Mixing Chemistry, Using Percentage Solutions

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Mahombi
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Joined: Fri Jul 14, 2006 3:37 am

Mixing Chemistry, Using Percentage Solutions

Post by Mahombi » Fri Jul 14, 2006 5:01 am

I have started to mix my own chemistry. Some recipes require the addition of ingredients in percentage solutions. The Darkroom Cookbook describes how this works by saying that "a 10% solution has 10 grams of a chemical dissolved in 100ml of water". It continues, "that means that every 10ml of a 10% solution contains 1 gram of a chemical".

Now, I am a bit dumb at maths, but that doesn't sound right to me. If you have 100ml of water and you add 10 grams of something to it, surely you end up with more than 100ml of that solution. Therefore, there can't be 1 gram within a 10ml amount. Or can there?

If you are still with me on this, my problem is that my recipe, (for an alkaline fixer, by the way!) calls for 800ml of Ammonium Thiosulphate 57-60%. Are there any maths experts or chemists out there who can help?


Keith Tapscott.
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Re: Mixing Chemistry, Using Percentage Solutions

Post by Keith Tapscott. » Fri Jul 14, 2006 5:48 am

Mahombi wrote:I have started to mix my own chemistry. Some recipes require the addition of ingredients in percentage solutions. The Darkroom Cookbook describes how this works by saying that "a 10% solution has 10 grams of a chemical dissolved in 100ml of water". It continues, "that means that every 10ml of a 10% solution contains 1 gram of a chemical".

Now, I am a bit dumb at maths, but that doesn't sound right to me. If you have 100ml of water and you add 10 grams of something to it, surely you end up with more than 100ml of that solution. Therefore, there can't be 1 gram within a 10ml amount. Or can there?

If you are still with me on this, my problem is that my recipe, (for an alkaline fixer, by the way!) calls for 800ml of Ammonium Thiosulphate 57-60%. Are there any maths experts or chemists out there who can help?
You mix the component in about 75% of the water required and then when it is completely dissolved, add more water to make up the final volume.
If you require 100ml of a 10% solution, dissolve in 75-80ml of water first, then top up with water to make 100ml.

Mahombi
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Joined: Fri Jul 14, 2006 3:37 am

Post by Mahombi » Sun Jul 16, 2006 5:23 am

Thanks for your response Keith. Your description seems to go along with my idea. The way it's described in the Darkroom Cookbook doesn't sound right. But in saying that, I did read somewhere the other day, that some things when dissolved in water, don't add to the overall volume. They apparently, on a molecular level, fill in the gaps left by the water molecules. So I guess that's what was meant in the text. I didn't think mixing chemistry was going to be quite as complex as this!

Lowell Huff
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Post by Lowell Huff » Mon Jul 24, 2006 4:23 pm

"Its not that I believe scientists to be always correct; but non-scientists to be wrong."

Isaac Azinoff

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