> I suppose that an hydrometer would be useful to verify the concentration
> purchased if there is any doubt, but once the starting concentration is
> known, the math is simple and no hydrometer should be required.
>
> I have figured it with algebra before, but the math was not so simple for
someone out of school for 44 years.  I am usually diluting drums of 85
percent to 65 percent.  A simpler non algebraic way to calculate might be as
follows:

a litre of 90 percent contributes .9 litre of pure formic
two litres of 50 percent requires 1.0 litres of pure formic.
so it requires the addition of .1 litre more of 90 percent and .9 litre
water
So the ratio of 90 percent to water to use for making 50 percent is
1.1 to 9

(There is a slight approximation here, as the .1 litre is actually only .09
pure, but as Allen noted, environmental factors far outweigh this.  However,
1.1 to 9 is significantly different than 1 to 1, and this method works for
any concentrations). No doubt someone will now post the proper algebra and
render this exercise obsolete.

But I like the hydrometer for one use.  I have a drum that got the valve
broken by accident and went for months poorly sealed (fortuneately it was
standing so no leakage of liquid, only fumes).  I could use it for diluting
stronger acid without knowing the concentration that is left in the drum.

Stan

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