Posted: 9/27/2014 3:50:59 PM EDT
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I haven't had to think about this type of math for over thirty years, now I can't remember how to do this stuff.
I need to make 15 mL of a .09 percent saline solution from a 20 percent stock solution. How much stock solution do I need? If there is a formula for this (I'm sure there is, there's a formula for everything), that would be quite helpful. TIA |
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Quoted:
I haven't had to think about this type of math for over thirty years, now I can't remember how to do this stuff. I need to make 15 mL of a .09 percent saline solution from a 20 percent stock solution. How much stock solution do I need? If there is a formula for this (I'm sure there is, there's a formula for everything), that would be quite helpful. TIA [[ .09% is just under 1/1000th, fyi ... .09 = 9% ... .0009 = .09% ]] .09% of 15ml = .0009 * 15ml = .0135ml ... so that's how much salt you want .0135ml / 20% = .0675 ml ... that's how much of the 20% solution contains .0135 salt. [[ Reverse check: .0675ml * .2 = .0135ml ]] 15ml - .0675ml = 14.9325ml ... that's how much water you add to the .0675ml of your 20% solution. Add salt to taste. EDITED ... I had slipped a decimal, ugh. |
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Another way to look at it ...
You have 20% and you want .09%. So you need to dilute by .09/20 = .0045 = just under a half a percent, or just under 1 in 200 ... to be more precise, 1 in 222 with a little rounding. So you need 1 part of the 20% solution and 221 parts of water. 1/222nd of 15 ml = .0045*15ml = .0675ml of 20% solution 221/222nds of 15ml = .9955*15ml = 14.9375ml of the good pure water, not cheap tap water, I mean the pricey bottled kind or at least filtered. And then the part about add to taste. |
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I may be way off, but I am getting 6.75ml of 20% + 8.25ml of water to make 15ml of 9%. 15ml * .09 = 1.35ml of saline is what you want 6.75ml * 20% = 1.35ml of saline 15ml - 6.57ml = 8.25ml of water needed. I'm no chemist or mathematician but this is what I get. You are basically cutting the amount of salt in half plus a bit more water. |
