Hello,
pH by definition (and as measured) is a negative log of 'apparent
concentration' of protons in the solution.
In a simplified version, 'apparent concentration', aka 'activity' is a
function of the 'activity coefficient' times theoretical concentration.
The activity coefficient is not a constant, it depends on many little
things. The major factor is the ion itself (proton in the case of pH
discussion), but it also depends on the solution's ionic strength,
temperature, organic solvent(s) content, presence and concentration of
non-dissociating compounds etc., etc. In case of pure water, it's just
properties of proton itself, and the temperature (atmospheric pressure
is a factor too, but very minor).
Most importantly, the properties of proton are such that it's activity
coefficient is significantly greater than 1.
For this reason, pure water is NOT neutral, it is slightly acidic
(proton IS an acid), and the pH value of pure water is around 5.6
If everything is perfect, the value that you read from a pH meter should
be around 5.6 if the water is as pure as it's expected to come out of
the milliQ. However, the reading also depends on many little things:
temperature, pressure, air quality, the electrode's age, the level,
quality, composition and the age the reference (filling) solution,
quality and electric connections of your pH meter, quality and age of
your calibration buffers, method of calibration, pH meter stability,
internal firmware and/or software, etc., etc., etc.
And, in addition, water is NOT a buffer, so the reading will fluctuate
even with tiny environmental changes. Just shake the water with the
electrode in it and observe the reading change.
In practice, if your pH reading of pure water is between 5 and 6, the
water is just fine. Assuming the electrode is calibrated correctly,
washed extremely well and was sitting in your water sample for at least
10-15 minutes, you should be a lot more concerned if the measured pH is
above 6. If this is the case, your milliQ needs service very urgently
(the water contains salts). If your reading is below 5, boil the water,
cool down in a tightly closed tube and measure again. If the reading
goes to around 5.6 your water is fine, but you need to worry a little
about the lab's air (high level of CO2 as in cell culture labs, or if
there is a bin with dry ice around, and also if HCl is used around to
adjust pH of buffers). However, if after boiling pH stays below 5, them
milliQ also needs a service.
Hope this helps.
================================
Dmitry Bochkariov, Ph.D.
Principal Scientist
Advansta Inc.
1505 Adams Drive, Suite B1
Menlo Park, CA 94025
(650) 325-1980 x530
================================
On 6/6/2017 7:27 PM, Wolfgang Schechinger wrote:
Dear Sudheer,
pure water has a theoretical pH of 7. What you are measuring, is the
effect of trace amounts of impurities, e.g. dissolved CO2 from the
air. As there is (virtually) no buffering capacity, what you are
measuring is irrelevant for buffer preparation etc.
Regards
Wo
On 2017年06月06日 22:56, Sudheer Sangeetham wrote:
HI
can someone tell me what is the milliQ water pH? In my working
institute I
have two departments, I collected milli Q from both departments and
checked
the pH. I have got pH 4.3 and 5.4, respectively . I donno which one I
should believe.
Thanks
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