What is pH?
pH appears on raw water, treated water and network results alike, and it's one of the first numbers operators check each day. Most people know that low is acidic and high is alkaline. Fewer know what the number is measuring, or why a small change can matter so much.
What is pH actually measuring?
pH is a measure of hydrogen ions in water.
A very small fraction of water molecules naturally split into a hydrogen ion (H⁺) and a hydroxide ion (OH⁻). In pure water there are equal amounts of each, and the water is neutral. Add an acid and you add hydrogen ions. Add an alkali (a base) and the extra hydroxide ions combine with hydrogen ions, leaving fewer of them.
The concentrations involved are tiny. Neutral water contains about 0.0000001 moles of hydrogen ions per litre. Rather than write numbers like that, chemists use the pH scale: the "p" means "the negative logarithm of", so 0.0000001 (10⁻⁷) becomes pH 7.
That has two consequences worth remembering:
Lower pH means more hydrogen ions. The scale runs backwards to what you might expect.
Each whole unit is a tenfold change. Water at pH 6 has ten times more hydrogen ions than water at pH 7, and a hundred times more than water at pH 8.
pH also only tells you how acidic or alkaline water is right now. It doesn't tell you how much acid or alkali it would take to change it. That's measured by alkalinity, which we cover separately in this series.
How is it measured?
Most pH measurement uses a glass electrode. A thin glass membrane develops a small voltage that changes with the hydrogen ion activity of the water, about 59 millivolts per pH unit at 25°C. The meter compares this with a reference electrode and converts the difference into a pH reading.
Electrodes drift as they age and foul, so they're calibrated against buffer solutions of known pH. Simpler methods, such as colour indicators in pool test kits, work on dyes that change colour with pH, but they're less precise.
pH is best measured in the field. Carbon dioxide dissolves in water to form a weak acid, so a sample that gains or loses carbon dioxide on the way to the lab can read differently from the water it came from.
As a guide
pH | Everyday example |
About 2 to 3 | Lemon juice, vinegar |
About 5.6 | Rainwater (from dissolved carbon dioxide) |
About 6.5 to 8.5 | Most drinking water |
About 8.1 | Seawater |
About 9 to 10 | Water standing in a new concrete tank |
About 11 to 12 | Household ammonia cleaner |
How is pH used in the water industry?
Controlling treatment chemistry
Almost every chemical step in a treatment plant depends on pH. Coagulants such as alum and ferric chloride form floc best within a particular pH window, and plants adjust pH to stay in it. Removing iron and manganese needs the pH high enough for them to oxidise and drop out. After treatment, lime, soda ash or caustic soda is often added to bring pH back to a set point before the water leaves the plant.
Making disinfection work
When chlorine is added to water it forms hypochlorous acid, a strong disinfectant, and hypochlorite ion, which is much weaker. The balance between the two is set by pH. At around pH 7.5 they're roughly equal; as pH rises, more of the chlorine shifts to the weaker form. This is why pH is one of the inputs in C.t calculations: the same chlorine dose and contact time achieves less disinfection at higher pH. Chloraminated systems have their own optimum, with monochloramine forming best at around pH 8.0 to 8.4.
Protecting pipes and plumbing
Low pH water dissolves metals and cement. It can strip copper from household plumbing, release lead from brass fittings, and attack cement-lined mains and concrete tanks. High pH water tends to deposit calcium carbonate scale. Utilities set pH targets, often together with alkalinity and hardness, to keep water stable as it moves through the network.
Spotting change
pH responds quickly to what's happening in a system. In reservoirs, algae consume carbon dioxide during the day and push pH up, then it falls back overnight. New concrete tanks and pipes can raise pH for months. In chloraminated networks, a falling pH can be an early sign of nitrification. Online pH monitoring with alarms lets operators see these changes as they happen.
Where the guidelines fit
The Australian Drinking Water Guidelines (ADWG) recommend a pH between 6.5 and 8.5, based on limiting corrosion and scaling rather than direct health effects. They also note that chlorine disinfection becomes less effective above pH 8.0.
Key points
pH measures hydrogen ions. Lower pH means more hydrogen ions and more acidic water.
The scale is logarithmic: each whole unit is a tenfold change.
pH controls how treatment chemicals work, how effective chlorine is, and whether water corrodes or scales pipes.
pH tells you where the water is now, not how easily it will change. That is alkalinity.
pH sits underneath most of the other parameters in this series. Understanding what it measures makes the rest of a water quality report much easier to read. If you'd like a hand interpreting your pH data, or setting sensible operating targets, the Viridis team is happy to help.
References
Australian Drinking Water Guidelines, pH fact sheet. NHMRC, NRMMC (2011, updated).
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