Info from Clorox on using their bleach for pools

[chem geek]

Aren't these home-brewed tests of chlorine strength hopelessly inaccurate? Aren't the errors inherent in diluting a sample to this level simply huge?

We tell people to dilute water samples by 3 or 4 to 1 so they can test higher chlorine levels with an OTO test but then warn them that the results are just ballpark. Now people are diluting chlorine samples by 20,000 to 1 (or so) and talking about tenths of a percent precision? I dunno...

You are right that unless one uses a calibrated pipette, the errors will be quite large. However, having a large dilution ratio does not contribute to the error. An error in a ratio, which is what a 20,000 to 1 dilution is measuring, is only equal to the percentage or proportionate error in the numerator and denominator. If the "1" unit were one drop and this was only accurate to within half a drop, then this is a 50% error and is huge, but if the "1" unit were 1 ml and were measured in a calibrated pipette, then the error could be as low as 1%. The "20,000" large volume can be fairly easily measured to within 1%, again assuming one has an accurate large volume measuring tool (for liters or quarts, for example).

I'm not sure how fcfrey is doing the measurements, but you bring up a good point about accuracy. The easiest way to determine the rough accuracy of a measurement is to try and repeat the measurement and see if you get the same result, especially if you go on the low/high side for the small/large volume measurement and then repeat going on the high/low side.

Richard