enjoys this subject from an engineering point of view!

 

[BetterSense]

My provision of the salt, which had never been renewed since the date of the first experiment, began to run low. I sent out for a fresh supply, and mixed the draught; the ebullition followed, and the first change of colour, not the second; I drank it and it was without efficiency. You will learn from Poole how I have had London ransacked; it was in vain; and I am now persuaded that my first supply was impure, and that it was that unknown impurity which lent efficacy to the draught.

From The Strange Case of Dr. Jeckyll and Mr. Hyde

 

[Ray Rogers]

In Medicine, there are also many cases where associated isomers, precursors, derrivitives etc. may infact share some of the desired target effects, or are able to manipulate them in certain ways; an inappropriate rough example might be salicyclic acid and acetylsalicylic acid: they both have antipyretic (antifever) activity. It is not absolutely necessary that the "impurity" be the main active ingredient, but it could be.

My uninformed attempt to make a phosphorescent substance failed due to materials that were "too pure", Now, seen from the other side of the window, failure was due to the absence of a necessary impurity or "active ingredient". The problem, here, is that the "active ingredient" does nothing by it self, nor do the other materals... it is only when one is present in at a certain extreamly low level of concentration, that the combination comes alive and begins to phosphoresce. So "impurity" can be different from "active ingredient"
I think they are called "active impurities" because:
1. They are not acting alone.
2. They are "active" in quantities that one might expect to remain after a certain amount of "purification" has already taken place.

 

[Murray Kelly]

Kirk,

Patrick is still in the hospital, and still pretty weak.

Sandy

My God! What happened - can you tell us here? PM if not, please.
Murray

 

[Ray Rogers]

Kirk,

Patrick is still in the hospital, and still pretty weak.

Sandy

If you are in more or less direct contact, and its not inappropriate, please let him know he is in our thoughts.... and we wish him a strong recovery.

Ray

 

[Photo Engineer]

There is a long thread updating us on the situation that was started by Sandy King a few weeks ago. You may wish to visit that thread. There is more information there.

PE

 

[Murray Kelly]

There is a long thread updating us on the situation that was started by Sandy King a few weeks ago. You may wish to visit that thread. There is more information there.

PE

Found it! Thanx.
Murray

 

[Martin Reed]

.....“Very cautious persons demand that photo-chemicals be of the `for analyzing´ quality [the highest german commercial grade]. This however is over-the-top, .....

A problem is that increasingly the only available option in some chemicals is 'analytical grade', with consequently significantly higher prices. As an example, ammonium dichromate - we've been chasing around many suppliers finding that we're always buying their 'last available' stock of general purpose grade, soon there will be no alternative to 'Analar' on many chemicals.

 

[Kirk Keyes]

" Analar" - that's an interesting term.

I the USA, the abbreviation "AR" (Analyzed Reagent) is more commonly used for chemicals that have been analyzed and the results of the analysis (in regards to the specifications for that grade) are supplied with the chemical, typically printed on the label of the bottle.

The next grade down is called Reagent or ACS Reagent (for American Chemical Society) and is typically the same chemical as AR, but without the supplied analysis report.

Both of these are typically much purer than Technical or lesser grades. But then it all depends on the specs of the grade as to what is checked and what is not...

 

[tkamiya]

I am neither a chemist nor a scientist, but if a given grade of a chemical works and a pure version of the same chemical doesn't, doesn't that mean there is an unidentified necessary component contained in the first chemical? Said differently, a chemical sold as "A" may contain small amount of "B" and "C" but as impurity to the chemical "A". For the purpose given, both "A" and "B" is required. If you get a pure version of "A", "B" would be missing.

In that case, it's not the purity of the chemical that is in question. It is necessary but unidentified component missing that is causing a problem.

 

[Photo Engineer]

In going over all of my years of experience, I have found virtually no cases of an "impurity" causing a given reaction in any sort of chemical reaction. In fact, the opposite is true. Impurities in most all cases tend to inhibit the desired reactions.

Cases in point include halide salts where NaCl contains some Iodide, and therefore the photographic is changed. Varying Iodide content from batch to batch varies the effectiveness of NaCl. The same holds true for Bromide salts, Borate salts and Carbonate and Sulfite salts. A trace of Sulfate in Sulfite changes the effect of Sulfite as a preservative and silver halide solvent.

This overlaps into conventional organic chemistry where traces of water in Ether or on Magnesium will totally inhibit the Grignard reaction.

The list goes on, but in very few (vanishingly few) cases does an impurity improve a reaction and when it does, then it is isolated and used as the real reagent in a purified form.

PE

 

[Murray Kelly]

Isn't 'AnalaR' a brand name?

As for contaminants that actually assist the process, I think the K2CO3 cryst. that Crawley insists on for his FX-2 because it has some bicarb in it as a helpful contaminant is the photo talk-fest starter best known to me.
Murray

 

[Ian Grant]

Another chemical where purity is sometimes important is Sodium Metabisulphite which is sold as the less pure Bisulphite form in the US (a mixture of Bisulphite/Metabisulphite). Henn and Crabtree published a paper on the substitutions in Kodak formulae. The major differences are to the pH of a developer solution.

Ian

 

[Aurum]

Isn't 'AnalaR' a brand name?

As for contaminants that actually assist the process, I think the K2CO3 cryst. that Crawley insists on for his FX-2 because it has some bicarb in it as a helpful contaminant is the photo talk-fest starter best known to me.
Murray

Analar is a brand name, originally registered by BDH, a venerable chemical supply house in the UK. Stands for "Analytical Reagent" and denotes typically 99+% pure, rather than GPR which is the normal industrial 95% grade. BDH is now part of VWR international