Understanding pH: A Simple Scale With Big Consequences
If you’ve ever tasted “metallic” tap water, fought limescale in your kettle, or wondered why some water feels “softer” than others, you’ve already had a close encounter with pH – whether you realised it or not.
pH is a measure of how acidic or alkaline (basic) a solution is. It runs on a scale from 0 to 14:
- pH 7 = neutral
- Below 7 = acidic
- Above 7 = alkaline (basic)
Pure water at 25°C has a pH of around 7. But the water that reaches your tap has travelled through rivers, soils, rocks and pipes, picking up dissolved minerals, gases and, in some cases, contaminants such as PFAS, heavy metals and disinfectant by-products. All these can influence its pH.
Why does this matter? Because pH quietly affects:
- How water tastes and feels
- How it interacts with your plumbing
- How effective your water treatment and filtration systems are
- And even how other contaminants behave once they’re in your water
Let’s unpack that, starting with the basics.
What Is the pH of Drinking Water Supposed to Be?
Most drinking water guidelines recommend a pH range between 6.5 and 9.5. This isn’t just for comfort; it’s for pipe integrity and public health protection.
Here’s how major authorities frame it:
- World Health Organization (WHO): No direct health-based limit for pH, but recommends 6.5–8.5 for palatability and to minimise corrosion and scaling in distribution systems.
- UK Drinking Water Inspectorate (DWI): Typically aligns with 6.5–9.5 at the tap, primarily as an indicator parameter, to keep the distribution network stable and safe.
- US EPA (Secondary Standard): Suggests 6.5–8.5 for aesthetic and operational reasons.
So, pH itself is not usually a toxic contaminant in the traditional sense. You won’t get “pH poisoning” from slightly acidic or alkaline tap water. But pH strongly controls:
- What your water can dissolve (metals, minerals, pipe material)
- How disinfectants like chlorine behave
- How likely contaminants are to stay dissolved or settle out
In short, pH sets the chemical “mood” of your water – and other substances respond to it.
How pH Affects Your Health
It’s common to see claims that “alkaline water” can detoxify your body, prevent cancer or “neutralise acidity” from your diet. From a scientific and medical standpoint, those claims are not supported by robust evidence.
Your body has strong buffering systems (especially your lungs and kidneys) that keep your blood pH in a very narrow range around 7.35–7.45. Drinking slightly acidic or slightly alkaline water within normal drinking water ranges does not override that.
However, pH can still influence health indirectly in several important ways.
When Water Is Too Acidic: Corrosion, Metals and Sensitive Stomachs
Water with a pH below about 6.5 is considered “aggressive” or corrosive. It’s more eager to dissolve metals and minerals from pipes, fixtures and storage tanks. That can have health implications.
Key risks of overly acidic water include:
- Increased metal leaching: Low pH can pull metals such as copper, lead, iron and zinc from plumbing.
- Higher risk of lead exposure: In older buildings with lead pipes, solder or brass fixtures containing lead, corrosive water can significantly raise lead levels at the tap.
- Digestive irritation in some individuals: Very low pH (well outside normal drinking water ranges) can be irritating to the mouth, throat and stomach lining.
There is a well-documented example of this. During the Flint, Michigan water crisis, changes to water treatment led to more corrosive water entering the distribution system. This stripped protective scales from pipes and released lead into drinking water. While the crisis wasn’t caused by “low pH” alone, corrosion control – closely related to pH management – was at the heart of the problem.
In everyday UK or EU tap water, you’re unlikely to see extreme pH values. But slightly acidic water combined with old plumbing can still be a concern, especially for:
- Infants and young children (more vulnerable to lead exposure)
- Pregnant people
- Homes with very old pipes or unknown plumbing materials
When Water Is Too Alkaline: Taste, Skin and Mineral Build-Up
On the other side of the scale, water above about pH 8.5 can bring its own set of issues, although again, these are mostly indirect rather than toxic in the strict sense.
Potential effects of very alkaline water include:
- Unpleasant taste: Water can taste bitter, “soda-like” or feel slippery.
- Skin and hair dryness in some people, especially if combined with high hardness (calcium and magnesium).
