U of S | Mailing List Archive | alt-photo-process-l | Re: Stock Paint:Gum Solutions?

Re: Stock Paint:Gum Solutions?


We totally agree on the subject of concentration... But I wasn't referring to concentrations, but to production recipes/stock solutions... You see that a lot in alt.photo, but also in just regular chemistry. For example: Some textbooks that mention preservatives for gumsolutions, give thymol as an example of such a preservative. Most often, it is referred to as a 100% solution, meaning that you add 100 gram thymol to 100 ml isopropylalcohol (acetone). Sometimes, not even a production recipe is given how to make a 100% solution. When you try to make sense of this, from a concentration point of view, you immediately run into confusion. Since a 100% thymol concentration can only be considered to be pure thymol. But since thymol is a solid, referring to a 100% thymol solution, automatically indicates that either you make a solution of 100 grams of thymol in 100 grams of acetone, or 100 grams of thymol in 100 ml of acetone.
From a concentration point of view, this can be confusing. Since the density of acetone is 0,8 g/ml, you could be precise:

the concentration of a 100% thymol-solution would be
- 55,6 % w/w thymol in acetone, or
- 50% w/v thymol in acetone.

When you refer to a solution of thymol in acetone with a concentration of 55,6% w/w, it is the same as referring to a 100% thymol in acetone solution. The first referral is based on concentration, the second referral is based on way of production. Since you only refer to the wiki-page of concentration, of course there will be no indication of how to interpret the 100%-solution 'recipe'...

Look at most books on how-to-make your own solutions. Almost always, you read things like 'make a 1-5% solution of oxalic acid (add 1 to 5 grams of oxalic acid to 100 ml of distilled water). They almost always refer to stock-solutions, and not of concentration of chemical solutions.

But is is a good point Kees started the discussion on the recipe people use, or the concentration of gum solutions. These discussions can cause some clarifications between 'hardcore' chemists, and the people who are not really 'into' chemistry. It seems that this discussion has helped clarify the importance of being very clear in what you mean when giving directions for stock-solutions (and then I won't even get started on the difference of using tubes of 15 ml of paint vs the weight of the different colours ;-), or other directions for the use of certain chemicals....


2008/10/24 Alberto Novo <alt.list@albertonovo.it>
a broad definition of concentrarion is rhe amount of one substance in the body that is containing such a substance. With the exception of molality (not molaRity), the body is composed by the solute plus the solvent.
So 30 g of gum in 100 g of water cannot be defined a 30% solution. To be more clear, we would refer to as a ratio like 30:100, or 30+100.
BTW, Wikipedia has a very long page about concentration.
When *making* a 30% solution, you weigh 30 grams, and dissolve this into 100
grams of solvent (water, in the case of gum).
The final solution, however, is not a solution with a concentration of 30%
w/w gum. You indeed are are correct that my 30% solution in fact has a 23% *
concentration* of gum in water. So, the concentration of gum in water (w/w)
is 23%. The solution itself, though, remains a 30% gum solution (and indeed,
this does not reflect the *concentration* but the '*recipe'* for making the
just to make things easier for the non-chemistry people around here ;-)
2008/10/24 Alberto Novo <alt.list@albertonovo.it>
This discussion about baume, concentration etc. periodically returns on the
list. I myself have had some difficulties the first time I started printing
gum, because very often the concentration is expressed in a fake percentage,
like 30% for a mix made from 30 g of gum plus 100 mL of water. For me, as a
chemist, this was very puzzling because this is not 30% but 30/(30+100)= 23%
weight/weight. Also very confusing is parts/parts, because the units of such
parts are not declared. Is it weight/weight, or volume/volume? In the first
case, 1+3 leads to 25% w/w, and 1+2 leads to 33% w/w.
Dirk-Jan: your 30% gum, which is for me a 23% w/w, is very close to Kees'
1+3 (if both parts are weights, it is my 25%) and the respective Baume
degrees are quite consistent.