Crude pH Estimates
By Matt Grimm
Originally Published: CSTN – March 2004
Have you ever been stuck running samples without the use of a reliable pH
meter? Probes can easily be in or out of calibration when you need to run a
pH to make sure a system in treated properly. There are also times when you
stick the pH probe in a sample and scratch you head. Luckily for us, the field
test kits that are supplied to Water Management Specialists are stocked with
a variety of different methods to obtain a close pH estimate.
For example, I was given a sample and asked to run several tests. I stuck the
pH probe in the sample and watched it drop all the way to 3.8. There I was
scratching my head thinking, “This pH shouldn’t be this low, should it?” So,
I put 50 ml in the casserole dish and grabbed the total alkalinity indicator.
Drop. Drop. Drop. “Oh, no! It is red. It is my second week on the job and I
am doing something wrong with the first sample somebody asked me to run.
What is wrong?” I ran it again, this time rinsing the dish out several times
and still it went red. At this point, I figured I had to be doing something
wrong so, naturally, I ask questions. To my relief, the pH was that low and
the lack of total alkalinity was my proof. Since I learned about this pH
double check, I looked into other ways to verify pH if you don’t trust or can’t
use your pH meter and here they are:
Phenolphthalein Indicator
This versatile reagent can also let you know if your sample is above or below
a pH of 8.3. When Phenolphthalein is added to water with a pH greater that
8.3, it turns pink. When it is added to a sample with a pH below 8.3, it
remains clear.
Total Alkalinity Indicator
Its not just for alkalinity testing, it also a rough pH indicator. Adding Total
Alkalinity Indicator to a sample with a pH below 4.3 will result in a
“salmon” color. When added to a sample with a pH greater than 4.3, it turns
green.
Bromothymol Blue Indicator
A pH slide method. In this test, mix a few drops of Bromothymol Blue
Indicator in a small sample cell and compare it to the standards in the slide.
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The sample will appear close to the colors of the slides if the pH is between
6.0 and 7.6. To ensure accuracy of this test, make sure to use the whitest and
brightest background light available and that the Bromothymol Blue
Indicator is not too old.
Meta Cresol Purple Indicator
Another pH slide method. In this test, mix a few drops of Meta Cresol Purple
Indicator in a small sample cell and compare it to the standards in the slide.
The sample will appear close to the colors of the slides if the pH is between
7.6 and 9.2. To ensure accuracy of this test, make sure to use the whitest and
brightest background light available and that the Meta Cresol Purple
Indicator is not too old.
pH Paper
All you have to do is dip a pH paper in your sample and read it against a
color chart. This may seem like an easy method at first, however, deciphering
the proper colors may prove a bit frustrating. The colors in certain ranges can
seem very similar and difficult to tell apart.
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