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  1. #1

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    I'm not a chemist - help Mr. Wizard.

    Please help me with my dumb question. When mixing chemicals and you are supposed to mix a certain percent solution, how much powder chemicals do you mix with how much water to get the percentage you want. In other words, to get a 1% solution, how much powder by weight do you mix per liter (or per gallon) of water to get that?

  2. #2

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    1 gram of anything in 100 ml of liquid makes a 1% solution. As you guessed, you'd use 5 grams in 100 ml to makes a 5% solution.

    -Dana

  3. #3

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    Thanks Dana - I knew that there was a real simple answer to this.

  4. #4
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    The real chemists have a minor fit about this because it's not figuring out the solution based on mole but for our purposes in photography it's good enough and it's a standard (kind of) across photography chemistry.
    ~Heather
    oooh shiny!
    http://www.stargazy.org/

  5. #5
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    Ok so if you want to mix 50 ml of a 27% solution of ferric oxalate and you just add 13.5 grams of FO to 50 ml of H2O(+ 1 gram or so of edta) you end up with more than 50 ml of solution. So would you add 13.5 grams to say 35ml of H2O and then add H2O to bring it to 50ml? The reason I ask is I've done it both ways without any adverse effects that I could notice. Also there is the matter of measuring water at 68 degrees and then remeasuring it after it has been heated to 180 degrees. Doesn't water expand when heated and also expand when frozen? I know this is splitting hairs for what we do but I am no chemist also. These are just a few things I notice when mixing solutions.
    Last edited by RobertP; 01-21-2009 at 04:23 AM. Click to view previous post history.

  6. #6

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    Actually, I'm a "real chemist" and we use percentage solutions all the time when exact stoichiometry (getting a certain known number of molecules) is not necessary.

    The best way to make 50ml of a 27% FO solution would be to add the solid first to about 35ml, dissolve it, then make it up to the 50ml. The density changes of water within the temperature range needed can probably be ignored -- they're no more than a few percent.

  7. #7
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    I'll have to go with what Jordan says. All of the others are just approximations, and only that. They work at very low concentrations of 1% or less on average. They become increasingly in error as percentage increases.

    A weight of solid placed into a liquid is known as wt/vol. If you weighed the water as Jordan alludes to, then the solution would have percentage expressed as wt/wt or weight for weight. This corrects for density.

    PE

  8. #8
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    Indeed, per the original definition of the mL, the density of water is 1 g/mL.
    "Only dead fish follow the stream"

    [APUG Portfolio] [APUG Blog] [Website]

  9. #9
    RobertP's Avatar
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    Thanks for clearing that up guys. So the bottom line is when all the materials are combined you actually end up using a little less than 50ml of water to make 50 ml of a 27% FO solution. As I said I have done it both ways, so I assume that the 27% mixed either way is a solution with enough saturation that regardless of the extra vol. of water there is suffient FO to reduce the platinum/palladium metal salts in Pt/Pd print. I usually mix it as you have said, by mixing in 35ml then topping it to 50 ml.The time I mixed it the other way I made a mistake and started with 50 ml of water but it didn't seem to effect the print.

  10. #10
    Akki14's Avatar
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    Maybe I'm missing something here but when something goes into solution, you don't get "more" or "less" water/solution. As long as it's dissolved fully it should still be 50ml. In both cases you should be ending up with 50ml... right? The two stage dissolving process is merely to help you dissolve the chemical then when you top up with water, you are sort of washing down the sides of the beaker/graduated cylinder/container. Basically, so you don't slop stuff out, yes?
    ~Heather
    oooh shiny!
    http://www.stargazy.org/

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