PH over 15 - what reduced ph?

[waterbear]

adding more or less indicator (or diliuting) will not change the accuracy of the reading, only the intensity of the color obtained. Phenol red is the indicator used for pH in pool tests. It has distinct colors at different pH:
yellow 6.8 and below
yellow orange 7.0
light orange 7.2
orange 7.4
red 7.6
red violet (sort of pinkish) 7.8
violet 8.0
purple 8.2 and above.

If you learn these colors your con't even need the color comparator to test pH! The color for a specific pH is always going to be the same no matter who manufactures the test, only the intensity of the color might differ.

Depending on who made your color comparator and indicator solution the colors obatained might be very intense (such as seen in the better kits from LaMotte and Taylor) or very pale pastels that are often seen in the cheap kits with more dilute indicators. Also, the number of drops added will affect how deep or pastel the colors are. Some kits tell you to add only 1 or 2 drops, others have you add 5. You are looking at the color, not how deep or pastel it is! 7.4 will be orange! It might be a deep orange or a very pale orange but it will be orange. Ditto for the other colors and pH.

Hope this explains it!

[4KidsSwim]

Thanks all! Yes, I was doing the test like I would for high chlorine, assuming same type of results. That's why I stop here first!!! Can't believe I forgot about the extreme range of Ph - good thing I homeschool my kids and we just covered that a few months ago:-) Failed that retention test!

My kit is Ben's kit from last season. I'm going to order something online as soon as I figure out what, but I live over an hour from the nearest shopping and nowhere near a professional pool store. So I just used what I had.

Muriatic acid - duh! I should have remembered that when I was pulling my really high alk down a bit early last year.

[chem geek]

Carl was more on the right track in how diluting with "neutral" water affects pH. If there were no pH buffers in the water, then diluting by adding an equal amount of water at pH 7 would roughly cut in half whatever was in excess in the water you were measuring -- in this case an excess of hydroxyl ions since the pH was above 8. pH is a logarithmic scale of the concentration of hydrogen ion and that concentration is in inverse proportion to hydroxyl ion. So in this case with a pH of, say, 8:

pH = 8 = -log10([H+]) = -log10(10^(-14)/[OH-]) = 14 + log10([OH-])

so [OH-] = 10^(-6) so cutting this in half would be 5x10^(-7) which would result in a pH of

14 + log10(5x10^(-7)) = 7.70

In general, diluting in half with water that is a pH of 7 will result in moving the pH towards 7 by about 0.3 units (-log10(0.5) = 0.3).

As for what happens when there is a pH buffer, the dilution cuts all the chemical species in half (assuming the diluting water has nothing in it) so the carbonate buffer resists the change in pH so the pH change might be even less.

The bottom line is that you can't use dilution for the pH test.

Richard

[CarlD]

I figured Richard could give a better and more detailed analysis.

My ability to follow log-based mathematics is sadly, so ancient as to be gone... But the basic concept of dilution without a pH buffer versus with a pH buffer is perfectly logical.

WB's alternate path is equally useful.