School me on the different dichromates

[mark]

I aware of the “this s#%t will kill you” warnings which I am very aware of, so no need to get into, but I just noticed that there is potassium, sodium, and ammonium.

what are the differences?

 

[koraks]

For practical purposes, molar mass and solubility.

 

[mark]

For practical purposes, molar mass and solubility.

For photographic purposes? I see potassium and ammonium dichromate in instructions for sensitizing but I don't see a difference between the two being explained. I have never seen sodium dichromate listed anywhere but Bostick and Sullivan carry it. More of a curiosity question than anything else.

 

[Martin Rickards]

Other than at high pH, where presumably the ammonium salt will release ammonia, not much difference as far as photography is concerned. The ammonium salt is likely to need more "delicate" handling as well.

 

[Andrew O'Neill]

I use both potassium and ammonium dichromates. The latter is more sensitive to exposure.

 

[mark]

I use both potassium and ammonium dichromates. The latter is more sensitive to exposure.

Does this effect contrast or exposure time in carbon printing?

 

[Andrew O'Neill]

Does this effect contrast or exposure time in carbon printing?

As a carbon printer myself, I use AD as it provides faster exposure times... I've never used PD for carbon... only for gum.

 

[mark]

Thanks Andrew.

 

[Vaughn]

Chemically, the dichromates all work the same. One difference will be the amount of dichromate per gram. The weight of the Ammonium atom being less than Potassium's, Ammonium dichromate (AD) will actually have more dichromate per gram than PD, thus AD is more 'active' by weight. This difference can be corrected for easily, but I do not have the conversion factor handy. Standardizing on one will keep things simple.

The other property that differs in solubility -- AD will form a higher concentrated solution than PD.

 

[Hexavalent]

Sodium dichromate is deliquescent (absorbs water from the air) and is therefore not convenient for lab use

 

[Andrew O'Neill]

Chemically, the dichromates all work the same. One difference will be the amount of dichromate per gram. The weight of the Ammonium atom being less than Potassium's, Ammonium dichromate (AD) will actually have more dichromate per gram than PD, thus AD is more 'active' by weight. This difference can be corrected for easily, but I do not have the conversion factor handy. Standardizing on one will keep things simple.

The other property that differs in solubility -- AD will form a higher concentrated solution than PD.

I think the conversion factor is 1.2...

 

[TheMissingLink]

294 g/mol vs. 252 g/mol. Makes a difference of 15%

 

[J 3]

The big change as mentioned is really solubility. Since dichromate concentration is one of the main ways to control contrast in a carbon transfer, or a gum print this can be significant. A must stronger solution of dichromate can be made in alcohol or water with Ammonium dichromate. I've not run into needing Ammonium with carbon but your millage may vary. My understanding is people doing gum prints are more likely to need this because they are often hitting closer to the solubility limits.

It should also be mentioned that dichromates have been banned in the EU and because of that many have switched to a hard to find diazo called CAS ( Diazidostilbene . It's full name is longer). It's expensive ( around a dollar a gram. You mix it to a 2.5% solution as best I've read ) but does not have the same health concerns. Carbon printers note it being lower contrast than dichromates.