- Gastrointestinal discomfort for those unaccustomed to high-pH water, particularly if there’s also very high mineral content.
At typical tap water pH levels, these effects are usually mild. The main challenges of high-pH water show up not in your body, but in your home infrastructure.
pH, Hardness and That White Crust on Your Kettle
If you live in a hard water area, you already know the symptoms:
- Scale in kettles and coffee machines
- White crust on taps and shower heads
- Soap that doesn’t lather well
- Cloudy film on glasses after washing
While hardness (mainly calcium and magnesium) is a separate parameter from pH, they interact. Alkaline conditions can encourage scale formation, because calcium carbonate is less soluble at higher pH. This won’t harm your health – in fact, calcium and magnesium are essential nutrients – but it does:
- Shorten the life of appliances
- Reduce energy efficiency in boilers and water heaters
- Increase the need for descaling and cleaning products
From a water treatment perspective, utilities often adjust pH deliberately to strike a balance: not so low that pipes corrode, not so high that scale becomes unmanageable.
Why pH Matters So Much for Plumbing and Pipes
Your plumbing system is essentially a long, reactive test tube that your water runs through every day. pH determines whether that test tube stays intact or slowly dissolves into your glass.
Here’s how pH affects common materials in pipes:
- Copper pipes: Low pH (<6.5) can accelerate copper corrosion, leading to blue-green stains and, in extreme cases, pinhole leaks. High levels of copper in water can cause gastrointestinal upset.
- Lead pipes and solder: Corrosive water can strip protective mineral layers and release lead, a potent neurotoxin, especially hazardous to children.
- Galvanised steel: Acidic water speeds up rusting and can cause discoloured (brown or orange) water due to iron release.
- Concrete or cement-lined pipes: Very soft, acidic water can slowly dissolve the cement matrix, altering pH and contributing to turbidity and mineral imbalance.
On the other hand, water that’s too alkaline and rich in minerals can form thick scale layers:
- Reducing pipe diameter and flow
- Creating rough surfaces where bacteria can attach and grow
- Distorting water flow and potentially affecting disinfection efficiency
Water utilities manage these risks by adjusting pH and using corrosion inhibitors (such as orthophosphates). But what happens after the water reaches your property – including how your own filters and softeners alter pH – is often your responsibility.
pH and Contaminants: Including PFAS
Where does pH fit into the bigger picture of chemical contaminants like PFAS, nitrates, or heavy metals?
Two key points:
- pH affects solubility and speciation: Many metals (e.g. lead, copper, iron) dissolve more readily at lower pH. This means that even if treated water leaving a plant is low in metals, corrosive conditions in pipes can raise levels again by the time water reaches your tap.
- pH influences treatment efficiency: Processes like coagulation, adsorption and advanced oxidation often have an optimal pH range. If the pH drifts, contaminant removal can become less effective.
With PFAS (“forever chemicals”), pH doesn’t usually change how much is present in your tap water as dramatically as it does for metals. However, pH does matter for:
- Performance of certain PFAS treatment technologies (for example, some ion-exchange resins and advanced oxidation systems).
- Behaviour of co-contaminants that can compete with PFAS for adsorption sites in filters or resins.
In short, pH is part of the broader chemical context that determines how easily your water can be cleaned up – or re-contaminated as it travels through pipes and fixtures.
How to Test the pH of Your Water
If you’re curious about your water’s pH, you don’t need a full laboratory. Several options are accessible to households:
- Water supplier reports: In the UK and many other countries, your water company publishes quality data, including pH ranges, by postcode. This gives you an overview of the water leaving their system, although it may not capture what happens in your home plumbing.
- pH test strips: Cheap, easy and widely available. They give a rough range (e.g. 6.0–7.5) but are less precise and can be influenced by colour perception.
- Digital pH meters: More accurate and reusable. They need calibration with standard solutions and proper care, but offer better resolution (e.g. 0.01 pH units).
- Professional lab testing: If you’re already testing for metals, nitrates or PFAS, pH will usually be measured at the same time. This is the most accurate, but also more expensive and slower.
