Chemical Purity Grades: Acceptable Levels of Insolubles

[Rafal Lukawiecki]

I've read some 500+ posts here about purity grades for some photographic chemicals, mainly borax and sodium sulphite. I followed heated yet informative debates on the subject, from 2007. I'd like to get a better grasp of when a technical grade of borax, sodium sulphite, and sodium carbonate is acceptable to us. My main target is Ansco 130, my standard paper developer, sodium sulphite also into HCA, for paper and film. Ideally, all would be pure enough for film developers, though I'm happy with Xtol.

From what I read, I gather a low level of heavy metals is important, as Pb < 0.002%, and Fe < 0.005%. Freedom from water insoluble impurities, which could get embedded in the emulsion, comes next. However, I could not find any statements that explain what makes an acceptable level of insolubles. Can that be expressed numerically, or is it a matter of testing/risking? My apologies, if I have missed a post.

There are no suppliers of photo grade chemicals in Ireland, I have to ship everything. I really like Photographers Formulary, but air shipping to Europe of the bulkier chemicals is very expensive. Silverprint in UK don't ship outside of UK. Some European photo suppliers offer technical, or are unclear about their grades. I am resigned to buying higher lab grades, from Sigma Aldrich, Acros, Lennox, or Fischer Scientific of items Formulary will not ship to Europe, like hydroquinone. The bulkier ones, like Na2SO3, are expensive from lab suppliers, $60/kg+.

I am not sure what else to do, but to try and understand the purity levels and grades, make the best choice I can, and just try and see what happens.

For example, I am looking at a spec of light sodium carbonate (made by Tata), which lists Fe < 0.003%, Pb < 0.0001% (same for As, Cd, Cr, Co, Cu, Mn, Ni, Ti, V), assay 99%, bicarbonate < 0.7%, chloride < 0.3%, and Na2SO4 < 0.04%. Insolubles in Water at 20˚C < 0.02% (or 200 mg/kg if you prefer). For completeness, it also mentions loss on heating < 0.5%, and pouring density of 480 min to 600 max kg/m3.

Or, I am looking at Borax, made in Turkey I think, with a less complete spec: Fe < 0.0003%, does not list Pb, SO4 < 0.007%, insolubles in water < 0.008%, B2O3 36.99%, Na2O 16.40% and pH 9.38. Strangely, it shows purity as 101.31%, which doesn't make sense to me. It also shows a sieve analysis of 0.63% at +1000mm and 3.28% at -0.068mm.

Or, looking at a food-grade sodium sulphite anhydrous certificate, from NaturalSpices.nl, it all looks good, except it does not list insolubles at all: > 98% Na2SO3, > 49.8% SO2, thiosulphate < 0.1%, heavy metals as Pb < 0.001%, Pb < 0.0001%, Fe < 0.001%, Hg < 0.00001%, As, Cd, Cr, Cu, Ni, Se, Zn < 0.0001%, Sb < 0.0002%, chloride < 0.003%, Na2S2O3 < 0.02%, Na2SO4 is the "remainder". The supplier states, however, that if any anticaking agents where added, they would have to be listed on the certificate. Hmm.

I'd be grateful if you could point me in the direction of a better understanding of these grades. Any other suggestions would be most welcome.

 

[Photo Engineer]

The size and amount of insoluble materials in a chemical is determined by the use in photography. If you are processing 35mm film, an insoluble particle of any size is more important than with 4x5 or 8x10 film and is virtually unimportant if you are only printing. But then, you can also filter a developer or fixer.

So, insoluble materials are judged on use as long as they do not "dilute" the effectiveness of the active ingredient by being present in huge quantities. Usually, insoluble materials are considered safe if filterable and if below about 1% of the total mass of the ingredient.

Worry instead about soluble impurities. Tin, Lead, Zinc, Iron, Mercury and other heavy metals are nasty to you and to most developers and fixers.

PE

 

[Rafal Lukawiecki]

Ron, thank you, very much, for your speedy reply—I have always found your posts very helpful. I hope I am not asking for too much, but would you consider the levels of Tin, Lead, Zinc, Iron and Hg, in the three examples in my post, excessive?

