Measuring pH:
Discussion Points and Handy Hints
Physical-Chemical Water Quality Refresher Day, 4 March 2004
Notes and comments provided by Sara Johnson,
Waterwatch Victoria Science Coordinator
Below are a number of points of discussion from the training day. All
points end with a ‘Helpful Hint’ that are worth considering when
monitoring pH, and/or interpreting monitoring data.
What is pH and why is it an important environmental indicator?
How is pH measured?
What factors affect pH strips? How should I look after them?
What factors affect pH meters? How should I look after them?
What is Automatic Buffer Recognition (ABR)
What is Automatic Temperature Compensation (ATC)?
Should I perform a one-, two- or three- point calibration?
Why do some instruments use pH 6.86 or pH 7, and/or pH 9.18 or 10 for
calibration? Can they be used interchangeably?
How long should I keep my calibration buffer solutions?
How long should I leave my probe immersed in the water sample before I
record the pH value?
Why does the pH of a stream change slightly over the day?
What is alkalinity and how is it linked to pH?
If rain is acidic, why aren’t all surface waters acidic?
What is pH and why is it an important environmental indicator?
See any one of Waterwatch Victoria’s monitoring manuals for further
information.
How is pH measured?
Waterwatch groups commonly monitor pH with two types of instruments:
pH strips and pH meters.
pH Strips
pH strips are paper strips coated with a range of indicators. Indicators are
organic molecules that change colour in an acid or a base. When an indicator
is placed or chemically bonded to paper or plastic strips, it provides a fast way
to determine if a substance has acidic or basic properties.
Most modern pH strips can be left in the solution until the final colour is
completed. Specially developed indicator dyes provide for sharper
differentiation of individual pH values. Colours of individual colour fields
shouldn’t run into each other.
pH Meters
A pH meter is a device that measures the voltage from electrodes and
converts it to a pH reading on a display.
Many modern pH probes are called combination electrodes. That is, they
combine the two components of a pH, a H+ sensing glass bulb and a
reference element, into one electrode. When a pH probe comes in contact
with a sample, a potential difference is created across the sensing membrane
surface of the glass bulb electrode. This membrane potential varies with the
pH. Measurement of pH requires a second, unvarying potential (independent
of H+ ions) to quantitatively compare the changes of the membrane potential.
The reference electrode serves this purpose by oozes a saturated salt
solution (commonly potassium chloride (KCl)) through a porous junction or frit
into the sample solution, completing the electrochemical circuit.
Helpful Hint:
Become familiar with your equipment. If you have a pH meter, know where
your reference junction is located. In pHScan and Hanna combo probes, the
junction is visible and located very close to the glass bulb and looks like a tiny
piece of tape sticking out of the probe. It can look very different and be
located in different positions in other probes.
What factors affect pH strips? How should I look after them?
Extreme heat, direct sunlight and moisture will all shorten the life and use of
your pH strips. Most manufacturers will recommend that you store your strips
in a dry location, at room temperature and do not expose them to direct
sunlight.
What factors affect pH meters? How should I look after them?
There are four components of a pH meter that can affect the instrument’s
performance: the glass bulb electrode, the reference junction, the electronics
within the meter itself and batteries.
Glass bulbs
Potential problems
Because the glass bulb is sensitive, it can break very easily.
Also, the glass bulb can become coated with insoluble and colloidal deposits if
it isn’t cleaned regularly. These contaminants affect the movement of H+ ions
across the glass bulb membrane.
Handy Hints - Maintenance
Conditioning your glass bulb by soaking new and/or sluggish meters in pH 7
solution is important to activate the instrument.
Avoid touching the glass bulb with fingers (oils, creams, etc will contaminate
the glass bulb). If the bulb isn’t functioning because the surface is clogged
with oils, etc, soaking it briefly in a weak acid before rinsing it thoroughly with
warm water may help. Some manufacturers provide different types of
cleaners for protein, inorganic and grease/oil build-up.
Never rub the surface of your glass bulb when drying a glass bulb (eg. after
rinsing with deionised water). Dab the glass bulb dry.
