Diluted ph test

Ok, this might be a dumb question, but here goes... When doing a diluted ph test (for example when fc is over 10ppm), the instructions are to dilute the pool water with an equal amount of distilled water. So, if the pool is at a ph of 8 for example, and it is diluted with distilled (ph of 7), won't that produce a result of 7.5? Other diluted tests require doubling the reading to account for dilution... Don't we also have to double the amount +\- 7 in order to get an accurate ph (in this example, the results are .5 over 7, so double the .5 and add to 7 resulting in the correct ph of 8)?

The dilution for testing pH is to reduce the chlorine level in the sample. High chlorine (over 10ppm) can cause inaccurate pH results. pH is not like the other tests we run on our pool water. Most tests measure how much of something is in the water. pH is a measure of the net result of the effect of all the acids and bases and buffers in the water and has more to do with the ratios of pH influencing factors than actual amounts. Distilled water has no pH influence so the ratios stay about the same.

Did this make sense?

@Dave: that's not quite right, but I can't explain it well either. I'd ask Chem_Geek, but I don't think anyone would understand his explanation.

I'll try to get my son (bio-chem degree) to write an explanation that we can all understand.

@ Eric: I can't explain it, but I have verified it with a digital (and calibrated) pH meter. I should add, this ONLY works with distilled water; if you use tap water, the result will be closer to what you intuitively expected.

Ok, I'll take your word for it that it works. It was purely a matter of curiosity anyway. I would still like to see an explanation if you can get your hands on one though... 'Cuz now I'm even more curious!

Diluting to change the pH range is tricky and not very effective for two reasons. First is that pH is a logarithmic scale which means that it takes a lot of dilution to move the scale. Every factor of 2 dilution using distilled or deionized water would move the pH of unbuffered water by 0.3 units towards 7.0.

However most water is buffered and dilution barely budges the pH. TA is a measure of some of that pH buffering. The pH in buffered water is largely determined by the ratio of the two chemical species of the buffer and diluting the water simply reduces both concentrations equally so the ratio remains constant and the pH essentially remains unchanged. Now if you were to dilute using buffered water, then you can change this ratio relationship and move the pH, but the calculations to figure out what the heck went on become somewhat complicated.

If you want to move pH to get it into a range you can measure, then you can use acid or base demand drops. You can then calculate what the original pH was if you know the buffering water parameters (mostly TA, but also CYA and borates). To a large extent, it's irrelevant since if the pH is that far out of range you just need to add acid or base to get it into range.

The only quick and dirty indicator for an extreme pH is the TA test where an immediate red means the pH is at or below roughly 4.5. You can also use wide-range pH test strips if you really want to know a very out-of-range pH.

Ok then. I followed most of that (enough to get the gist of it). Thanks for the explanation!