Diluting phosphoric acid

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hotwatermusic

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This subject has been covered before but I am still unclear on a few points. I bought 950mL of 85% phos acid and am planning on diluting it.
1. Is the only reason for dilution, safety? At what strength does phosphoric become reasonably safe?
2. Is there any reason not to use deionized water?
3. Is it better to dilute by weight or volume, assuming the accuracy of measurement were equal.
 

ajdelange

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1. Is the only reason for dilution, safety? At what strength does phosphoric become reasonably safe?
There are other reasons as well. As for safety it seems pretty clear as LHBSs sell 10% but not 85% that the former is safer than the latter.
2. Is there any reason not to use deionized water?
There are reasons for not using anything except DI water.
3. Is it better to dilute by weight or volume, assuming the accuracy of measurement were equal.
That depends entirely on how you intend to use it. If you are going to weigh out the diluted acid when using it then clearly you want to know mEq/gram. If you are going to use a cylider or syringe then you'll want to know mEq/mL. As a 10% solution is pretty dilute (density 1.05 g/cc) they are going to be pretty close.
 
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hotwatermusic

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Thanks. I have only seen mention of "water", in other threads but assumed D.I. water would be better at not introducing extraneous ions. I was planning on diluting to a concentration of roughly 21.25% but might as well cut by one more half to be closer to that home brewer strength.
 
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