‘NA2’ is the shorthand name for a contrast control agent (restrainer) that’s often used with palladium prints. Its proper name is sodium hexachloroplatinate (Na2PtCl6). As you can see from its chemical formula, it contains platinum. This leads to a reasonable and fairly common question:
“How much platinum does ‘NA2’ leave in a palladium print?”
My short answer is: somewhere between almost none and none whatsoever. read on for the long answer…
Understanding why there’s little or no platinum in the print requires answers to three additional questions:
- How much ‘NA2’ should be used when printing?
- How much platinum does this amount of ‘NA2’ add to the sensitiser?
- How much of the platinum in the sensitiser actually makes it through to the finished print?
How much ‘NA2’ should be used when printing?
A small quantity of ‘NA2’ has quite quite a large effect on a print’s contrast.
If you have good quality negatives then there is no reason why you should need much (or any) ‘NA2’ in your sensitiser. A single drop of 5% ‘NA2’ may be sufficient to tune the contrast of an 8×10 print.
‘NA2’ should never be used as a standard additive in your sensitiser. This is an excellent way to flush your money straight down the drain, but is completely unnecessary for printing. If you find that you consistently need to add ‘NA2’ then most likely your negatives are too thin or your working space has too much light: fixing these problems will save you a tremendous amount of money (and improve your prints).
If you really, really want to use a restrainer as standard, then either:
- Add a little dichromate to your developer
- Add a little potassium chlorate to your ferric oxalate
Both are considerably cheaper than ‘NA2’ and will have the same effect in small quantities.
How much platinum does ‘NA2’ add to the sensitiser?
For the calculations that follow I have assumed 1 drop of ‘NA2’ is used in 1ml of palladium solution, which is reasonable for an 8×10 print. You may use slightly more or less than this, but, as you’ll see later, that doesn’t really make much difference to the quantity of platinum in the finished print.
The standard palladium solution, sodium chloropalladate (Na2PdCl4), should have a concentration of 0.5M (approximately 15% wbv). Actual concentrations vary somewhat but this is a good starting point. 1ml of 0.5M sodium chloropalladate solution contains 500μ︎mol of palladium (1×10-3 litres x 0.5 moles/litre = 500×10-6 moles).
A 5% solution of sodium hexachloroplatinate has a concentration of 0.09M. Assuming that one drop has a volume of 0.05ml, then 1 drop of 0.09M ‘NA2’ contains 4.5μ︎mol of platinum (0.05×10-3 litres x 0.09 moles/litre = 4.5×10-6 moles).
Therefore, a sensitiser made from 1ml of palladium solution with 1 drop of 5% ‘NA2’ has a palladium to platinum molar ratio of 500:4.5, or about 100:1. In other words, platinum constitutes a little bit less than 1% of the total platinum and palladium molecules in the sensitiser.
How much of the platinum in the sensitiser actually makes it through to the finished print?
With a normal platinum solution, for example potassium tetrachloplatinate (K2PtCl4), the developer reduces the platinum salt and thereby deposits particles of pure platinum to form the image. At first glance, it’s reasonable to suppose that the developer would reduce the ‘NA2’ in the same way, and that this would leave some platinum in the image. However ‘NA2’ is not a normal platinum sensitiser. It has two more chlorine atoms than a normal platinum sensitiser (i.e. PtCl6 versus PtCl4), and chlorine inhibits (slows down) the developer’s reduction reaction.
Developing a platinum/palladium print is a chemical race against time. While the developer is reducing the platinum/palladium salt to form the image, it is simultaneously dissolving the sensitiser and washing it out of the paper. If the first action takes too long then the platinum is washed out of the paper before the reduction reaction is completed.
And that’s exactly what happens with ‘NA2’: the extra chlorine inhibits the reduction reaction, and much of the partially reduced platinum salt is washed away before an image can form. (This is most likely why early attempts to print with hexachloroplatinate salts failed to produce an acceptable image.)
This washing out of the partially reduced ‘NA2’ is what causes its contrast enhancing effect, so it’s actually a good thing. It’s only bad news if you were hoping for it to leave behind significant amounts of platinum in your print.
It is likely that there is some platinum in the image, but not much; and undoubtedly very much less than if a normal platinum solution had been used. I’m not aware of any experimental data about this, but my guess is that the finished print will have a palladium:platinum ratio closer to 1000:1 than the 100:1 that was in the sensitiser.
In conclusion, when using ‘NA2’ as a restrainer, platinum plays no meaningful part in the finished print.