Incidentally, I use 4x5 most of the time, and 6x6 for the rest. My water is very soft, and I pre-filter it, as it has naturally occurring peat (Ireland).

Many thanks and regards from Europe.

 

[Photo Engineer]

Rafal;

I just do not remember the quantities of those impurities. Suffice to say that most Photo Grade chemicals meet or exceed the requirements for heavy metals. Many food grade chemicals do as well (at least for heavy metals), but they do not meet specs for insoluble materials as one example. Many food grade chemicals are just fine, but contain silicates to prevent caking and other ingredients. Table salt is just fine for heavy metals, but may contain iodides which are nasty in some developer and fix formulas.

So, it is difficult to answer. I order nothing but Photo Grade or better.

PE

 

[Gerald C Koch]

I would like to stress that photo grade promises only one thing. That there are no impurities that will have any adverse effect on photographic materials. This grade may actually be less pure that the other grades.

As far as insolubles are concerened any photo solution should be filtered before use.

The two main metals that can cause problems are iron and copper which may hasten oxidation particularly with ascorbate developers. For this reason a chelating agent should always be used with ascorbate developers. Not all chelating agents are suitable for this purpose. High carbonate or sulfite developers should contain a chelating agent such as sodium hexametaphosphate for calcium.

 

[hrst]

Also note that for a chemical you only use a very tiny amount in a formula, for example, sodium or potassium bromide, a rather high percentage of contaminants can be OK compared to a chemical used at higher amounts (sulfite, borax, carbonate...) The difference of the amounts used in formula can be a whopping 100x so what is important is the contaminant concentration in the final solution. This is why water is also important, it is used in the highest amount.

It seems to be common wisdom that soluble iron in tap water of some areas is the cause of XTOL sudden death syndrome, and this happens even though Kodak has surely formulated best possible chelating agents they can think of. But the amount of iron in tap water can be surprisingly high as it does not cause problems to us like lead or mercury would do.

 

[Photo Engineer]

Please note that tin (stannous ion) is a fogging agent and should not be present in developers. This is found in galvanized materials.

PE

 

[desertrat]

If you want to save money by using a tech grade chemical, you can mix a concentrated stock solution of that chemical in water and let it sit for several days. The insolubles will settle to the bottom of the container, although some may float. The stock solution can be pipetted or siphoned out of the middle of the storage container to another container just before dilution and use. This won't work for developing agents or other ingredients that can react with oxygen in the water. But it works fine with chemicals like borax or sodium carbonate.

 

[PhotoJim]

It seems to me - and mind you, I am not a chemist - that experimentation might be in order. This presupposes that the quality of these ingredients will be consistent. If that assumption is valid, trying some of these photos on non-essential work (e.g. test rolls) would give you a lot of information.

One other option might be to have Silverprint ship you some photo-grade chemicals to Northern Ireland, and going there to get them or having a contact there forward them to you. Alternatively, as someone suggested to a Scottish poster with a similar problem a few weeks ago (he didn't want to pay the shipping cost for small quantities of chemicals) a trip to London is a pretty fun way to spend a weekend. You can buy some pretty large quantities of most of these chemicals, so that such hassles will be very rarely needed.

 

[Gerald C Koch]

Here in the US when I am presented with a chemical of unknown purity and one marked USP (United States Pharmacopoeia) I will pick USP grade. In Great Britain you have the British Pharmacapoeia but I don't know if chemicals for human use are marked the same way as here. For example, Arm & Hammer adverises that their Baking Soda is USP grade. The USP also lists the allowed limits for certain imputities. In general anything for human use is probably fine for photographic use.

 

[Photo Engineer]

Jerrry;

I would omit table salt with KI in it and a number of other chemicals from this. It is virtually on a case by case basis.

PE

 

[Gerald C Koch]

Jerrry;

I would omit table salt with KI in it and a number of other chemicals from this. It is virtually on a case by case basis.

PE

I agree, hence my caveat of "in general." Perhaps I should have said for "many chemicals." For sodium chloride I would recommend using pickling salt. I personally do not think of iodized salt as a pure chemical since the label clearly states that there is an additive which is chemically active for emulsions.