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Contents

- What is pH? - Why is pH important? - Measuring pH - Typical applications

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What is pH?

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1e-14 1e-13 1e-12 1e-11 1e-10 1e-09 1e-08 1e-07 1e-06 1e-05 1e-04 0.001 0.01 0.1 1.

pH

Hydrogen Ion Concentration [H+]

Pure Water

Wastewater (before treatment)

Wastewater (after treatment)

SoilMilk

CheeseMeat

Red WineWhite Wine

Beer

A neutral solution is a solution in which there are equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-)

A neutral solution, such as water, has a pH of 7.

pH is defined as the activity of hydrogen ions in a solution, as shown by the equation: It is used to determine how acidic or basic a solution is.

pH = - log10 aH+

In dilute solutions, activity is equivalent to concentration, so it is effectively a measurement of the concentration of hydrogen ions in solution.

The more hydrogen ions present in solution, the lower the pH, and therefore the more acidic the solution.

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Why measure pH?

Optimisation Properties of products, such as appearance and taste, will be affected by acidity during production. In order to produce the optimal product, the pH needs to be monitored during production and adjusted to the appropriate pH. E.g. in the beverage industry, wine will taste different depending on its pH, and will taste sour if the product is too acidic.

Costs of production can be minimised, as having the optimal pH during growth of a product will result in a maximum yield of the product. E.g. in hydroponics, the pH of the solution the plants are grown in should be kept at a certain pH in order to optimise the growth of crops, and so produce the greatest yield.

Protection Sometimes, if the pH goes outside the optimal range, the product can become harmful to the consumer, so it is essential to maintain an optimal pH to prevent this. E.g. chlorine levels in swimming pools need to be high enough to kill any micro-organisms, but not so high that they cause irritation to the eyes of those swimming. This can be determined by the pH of the pool.

Equipment can be damaged by corrosion from chemicals if they are outside the desired pH range, so the pH must be monitored and maintained.

Legally, some industries have to keep pH levels within a certain range. E.g. wastewater must be at a suitable pH before it enters the sewage system.

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Measuring pH

The most accurate method of measuring pH of a solution is using a pH meter and a pH electrode.

The pH electrode must be used alongside a reference electrode as a standard. This can be using two separate electrodes, or a combination electrode, which includes both.

The electrode measures the activity of hydrogen ions in the sample solution and produces a potential output in mV.

The hydrogen ion activity is measured by the migration of hydrogen ions into or out of a gel layer that forms between the outer surface of the glass electrode and the sample solution being measured.

If the sample is acidic, the hydrogen ions will migrate into this gel layer, resulting in a positive charge on the outer surface of the electrode, and hence a positive potential output.

If the sample is alkaline, hydrogen ions will migrate from the gel layer into the sample, resulting in a negative charge forming on the outer surface of the electrode, and hence a negative potential output.

This potential output is converted to the pH scale, to give a reading in pH units, which is the more common scale used to measure pH.

Inner Tube Housing the Membrane Sensing Wire

ElectrolyteContaining Silver

Ag/AgClReference Wire

Ceramic Junction

Combination pH ElectrodeCombination pHElectrode

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Typical applicationsPure water - pH 7

Wastewater - pH before treatment typically 6.5-8.5 - pH after treatment 7.0-7.2

Soil - pH depends on what you are growing, best pH range for most plants is 6.0-7.0

Food/dairy products (meats, cheeses, milk etc.) - pH of milk is 6.3-8.5 - pH of cheese varies depending on type of cheese, but typically around 5.0-6.0 - pH of meat depends on type of meat, but typically between 5.0 and 7.0

Alcoholic beverages (wine, beer) - pH of white wine typically between 3.0-3.4 - pH of red wine typically around 3.3-3.6 - pH of beer typically about 4.0