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Colorimeter
A general definition of a "colorimeter" is an instrument to measure "color" as
the name suggests. This is usually done by preparing a sample according to
directions and comparing its color against a reference, or series of references.
These comparisons are done visually in some cases, and instrumentally at a fixed
wavelength of light in other instances. Typical applications are the measurement
of pH or hypochlorite level in a swimming pool. A spectrophotometer is a more
complicated instrument and there are several configurations. But the general
principle is that a light source is diffracted (that is the various wavelengths
are spatially separated by a grating or prism). The various wavelengths pass
through an entrance slit and the test sample (in some configurations the sample
and entrance slit are reversed). The sample selectively absorbs the various
wavelengths of light in varying amounts. The light then passes through another
slit, called the exit slit, and impinges upon a detector. The detector converts
the light intensity at the particular wavelength into an electrical signal that
is amplified and displayed on a screen or traced on a chart (light absorbed
versus wavelength). There are many variations on this basic design. For example,
in some cases the light passing through the sample is compared to a reference
signal that passes through some reference sample -- which may contain just the
solvent but no active absorber.In short, a colorimeter provides an over all
measure of the light absorbed, while a spectrophotometer measures the light
absorbed at varying wavelengths.
pH ELECTRODE
TRAINING GUIDE
SECTION 1 -
ELECTRODE CONSTRUCTION
What is pH?
It is
the unit of measurement describing the acidity or alkalinity of a solution,
measured over a scale of 0 - 14. The measurement is carried out using a pH
meter, a pH glass electrode and separate reference electrode or a combination
electrode which combines the pH and reference in the same body.
The pH Sensitive Membrane
The
most common type of sensitive membrane used on a pH electrode is a blown glass
bulb or rod. The glass used on our electrodes is suitable for most applications.
If, however, the application involves the constant monitoring of high
temperature liquids or high pH values (above pH 13), then an alternative glass
type can be specified. A bulb configuration will provide a fast response and
accurate results when used in a sample of low ionic strength whereas a rod or
bullet shaped membrane is very rugged and will be more resistant to breakage.
The Reference Cell
Housed
within the outer chamber of the pH electrode is a reference system which is
designed to provide a stable reference voltage for the sensor. This reference
'half-cell' will maintain a constant output in all liquids. Reference cells
consist of an internal element (usually a Ag/AgCl wire), an electrolyte (usually
KCl solution) and a liquid junction. The liquid junction provides a leak path
for the internal electrolyte to 'weep' into the sample chamber and provide an
electrical contact with the liquid to be measured. If the liquid junction is not
efficient then measurements will be inaccurate.
The Cap/Cable/Connector
Electrodes used in laboratories are usually fitted with 16mm diameter caps to
suit cantilever arm electrode stands. The cable used is a high grade, screened
coaxial type with low noise characteristics. Because of the high impedance of pH
electrodes, typically 100 megohms, connectors should always be kept clean and
dry. Detachable cable electrodes should not be used in very humid environments.
SECTION 2 - HOW
TO SPECIFY AN ELECTRODE
The
following check list, when used with the
pH electrode selection chart, will help to identify a suitable electrode for
any given application.
SECTION 3 -
CALIBRATION OF pH METER AND ELECTRODE
To
achieve accurate, reproducible results a great deal of attention needs to be
paid to the calibration method. A decision should be made on the accuracy
required for the measurement. This will enable the user to choose the type of
calibration required and the appropriate type of equipment to be used. The
following recommendations will ensure the best levels of accuracy.
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All solutions should be
stirred gently to ensure the sensor is measuring a true representation of
the beaker contents.
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Calibration buffers should be
chosen which have pH values either side of the expected sample value, i.e,
for a sample which has an expected pH of 5, pH buffers with a value of pH 7
and pH 4 should be used.
-
Always use a 'control' buffer
to keep a check on the drift of the electrode. A method commonly used is to
place the electrode into a buffer, which has a value close to the sample pH,
between measurements.
-
Fresh buffer solutions should
be used. Changing all solutions daily is a good practice.
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All solutions should be
maintained at an equal temperature.
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Rinse the electrode
thoroughly in deionised water between measurements.
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When calibrating the
electrodes, allow sufficient time to elapse for the reading to stabilise
before adjusting the meter. At least one minute, preferably longer.
SECTION 4 -
PROCEDURE FOR CALIBRATING THE pH METER
Equipment
Required
-
1 x high quality pH/mV meter
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1 x 100ml pH 7.00 buffer
solution
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1 x 100ml pH 4.00 buffer
solution
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1 x 100ml pH 5.00 buffer
solution
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1 x calibrated glass
thermometer
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1 x temperature controlled
water bath (required if the sample value is different to ambient)
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1 x combination pH electrode
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4 x 200ml beakers
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3 x Teflon stirrer paddles
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1 x magnetic stirrer
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1 x cantilever arm electrode
stand
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1 x fast flow wash bottle
containing deionised water
Method
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a)
Assemble all equipment.
-
b) Lower fill hole sleeve on
the electrode (if fitted) and thoroughly rinse the electrode tip.
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c) Lower electrode into
gently stirred pH 7.00 buffer and allow to stabilise.
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d) Check the temperature of
the calibration solutions and adjust the default reading on the pH meter, if
applicable.
-
e) After 1 - 2 minutes adjust
the calibration control on the pH meter to the appropriate pH value.
-
f) Raise electrode from
beaker and thoroughly rinse with deionised water.
-
g) Lower electrode into
gently stirred pH 4.00 buffer and allow to stabilise.
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h) After 1 - 2 minutes adjust
the slope control on the pH meter to the appropriate temperature corrected
value.
NOTE: Many modern microprocessor controlled pH
meters have automatic buffer recognition.
Please consult the instrument manual for specific adjustment information.
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i) Rinse the electrode and
repeat stages c) to h) to confirm calibration.
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j) Rinse the electrode and
lower into pH 5.00 buffer.
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k) After stabilising, record
the reading in pH 5.00 buffer.
-
l) Between measurements in
the sample, rinse and lower the electrode into the control buffer (pH 5.00)
for comparison with the recorded reading (remember to check temperature pH
versus pH values).
SECTION 5 - CARE
AND MAINTENANCE OF ELECTRODES
By
following this advice, it is possible to significantly increase the expected
life of an electrode and also to improve the quality of measurement results.
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pH electrodes must always be
stored wet. There are many opinions on which storage solution is the best.
Our electrodes are all supplied soaked in a saturated KCl solution with the
exception of double junction electrodes which are stored in the appropriate
refill electrolyte for their application.
-
For short term storage, soak
the electrode in KCl.
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For long term storage, fill
the soaking boot, fit over the end of the electrode and seal with parafilm.
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Electrodes should never be
stored in any of the following liquids:
Deionised water, sample solutions, solvents, hydrochloric acid, pH buffers
containing mercury based preservatives.
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Sensing tips should always be
rinsed after use.
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Reference cells should always
be kept regularly topped up with electrolyte.
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Connectors must be kept clean
and dry.
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If the electrode needs to be
cleaned physically, always use soft tissue soaked in a mild detergent or
propanol.
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Regularly inspect the glass
pH sensitive membrane for cracks or chips.
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