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 <email@example.com>