For most households, combining your supplier’s data with a simple at-home check is enough to understand whether your pH is broadly within the recommended range.
What If Your Water pH Is Outside the Ideal Range?
If you measure a pH significantly below 6.5 or above 9.5 at the tap, it’s worth investigating further. Start by asking:
- Is this result consistent across several measurements and days?
- Is the issue present in both hot and cold water? (Test cold water, after flushing for at least 30 seconds, for a more accurate reading.)
- Is there visible corrosion, staining or scaling on fixtures?
Then, practical next steps might include:
- Contact your water supplier: Ask for their latest data and whether there have been any treatment changes. They may be able to send an engineer or advise on specific local issues.
- Get a broader water test: Especially for metals like lead and copper, and for other problem indicators such as iron, manganese and hardness.
- Inspect plumbing materials: Identify whether you have lead service lines, old copper, galvanised steel or problematic fittings.
From there, adjustments may involve both system-level and household-level interventions.
Household Solutions: Adjusting pH and Protecting Your Home
If your water supplier’s pH is within the recommended range but you’re still seeing corrosion or scaling, the problem may lie in your internal plumbing or treatment devices.
Common household options include:
- Point-of-use filters: Many under-sink or countertop systems (e.g. activated carbon, reverse osmosis, ion exchange) can slightly change pH by removing minerals or altering ionic balance.
- Neutralising filters: Often used on private wells, these pass acidic water through media like calcite (calcium carbonate) to raise pH and add alkalinity, reducing corrosiveness.
- Water softeners: These primarily exchange calcium and magnesium for sodium or potassium. They don’t directly control pH, but they can change how water interacts with fixtures and soaps.
- Whole-house treatment systems: In some cases, a tailored combination of pH control, softening and filtration is necessary, especially in areas with very aggressive or very scaling-prone water.
If you’re also targeting PFAS or other persistent contaminants, check carefully how a given treatment configuration affects:
- pH at different points in the system
- Filter performance and lifespan
- Corrosion risk in both cold and hot water lines
A system that removes PFAS effectively but causes excessive corrosion might unintentionally increase your exposure to metals – trading one problem for another.
pH in the Bigger Environmental Picture
pH doesn’t just matter in your pipes; it matters in rivers, lakes, wetlands and groundwater. Aquatic organisms are often far more sensitive to pH shifts than humans. Many species thrive in specific pH windows, and even small changes can:
- Disrupt reproduction and growth
- Alter nutrient cycles
- Change how metals and pollutants are mobilised in sediments
Acid rain, mining effluents, industrial discharges and agricultural run-off have all altered water pH in various regions. In some cases, that makes metals and other contaminants more mobile, increasing exposure risks downstream – including at drinking water intakes.
PFAS add another layer to this story. They are extremely persistent and can travel long distances in water. While their stability means pH alone won’t break them down, the broader chemistry of the water – including pH, organic matter and co-contaminants – influences how PFAS spread, how they’re removed in treatment plants and where they accumulate in the environment.
Practical Takeaways for Your Home and Health
To make sense of all this chemistry in day-to-day terms, it helps to keep a short checklist in mind:
- Know your baseline: Look up your supplier’s water quality report and note the typical pH range, hardness and any corrosion control measures used.
- Test at the tap: If you have older plumbing or unexplained staining, scaling or metallic tastes, do a simple pH check and consider a broader lab test for metals.
- Watch for warning signs: Blue-green stains (copper), brown water (iron), frequent pinhole leaks, and chalky build-up all hint at underlying pH–corrosion–hardness interactions.
- Match treatment to the problem: Don’t install a strong softener or aggressive filtration system without understanding how it affects pH and corrosion. When in doubt, consult a water treatment professional, and ask specifically about pH and metal leaching.
- Think beyond taste: Water that tastes “fine” may still have pH-related issues, especially if you live in an older building. Taste is a useful indicator, but not a complete safety test.
pH may seem like a small number on a lab sheet, but it quietly shapes how your water behaves in your body, in your pipes and in the environment. Understanding it – and paying attention to how it interacts with contaminants from PFAS to lead – is a practical step toward safer, more sustainable water in everyday life.