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DCG Theory

Dark Reaction

Can you elaborate on dark reaction?

Jeff Blyth responds:

A good question ! ----- It needs a chemist to do it justice because it is complicated. Although it straightforwardly means dichromate reacts slowly with gelatin at room temperatures without light being involved, it does need some explaining. If you are not interested in the finer chemistry detail then stop reading here .

Most oxidizing agents such as oxygen in the air, nitrates (saltpeter as in gunpowder etc) , and chlorates can be mixed with combustible materials and just sit there inactive virtually forever unless something such as a lighted match gives them that vital spark which triggers the chain reaction and rapid burn up. So these oxidants need what’s called “activation energy” as a kick start. However in the case of dichromates the chromium is a member of what are known in the Periodic Table as “transition metals” . These are inclined to have the peculiar ability to indulge in low activity with little activation energy which is why they are used universally as catalysts for low temperature reactions. They have variable valency which is why we have referred in this forum to CrVI going to Cr V going to CrIII The transition metal iron in our blood is doing this sort of low temperature oxidation work for us of course too. The transition metal effect is to do with atomic orbitals where the electrons have a large array of complicated empty orbitals to whizz about in some of the time and to get through barriers without having to be kicked to jump over the normal activation energy barrier that non- transition metal ions have to do. A non- transition metal such as say Aluminum which is always Al III in our water based alum chemistry here and cannot be reduced or oxidized to a different valency but it does make complexes with the gelatin and hardens it (but not quite as strongly as CrIII). So back to dichromate which has 6 electrons missing from its uncombined metallic state. The electrons have been taken mainly by 3 oxygen atoms in a not very strong arrangement and these electrons are actively whizzing around the Cr atom’s empty orbitals as well as their main base around the oxygen atoms. Energy is gained for the system if 3 of them can return permanently to the Cr atom by being instrumental in getting the oxygens to swap them for other electrons in neighboring organic groups in the gelatin to give more stable arrangements producing partially oxidized gelatin. So the dark reaction of dichromate is primarily a matter of oxidation of the gelatin without a kick start with light energy or extra heat and it can be slowed down in a ‘fridge but needs to be in a freezer to really slow it down. Incidentally the less pure the dichromate the more it contains other transition metals such as copper and the more it enables this catalysed oxidizing effect to occur in the dark. More acidity also increases it which is why the more acidic ammonium salt in unexposed gelatin film gives it a shorter shelf life than the potassium salt.

Gelatin and Anti-Crystallization Properties

Let’s remember that yet one more of the great features of gelatin is its ability to hold quite concentrated solutions of salts within itself as a form of gelled solid solution. This is a great feature for us holographers because without it some of our valuable techniques would be spoiled by the normal crystallization processes which would transform glass clear film into the equivalent of frosted window glass.

This special anti-crystallization feature can be undermined if we allow concentrated salt solutions to crystallize on the surface of the gelatin because it can then encourage crystal seeding to occur within the gelatin.

So if one is making a “G307” system where the coated gelatin is dipped into a bath of say 6% potassium dichromate one needs to gently wipe off the excess droplets of dichromate salt off the surface before drying . (This system needs high dichromate salt concentrations for exposure to 532nm but NOT if one is exposing with blue wavelengths.)

This also applies to the diffusion system for making silver halide gelatin film, where careful removal of silver nitrate solution in surface droplets is needed before drying and immersing in the bromide bath. Another (but less common ) way that anti -crystallization property can me reduced is by excessive drying by overheating so that the inherently bound in water found in normal gelatin film is driven off or the salt-laden gelatin is stored under very low humidity.


Converting Dichromate to Chromate

I hope I can just clarify something about potassium dichromate versus chromate. You can readily convert dichromates into chromates by adding the right amount of alkali to a stirred solution of say 5% potassium dichromate until it gets to a pH of about 8 to 9. The chemistry really is straightforward enough. I will just run through it for future reference.

Potassium chromate is the potassium salt of Chromic Acid : H2CrO4 where the 2 acid H's are substituted for 2 K's

Now to see how potassium Dichromate (K2Cr2O7) is made up, please just note down the total formula from adding one potassium chromate (K2CrO4) to one chromic acid (H2CrO4). You then get a total of K2H2Cr2O8 . now just take away 1 H2O and you get K2Cr2O7. So dichromates are all just 1:1 combinations of chromate salts with chromic acid. There is of course no need to use potassium hydroxide to do the conversion, Na OH will work just fine.

I have just calculated that 100 ml of 5% K2Cr2O7 will require 1.36 g solid sodium hydroxide to convert it all to the chromate form. (The colour of the solution changes from the intense orange yellow to a light canary yellow when the conversion is complete.) Some of you will have already spotted that 5% K2Cr2O7 solution has got more chromium compound in it than the intended 5% K2 Cr O4 solution.

To make it equivalent you need to cut down the volume of solution used by a third. But I don’t think this is a critical issue .