Perfect pH

There are 100’s of different applications for measuring pH. When you consider the number of available instruments, electrodes, calibration standards, and individual techniques, it might be hard to find two users that measure pH the exact same way. Some users are able to achieve the best possible results, while many more users struggle to get accurate and repeatable pH measurements. 

Our 7 tips for perfect pH will help to take the guesswork out of your pH measurements, helping you get the most out of your pH system whether you have a pocket tester or an advanced benchtop meter. 


Store your electrode properly in pH storage solution.

  • Never store in distilled, deionized, or tap water.
  • With refillable electrodes, close the filling hole for storage.
  • Keep the electrode upright if possible.

pH-electrode

Clean your electrode regularly

Cleaning a dirty, clogged or coated electrode can restore proper electrode performance and prolong the useful life of the electrode. Use the correct cleaning solution appropriate to the samples being measured.

pH-clean-electrode

Never re-use buffers

A pH system is only as accurate as the calibration buffers that are used for standardization.

  • Calibrate regularly with fresh buffers.
  • pH 10 buffer, in particular, will drift in a matter of hours due to carbon dioxide absorption.
  • Check the expiration date on your buffers.

For best accuracy, use a minimum of two-point standardization; first with a buffer value close to the electrode systems zero potential (typically pH 7); and next with an acid or base buffer whose value brackets the expected pH value of the sample(s).

pH-buffers

Rinse electrode tip well between readings

Use distilled water or electrode rinse solution to thoroughly rinse the electrode and never rub or blot the bulb to dry it. Gently shake off the excess water.

Electrode bulbs should always remain hydrated, and a dry electrode bulb will cause more problems than one or two drops of water in your sample.

pH-rinse-electrode

Check electrode fill level

If you use a refillable electrode be sure to check your fill level before storing and measurement.

Ensure that the filling hole is closed for storage, but open for calibration and measurement.

pH-fill-level

Record temperature along with your pH

The temperature of your sample can have an effect on the pH. Ensure the temperature has stabilized on the probe before taking a reading and always record this along with the pH.

pH-temperature

pH values of buffers at various temperatures

0˚C 1.67 4.00 6.98 47.11 9.46 10.32 12.79
5˚C 1.67 4.00 6.95 7.08 9.40 10.25 12.73
10˚C 1.67 4.00 6.92 7.06 9.33 10.18 12.67
20˚C 1.67 4.00 6.87 7.01 9.23 10.06 12.52
25˚C 1.68 4.01 6.86 7.00 9.18 10.01 12.46
30˚C 1.68 4.02 6.85 6.98 9.14 9.97 12.36
40˚C 1.69 4.04 6.84 6.97 9.07 9.89 12.17
50˚C 1.71 4.06 6.83 6.97 9.01 9.83 11.96
60˚C 1.72 4.09 6.84 6.97 8.96 9.79 11.73
70˚C 1.74 4.13 6.85 6.99 8.92 9.78 11.47
80˚C 1.77 4.16 6.86 7.03 8.89 9.78 11.19
90˚C 1.79 4.21 6.88 7.08 8.85 9.80 10.89

Check the pH slope in pH 7 buffer regularly

The slope should be greater than 92% (53.5 mV/pH @ 20°C or 54.4 mV/pH @ 25°C). The offset (as measured in fresh pH 7 buffer) should be +/- 30 mV for new electrodes and +/- 59 mV for aged electrodes to enable auto-buffer recognition during calibration.

How to check your pH electrode slope

  1. Select the millivolt (mV) mode of your pH meter
  2. Using a pH electrode, obtain mV readings of two fresh calibration buffers (i.e., pH 4.0 and pH 7.0 are best)
  3. Determine the net mV change
  4. Determine the net mV change per pH unit change and compare using the chart below

pH-slope

Slope
 

at 20°C

at 25°C

Status
 
1 pH unit 3 pH unit 1 pH unit 3 pH unit
>102 >59.3 >178.0 >60.3 >181.0 Poor
102% 59.3 178.0 60.3 181.0 OK
101% 58.8 176.3 59.8 179.3 Very Good
100% 58.2 174.5 59.2 177.5 Ideal
99% 57.6 172.8 58.6 175.7 Very Good
98% 57.0 171.0 58.0 173.9 Very Good
97% 56.4 169.3 57.4 172.2 Very Good
96% 55.8 167.5 56.8 170.4 Very Good
95% 55.3 165.8 56.2 168.6 Very Good
94% 57.0 171.0 58.0 173.9 OK
93% 54.1 162.3 55.0 165.1 OK
92% 53.5 160.5 54.4 163.3 OK
<92 <53.5 <160.5 <54.4 <163.3 Poor

Store your electrode properly in pH storage solution.

