The water in one of my systems has a high ph, but unfortunately when I test it with high ph solution, the resulting color does not match any of the color swatches on my test kit. I am GUESSING by trying to interpolate between colors that the ph is about 8.3, but that's just a guess and probably not very accurate.

So I decided to mix equal parts of the unknown ph water with some rainwater that shows a ph of 6.0, thinking I could test the resulting mix, find the ph, take the difference in ph from the 6.0 water and the mixture, then add that difference back to the ph of the mixture to find the original ph. In other words, I was assuming the mixture will be the arithmetic average of the ph of the two solutions. But then it occurred to me that ph is on a log scale, not an arithmetic scale, so the arithmetic average method may not be correct.

So here's my question: what is the ph of an unknown solution which when mixed in equal parts with a ph 6.0 solution, results in a mixed solution with 8.0 ph? (I figure Urbanfarmer will know how to do this). And if anybody has the formula for this calculation, I would like to see it. (I can plug it into an excel spreadsheet to simplify this problem in the future.)

Thanks to anybody who can help!