U of S | Mailing List Archive | alt-photo-process-l | Re: bichromate gum

Re: bichromate gum

This is why I just LOVE you on the list, Loris. Because it is "thought" that sodium was much faster due to its solubility, but in step wedges I found it to be more like pot di in speed. So your science confirms that.
----- Original Message ----- From: "Loris Medici" <mail@loris.medici.name>
To: <alt-photo-process-l@usask.ca>
Sent: Friday, July 11, 2008 8:53 AM
Subject: Re: bichromate gum

As a side note:

One important factor which affects the speed / contrast of the coating
solution is the molarity of the sensitizer solution.

sodium dichromate's (NaDi) molar mass is 298.02g,
potassium dichromate's (KDi) molar mass is 294.19g,
ammonium dichromate's (NH4Di) molar mass is 252.07g.

Same molarity = same amount of dichromate ions, therefore if you
standardized before to say ... 10% KDi, you can use 10% NaDi as a
substitute at the same amount / with the same exposure time, to get
virtually similar results. (Since the difference between their molar mass
can be simply disregarded -> being only around 1.3%!)

Since NH4Di's molar mass is considerably less compared to KDi, you should
mix a lower strength NH4Di sensitizer (in order to balance the amount of
dichromate ions between the two):

source KDi Strength * (molar mass NH4Di / molar mass KDi) = target NH4Di

10% * (252.07 / 294.19) ~= 8.5%

So, 8.5% NH4Di should act almost exactly the same as 10% KDi (in terms of
speed and contrast).


11 Temmuz 2008, Cuma, 4:19 pm tarihinde, Christina Z. Anderson yazmış:
I never buy reagent grade chemicals and do just fine.  I also have used
three dichromates and they're all fine except sodium is sooo saturatable
that you can mix it over 100% and get a honey like consistency.