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Analytical · Glossary

What is pH?

0 to 14 scaleAnalytical

pH is a measure of how acidic or alkaline a water based solution is, based on the hydrogen ion activity.

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pH Explained

A pH of 7 is neutral at 25 °C; lower values are acidic and higher values alkaline. Each pH unit is a tenfold change in hydrogen ion activity.

Process pH is measured with a glass electrode and reference electrode, compensated for temperature.

pH Formula

pH = −log₁₀ aH+
aH+hydrogen ion activity, close to concentration in mol/L for dilute solutions
Example: [H+] = 10⁻³ mol/L gives pH 3.
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Calculators for pH

CalculatorpH CalculatorFree online calculator with formula and worked example.CalculatorpH Electrode CalibrationFree online calculator with formula and worked example.CalculatorDilution CalculatorFree online calculator with formula and worked example.

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Free downloadpH Meter Calibration ProcedureFree step by step procedure, Word and PDF.

Frequently Asked Questions

What is pH?

pH is a measure of how acidic or alkaline a water based solution is, based on the hydrogen ion activity.

Can pH be below 0 or above 14?

Yes, in very strong acids or bases, though process electrodes are not accurate there.

Is neutral always pH 7?

Only at 25 °C; at higher temperatures neutral water has a lower pH.

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Related Terms

Analytical termpH ElectrodeA pH electrode is a sensor with a pH sensitive glass membrane and a reference electrode, producing a…Analytical termNernst SlopeNernst slope is the theoretical change in electrode voltage per pH unit, 59.16 mV at 25 °C, rising with…Analytical termBuffer SolutionA buffer solution has a stable, known pH and resists change, and is used to calibrate pH electrodes.

More Analytical Terms

Analytical termReference ElectrodeA reference electrode provides a stable voltage against which the pH or ORP measuring electrode is compared.Analytical termpH Temperature CompensationpH temperature compensation corrects the electrode slope for sample temperature, so the analyser converts…Analytical termOxidation Reduction PotentialOxidation reduction potential is the voltage that shows whether a solution tends to oxidise or reduce,…Analytical termConductivityConductivity is the ability of a solution to carry electric current, which rises with the concentration…Analytical termCell ConstantCell constant is the geometry factor of a conductivity cell, the electrode spacing divided by effective area.Analytical termConductivity Temperature CompensationConductivity temperature compensation converts measured conductivity to the value at a reference…

All analytical terms · Full glossary A to Z

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