  • Never store in distilled, deionized, or tap water.
  • With refillable electrodes, close the filling hole for storage.
  • Keep the electrode upright if possible.

pH-electrode

Clean your electrode regularly

Cleaning a dirty, clogged or coated electrode can restore proper electrode performance and prolong the useful life of the electrode. Use the correct cleaning solution appropriate to the samples being measured.

pH-clean-electrode

Never re-use buffers

A pH system is only as accurate as the calibration buffers that are used for standardization.

  • Calibrate regularly with fresh buffers.
  • pH 10 buffer, in particular, will drift in a matter of hours due to carbon dioxide absorption.
  • Check the expiration date on your buffers.

For best accuracy, use a minimum of two-point standardization; first with a buffer value close to the electrode systems zero potential (typically pH 7); and next with an acid or base buffer whose value brackets the expected pH value of the sample(s).

pH-buffers

Rinse electrode tip well between readings

Use distilled water or electrode rinse solution to thoroughly rinse the electrode and never rub or blot the bulb to dry it. Gently shake off the excess water.

Electrode bulbs should always remain hydrated, and a dry electrode bulb will cause more problems than one or two drops of water in your sample.

pH-rinse-electrode

Check electrode fill level

If you use a refillable electrode be sure to check your fill level before storing and measurement.

Ensure that the filling hole is closed for storage, but open for calibration and measurement.

pH-fill-level

Record temperature along with your pH

The temperature of your sample can have an effect on the pH. Ensure the temperature has stabilized on the probe before taking a reading and always record this along with the pH.

pH-temperature

pH values of buffers at various temperatures

0˚C 1.67 4.00 6.98 47.11 9.46 10.32 12.79
5˚C 1.67 4.00 6.95 7.08 9.40 10.25 12.73
10˚C 1.67 4.00 6.92 7.06 9.33 10.18 12.67
20˚C 1.67 4.00 6.87 7.01 9.23 10.06 12.52
25˚C 1.68 4.01 6.86 7.00 9.18 10.01 12.46
30˚C 1.68 4.02 6.85 6.98 9.14 9.97 12.36
40˚C 1.69 4.04 6.84 6.97 9.07 9.89 12.17
50˚C 1.71 4.06 6.83 6.97 9.01 9.83 11.96
60˚C 1.72 4.09 6.84 6.97 8.96 9.79 11.73
70˚C 1.74 4.13 6.85 6.99 8.92 9.78 11.47
80˚C 1.77 4.16 6.86 7.03 8.89 9.78 11.19
90˚C 1.79 4.21 6.88 7.08 8.85 9.80 10.89

Check the pH slope in pH 7 buffer regularly

The slope should be greater than 92% (53.5 mV/pH @ 20°C or 54.4 mV/pH @ 25°C). The offset (as measured in fresh pH 7 buffer) should be +/- 30 mV for new electrodes and +/- 59 mV for aged electrodes to enable auto-buffer recognition during calibration.

How to check your pH electrode slope

  1. Select the millivolt (mV) mode of your pH meter
  2. Using a pH electrode, obtain mV readings of two fresh calibration buffers (i.e., pH 4.0 and pH 7.0 are best)
  3. Determine the net mV change
  4. Determine the net mV change per pH unit change and compare using the chart below

pH-slope

Slope
 

at 20°C

at 25°C

Status
 
1 pH unit 3 pH unit 1 pH unit 3 pH unit
>102 >59.3 >178.0 >60.3 >181.0 Poor
102% 59.3 178.0 60.3 181.0 OK
101% 58.8 176.3 59.8 179.3 Very Good
100% 58.2 174.5 59.2 177.5 Ideal
99% 57.6 172.8 58.6 175.7 Very Good
98% 57.0 171.0 58.0 173.9 Very Good
97% 56.4 169.3 57.4 172.2 Very Good
96% 55.8 167.5 56.8 170.4 Very Good
95% 55.3 165.8 56.2 168.6 Very Good
94% 57.0 171.0 58.0 173.9 OK
93% 54.1 162.3 55.0 165.1 OK
92% 53.5 160.5 54.4 163.3 OK
<92 <53.5 <160.5 <54.4 <163.3 Poor

Featured pH Products

Benchtop pH Meters

The ideal choice for dedicated pH testing in water, process, research, and QA/QC samples. The Thermo Scientific portfolio of pH Meters offer a range from simple & economical models to powerful & sophisticated pH meters, to help you analyze your samples with confidence.

Portable pH Meters

Our portable pH meters are designed to offer pH testing in the field for applications including environmental, drinking water, and wastewater analysis.

pH Electrodes

The Thermo Scientific electrodes portfolio spans a vast range of industries and applications, and are categorised into digestible classifications to help you choose which electrode is best for your application.

pH Reference Buffers

The quality of your measurement results depends upon the quality of your standards, buffers, and reagents. We offer pH buffers and test kits for a variety of applications.