If you can see air bubbles inside the glass probe of your instrument, give the
instrument a number of short flicks to move the internal liquid to the end of the
probe, and the bubble should return to the top part of the probe.
NEVER ALLOW YOUR pH PROBE TO DRY OUT AND NEVER STORE
YOUR PH PROBE IN DISTILLED OR DEIONISED WATER.
By storing a probe in distilled water, you will be encouraging H+ from within
the pH bulb to migrate out of the probe. This results in the probe becoming
unstable and sluggish. Always store your probe in whatever solution is
recommended by the manufacturer. For short periods of time, pH 7 is
commonly recommended. For longer-term storage, most manufacturers
recommend a type of storage solution. These storage solutions commonly
contain a high salt concentration, which prevents dilution of the glass bulb’s
internal fluid.
Reference junction
Potential problems
For a pH meter to work properly, it must be able to ooze the reference
solution (KCl) out of the reference electrode. Problems arise when either the
meter exhausts its storage supply of reference solution, or when the reference
electrode’s junction becomes blocked. Most pH probes on the cheaper end of
the market have a closed reference solution storage cell, which means that
you don’t know when you’re running low on reference solution. The first you’ll
know about it is if your meter becomes sluggish or unresponsive. One
advantage of more expensive pH meters is that you can often ‘top up’ the
reference solution reservoir, and also check to see if the junction is blocked.
If you are monitoring waters with high suspended-solids, oils or greases, your
reference junction has a greater chance of becoming clogged.
Handy Hints - Maintenance
Note that sometimes you may find white looking crystals on your probe if you
haven’t used the probe in a while. This is just some of the reference solution
which has oozed out between use. These crystals won’t harm your instrument
and can be washed off with water.
Know the age of your instrument and whether or not you can top up or replace
the reference solution in the probe. Some pH meters (eg pHScans) have a
fixed internal reservoir of reference solution. These reservoirs can’t be topped
up, but the sensors (including reference solution reservoir) can be replaced. If
you have the ability to replace/top up reference solution, flush out any crystals
that may have formed with more reference solution. Certain meters (eg.
Horiba U-10) have a liquid junction which, when pressed, can demonstrate
whether or not reference solution is oozing freely from the reference
electrode.
Meter electronics
Corrosion inside meters is very common if the meter is not waterproof and is
stored in a damp environment and/or immersed in solution. Note that
splashproof doesn’t mean waterproof. If a splashproof probe is emersed in
solution, expect some water to leak into the meter.
Batteries
Not all instruments will advise you that it is running low on battery power, and
many instruments with low battery power will produce erogenous readings. It
is useful to keep a spare set of batteries handy.
Handy Hint - Maintenance
Some meters give a particular error code when batteries are either running
low or need replacing. Know what this code is for your type of instrument.
What is Automatic Buffer Recognition (ABR)
Automatic Buffer Recognition (ABR) is a function of many meters to recognise
which calibration buffer it is immersed in. For example, when immersed in pH
7 during calibration mode, it registers that it is a pH 7 buffer and calibrates the
instrument accordingly. Some instruments have the ability to recognise
multiple buffer types (eg. Hanna combo probe recognises 5 pH buffers – 4.01,
6.86, 7, 9.18 and/or 10, although only two combinations are accepted. See
notes on buffers below)
What is Automatic Temperature Compensation (ATC)?
pH, like many other water quality parameters, changes with temperature. This
attribute is particularly important when considering calibrating your instrument.
For example, if a solution were pH 10.01 at 25oC, its value would become pH
10.18 at 10oC or pH 9.89 at 40oC. This change is not an error; this is the true
pH value of the solution at the new temperature.
Fortunately, many instruments have an Automatic Temperature
Compensation (ATC) function to make pH calibration easy. I will use an
example to explain.
I want to calibrate my pH meter, which has an ATC function. After calibrating at pH 7, I
immerse the pH probe in a pH 9.18 solution with a temperature of 15oC. Because my meter
also has an Automatic Buffer Recognition function, my meter recognises which pH standard it
is immersed in and calibrates at pH 9.18. I check my calibrated meter against the pH 9.18
buffer solution, and instead of reading pH 9.18, the meter reads pH 9.28.
