Quote Originally Posted by cschnurr
. . . it be easier just to put in
2 drops and multiply by 2.5 (5/#drops)
I don't know the mechanism by which this test works, but I do know that is does NOT behave the way you are suggesting. I experimented with the chlorine indicator in my test kit (hth OTO hydrochloric acid <10% Orthoiodine <1% apparently made by Arch Chemicals, Inc., Norwalk, CT.)

With a test sample containing a chlorine concentration of about 18 ppm, the sample was actually DARKER with 1 drop of the OTO solution than it was with the required 5 drops. That surprised me, so I tried it again and got the same results.

Then I tried it with a test sample containing a chlorine concentration of about 1 ppm. The sample color remained essentially the same regardless of the number of drops of the OTO solution (1 through 5 drops tested.)

On the other hand, the method that I suggested is definitely valid, at least in theory, although it is subject to loss of accuracy even when you carefully follow a procedure to minimize errors. The reason for it's validity is simple: by combining your test sample with distilled water you are DILUTING your sample - in other words reducing the concentration of chlorine. The amount of that reduction can be precisely calculated if you know the relative quantity of the distilled water to the quantity of the test sample.

Like I said, it's just a way to get a ballpark idea of high chlorine concentrations while waiting for a better test kit to arrive!