I frequently get asked about how to use and calibrate a pH meter. Let's start with use and then move on to calibration.
Measurement of mash pH is the use to which meters are most often put by
brewers. Assuming the meter is calibrated (see below) here is how that
is done.
1. Stir the mash thoroughly. This is especially important if the
measurement is to check on the effects of an acid or alkali addition.
Withdraw a small sample of the liquid. It doesn't matter if some grain
is included.
2. Cool the sample to room temperature, ideally the same temperature as
the buffers you used for calibration. This prolongs electrode life and
reduces the burden on ATC. If you use a small metal saucepan you can
achieve the cooling quickly by immersing it in cool/cold water.
3. Rinse the electrode with DI water, shake off and blot (see
calibration below) and insert the electrode into the sample. Move the
electrode around for a few seconds (sample rinses any water off bulb and
junction) then stop and wait until the reading is stable. Some meters
will decide when the reading is stable for you and beep to signal this.
Record both the reading and the temperature.
4. Repeat the process every 5 minutes or so until the readings stop changing. This usually 15 - 30 minutes after strike.
5. Rinse the electrode with DI water and return to storage solution or
just tap water for short term (i.e. between readings) storage.
For calibration the overall instructions are simple: follow the
manufacturer's instructions. For those who don't have a meter in hand
and want to have an idea as to what is involved or for whom the supplied
instructions are less than adequate the following is offered. Those who want to know what the meter is doing during calibration or who wish to put their meters in millivolt mode and do the calibration and measurement calculations themselves can go here.
Buffers
and samples should be at room temperature.
1. Store the electrode in a storage solution recommended by the
manufacturer. This will often be a saturated or nearly saturated
solution of potassium chloride.
2. Prepare fresh 4 and 7 buffer solutions using deionized water.
Several manufacturers sell capsules of powder which contain the buffers’
chemical components. These are simply added to a specified amount of DI
water (50 or 100 mL) just before use. Premixed buffers are also sold in
sealed packages (similar to the ketchup packages from fast food
restaurants). These work as well as the buffers one mixes on the spot
and are obviously more convenient but tend to be, because of the
packaging, more expensive. Premixed buffers are also sold in bulk i.e. 1
L bottles or 4 L jugs or cubitainers. If buffers in this form are being
used check that they are not beyond their expiration dates and pour
small amounts of each into a clean beaker or preferably, sealed
container, at the beginning of each brew day. Do not return used buffer
to the bulk storage.
3. Remove the storage cap from the electrode. If the electrode is the
refillable type, insure that it contains adequate fill solution, top up
if neecessary and, whether you top up or not, open the fill hole so air
can enter the electrode body allowing fill solution to freely flow out
through the reference junction.
4. Rinse the electrode with a stream of DI water from a wash bottle.
Blot dry with clean tissue or paper towel. Don’t touch the actual
electrode bulb when you do this. You don’t need to get all the adhering
water, just the bulk of it. Wicking of water into the paper is adequate.
5. Turn the meter on, allow it to stabilize for a few minutes, and then
lower the electrode into the first buffer solution. With most modern
meters it does not matter which one you go into first as these meters
have automatic buffer recognition. Following the manufacturer’s
instructions put the meter in calibration mode and initiate calibration
if necessary (e.g. press the ‘read’ or ‘Cal’ button).
6. Move the electrode around in the buffer a little to rinse any
adhering DI water off the bulb and away from the reference junction.
7. Wait until the reading stabilizes. Modern instruments tend to have
stability indicators which beep or otherwise alert the operator when the
reading is stable (hasn’t changed by more than a threshold amount in a
given period of time). These often also instruct the operator ro move on
to the next buffer when stability is detected. In others you may have
to determine when the reading is stable yourself and indicate this to
the meter by pressing a button. Follow the manufacturers instructions
and/or prompts on the meter’s display.
8. When instructed to move to the second buffer, remove the electrode
from the first buffer, shake adhering buffer off and rinse with a stream
DI water. Blot away as above and insert the electrode in the second
buffer. Move electrode around in second buffer.
9. When the second reading is stable, take whatever action is necessary
to complete the measurement as above. In some meters there will be an
option for a third buffer. In those meters you will have to do something
(e.g. press an ‘exit’ button) to indicate to the meter that calibration
is complete if you are doing a 2 buffer calibration.
10. The instrument will now calculate the calibration parameters (slope
and offset) and, in some cases, display these to you in the case of
slope either as a percentage (should be near 100) or a number like 57.3
which is the number of millivolts change per unit change in pH at some
reference temperature. The offset will be a millivolt number which
should be small i.e. a few millivolts (it can be negative). If the meter
presents those numbers, write them in your log book. They represent a
record of the rate at which your electrode is aging. Fancy meters will
automatically store the calibration data, tagged with time and date, in
the meter’s memory.
11. Take whatever action is necessary to indicate that the calibration
is to be accepted (e.g. press a ‘store’ or ‘exit’ or other button as
directed by the manual).
12. Remove the electrode from the second buffer. Shake, rinse and blot as before. Place in sample.
13. Press ‘read’ button if necessary. Otherwise monitor display. Move electrode around in sample.
14. When reading is stable (as determined by you or meter electronics)
record pH and temperature. Fancy meters will automatically store these
in memory and some will even transmit them to an external computer.
15. Remove from sample, rinse and blot dry as before. Move to next
sample. If finished, rinse extra thoroughly. After shaking and blotting
dry insure that cap contains sufficient storage solution to cover bulb
and replace cap. Turn meter off if finished for the day. If not finished
for the day the probe can be left in the last sample.
11b. As a check on the calibration you can measure the 4 and 7 buffers
again at room temperature. You may wish to do this after some time has
passed or even after you have finished measuring samples for the day. pH
values are often printed on the buffer package. Sometimes they are not.
If not and assuming you are using NIST traceable pH 4 and 7 technical
buffers the pH values of the buffers are:
pH 7: 1911.4/K -5.5538 + 0.022635*K - 6.8146e-6*K*K
pH 4: 1617.3/K -9.2852 + 0.033311*K - 2.3211e-5*K*K
where K = °C + 273.15 (i.e. K is the temperature in Kelvins).
The values you read should be close to those given by the formulas or on
the buffer package. If they are not then your meter is drifting.
Cool the 4 buffer to about 40 °F and measure its pH. Do this right after
completing calibration. If your meter reads off by more than a few
hundredths then its isolectric point is not equal to 7 and you must be
careful to measure buffers and samples at close to the same temperature
(ATC won't work well).