Why? Does this mean that the instrument didn’t calibrate properly?
No, the instrument above has worked perfectly. The ATC function means that
the meter recognises that the pH at 15oC (pH 9.28) is different to that at 25oC
(pH 9.18), and calibrates it accordingly. The temperature compensation factor
is very close to 0.003 pH/oC/pH unit away from pH7. Very acidic and very
basic buffers and solutions have the greatest variation. There is very little
variation around pH 7 (less than 0.1 pH units).
Not all pH meters have ATC. In this case, you must manually enter the
temperature corrected calibration value during calibration (using the example
above, knowing that the pH 9.18 buffer solution’s temperature is 15oC, you
would enter the pH value 9.28 during calibration, not 9.18). Most calibration
solutions provide a table of pH values of pH standard solutions at various
temperatures on the side of the bottle.
Handy Hints:
ATC in pH METERS ISN”T THE SAME AS ATC in ELECTRICAL
CONDUCTIVITY METERS. ATC for pH doesn’t standardise measurements to
25oC as it does for EC meters. It corrects for the natural change in pH with
temperature. Because of this, you should always report temperature with pH
measurements.
A good web site clarifying Automatic Temperature Compensation in pH
meters can be found at: http://www.eutechinst.com/techtips/tech-tips4.htm
Should I perform a one-, two- or three- point calibration?
Different instruments allow you to calibrate at one-, two- or three- calibration
points. More than one calibration point is recommended for accurate data.
If you rely on just one calibration point (eg. pH 7), you only provide one point
of reference for the instrument to use when programming the relationship
between different voltage outputs from across the glass bulb membrane (mV)
with pH values (known as the Nernst equation). Over the range of 14 pH
units, this leaves you open to error. By providing 2 or 3 calibration points (pH
7 + pH 4 +/or pH 10) over your expected pH range, the relationship between
voltage outputs and pH values strengthens.
Handy Hint:
Calibrate your instrument to reflect instream conditions. The range that you
calibrate over (eg. acidic – neutral (pH 4 –7) or neutral – alkaline (pH 7 – 10)
should capture the range of instream conditions you expect to find. If
monitoring over the full range of pH, consider purchasing a good meter with a
3-point calibration, or use two 2-point meters calibrated over the acidic and
basic range.
Why do some instruments use pH 6.86 or pH 7, and/or pH 9.18 or 10 for
calibration? Can they be used interchangeably?
Two types of buffers exist – primary buffers and secondary buffers. The main
difference is how they’re made, and whether or not the method meets
stringent statistical analysis.
Primary buffers are those made with direct traceability to the components that
make up the buffer. pH 4.01, 6.86 and 9.18 are primary NIST buffers. (NIST is
America’s National Institute of Standards and Technology). pH 4 buffer is
made from 0.05M potassium hydrogen phthalate, pH 6.86 is made from
0.025M KH2PO4 and 0.025 M Na2PO4 (neutral phosphate) and pH 9.18
buffer is made from 0.01 M Borax.
Interestingly, pH 7 and pH 10 buffers are considered secondary buffers,
because there aren’t standard NIST methods for their determination. PH 7 is
commonly made from 0.021 M of KH2PO4 and 0.029M of Na2PO4.
Secondary buffers are commonly verified with a highly accurate meter that
has been calibrated against primary buffers.
This said, choose the buffers that are compatible with other pH meters that
you might already use (that way you can purchase common calibration
solutions). Also, some instruments such as the Hanna Combo have two
groups of pH calibration values, either 4.01, 6.86 and 9.18, or 4.01, 7.01 and
10.01, and automatic buffer recognition based on these groupings. Problems
will arise if you are using and calibrating a meter in pH 7 BUFF mode with a
pH 6.86 calibration buffer.
How long should I keep my calibration buffer solutions?
