School me on the different dichromates

mark

Member
Joined
Nov 13, 2003
Messages
5,703
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?
 
OP
OP

mark

Member
Joined
Nov 13, 2003
Messages
5,703
koraks said:
For practical purposes, molar mass and solubility.
Click to expand...

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

Member
Joined
Dec 24, 2016
Messages
390
Location
Asturias, Spain
Format
35mm
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.
 
OP
OP

mark

Member
Joined
Nov 13, 2003
Messages
5,703
Andrew O'Neill said:
I use both potassium and ammonium dichromates. The latter is more sensitive to exposure.
Click to expand...

Does this effect contrast or exposure time in carbon printing?
 

Andrew O'Neill

Moderator
Moderator
Joined
Jan 16, 2004
Messages
11,956
Location
Coquitlam,BC Canada
Format
Multi Format
mark said:
Does this effect contrast or exposure time in carbon printing?
Click to expand...

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

mark

Member
Joined
Nov 13, 2003
Messages
5,703
Thanks Andrew.
 

Vaughn

Subscriber
Joined
Dec 13, 2006
Messages
10,079
Location
Humboldt Co.
Format
Large Format
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

Member
Joined
Nov 19, 2009
Messages
592
Location
Ottawa, Onta
Format
Multi Format
Sodium dichromate is deliquescent (absorbs water from the air) and is therefore not convenient for lab use
 

Andrew O'Neill

Moderator
Moderator
Joined
Jan 16, 2004
Messages
11,956
Location
Coquitlam,BC Canada
Format
Multi Format

I think the conversion factor is 1.2...
 

TheMissingLink

Member
Joined
Sep 30, 2004
Messages
178
Location
Hamburg/Germ
Format
Large Format
294 g/mol vs. 252 g/mol. Makes a difference of 15%
 

J 3

Member
Joined
Feb 1, 2016
Messages
147
Format
Medium Format
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.
 
You must log in or register to reply here.
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…