This question depends on a number of variables. Calibration solutions, when
purchased, should come with a batch number (or production date) and a
used-by-date. Storing containers tightly sealed and in the dark will maximise
the life of your buffers.
Reusing calibration solutions multiple times over an extensive time period
opens up a range of potential contaminations. Higher pH buffers, pH 9.18 and
10, can be unstable. If you are using the same portion of calibration solution
multiple times, I recommend that you have a second container of identical
calibration solution clearly labelled ‘rinse’. By dunking probes in this ‘rinse
solution’, you minimise the transfer of any contaminants into your ‘calibration’
solution.
Handy Hint:
Know the age of all calibration buffers. If distributing small amounts of
calibration solutions to groups, record the date that the new calibration buffer
was distributed, plus the expiry date of that solution. Ensure groups use clean
probes that have been blotted dry. If you want to look at whether or not the
buffer’s pH value has drifted over the distribution period, measure the old
buffer against a new buffer with a well-calibrated instrument. Keeping a record
of calibration solution drift will help to defend the robustness of your data, as
well as assist data interpretation.
How long should I leave my probe immersed in the water sample before
I record the pH value?
All pH meters will have a temperature sensor built into the instrument. You
may or may not be able to see this sensor. Temperature sensors built into
instruments can take a little time to stabilise when immersed into a water
sample.
Handy Hint:
If temperature can be read off your meter, wait until this value stabilises
before recording the pH value.
Why does the pH of a stream change slightly over the day?
The pH of a water body results from the ratio of H+ to OH-. In natural waters
this usually is dependent on the carbonic acid equilibrium. When carbon
dioxide from the air enters freshwater, small amounts of carbonic acid are
formed that then dissociate into hydrogen ions and bicarbonate ions, as
shown below:
Co2 + H2O -> H2CO3
H2CO3 -> HCO3- + H+
This increase in H+ ions makes the water more acidic and lowers the pH. If
CO2 is removed (as in photosynthesis), the reverse takes place and pH rises.
What is alkalinity and how is it linked to pH?
Alkalinity refers to the capability of water to neutralise acid. This is really an
expression of buffering capacity. A buffer is a solution to which an acid can be
added without changing the concentration of available H+ ions (ie without
changing the pH) appreciably. The carbonate system provides acid buffering
through two alkaline compounds: bicarbonate (HC03-) and carbonate (CO3--).
These compounds are usually found with two hardness ions: calcium (Ca++)
and magnesium (Mg++). It essentially absorbs the excess H+ ions and
protects the water body from fluctuations on pH.
So, generally, soft water is much more susceptible to fluctuations in pH from
acid rains or acid contamination. The presence of calcium carbonate or other
compounds such as magnesium carbonate, contribute carbonate ions to the
buffering system.
Total alkalinity is reported as mg/L CaCO3.
If rain is acidic, why aren’t all surface waters acidic?
The answer is geology. Some types of rocks can reduce (neutralize) the
acidity of the rain, whereas other rocks have little or no effect. Calcite
(CaCO3) and dolomite [CaMg(CO3)2] are two minerals that greatly mitigate
the effects of acidic rain; calcite and dolomite are the principal minerals that
make up the rocks limestone and dolomite, respectively, as well as marble.
Granite outcrops provide very little buffering, and rain-filled pools or streams
in this area may be lower than pH 7, and variable over the day.
Helpful Hint:
Have a look at how local geology compares with pH readings in your local
streams. You should be able to access Geology maps.
Useful Reading

A Community Monitoring Manual for Victoria

Waterwatch Victoria’s Equipment Manual, Data Confidence Manual and
Methods Manual.

A great reference website is http://www.ph-measurement.co.uk


Nollett, L.M., (2000) Handbook of water analysis. Marcel Dekker Inc, NY.
To add additional points to this discussion paper, or to clarify any points,
please contact 
Sara Johnson
Waterwatch Victoria Science Coordinator
123 Brown St
Heidelberg 3084
Victoria
sara.johnson@dse.vic.gov.au
Ph 03 9450 8748
Fax 03 9450 8799