What is the pH of distilled water?

Purified water from distillation, deionisation or reverse osmomsis purification systems will usually have an acidic pH. But why is the pH of distilled water not neutral pH 7? This does not mean the water is contaminated - we've taken a look at the chemistry that explains the acidity. [caption id="attachment_8125" align="aligncenter" width="374"] Deionised (DI) or distilled water often has an acidic pH - but why is this?[/caption]

Why is the pH of distilled or deionised water not neutral?

Both distillation and deionisation systems aim to remove impurities from water. The processes work in different ways and have different strengths and weaknesses, but remove many of the ions commonly found in tap water such as carbonates, nitrates and others. What is left should be only H₂O - pure water - which in theory has an exactly neutral pH of 7.0 In reality, as soon as the water comes into contact with air, CO2 (carbon dioxide) begins to dissolve into it. This dissociates to form a mild acid called carbonic acid (this is the same way that acid rain is formed) - as shown in the equation below;

CO₂ + H₂O  --> HCO₃^- + H⁺

This means that most distilled or deionised water will have an acidic pH below 7.0, sometimes as low as pH 5.5

How can I test the pH of distilled water?

Measuring the pH of distilled water is more difficult than measuring standard tap water, because there are so few ions present. This means that there is little available to interact with the electrode membrane and cause a response. Many standard pH electrodes will show large drift when trying to read distilled or deionised water - i.e. the readings move about and not settle. To test distilled or deionised water an electrode specifically designed for low ionic strength liquids should be chosen - for example the Blueline 13PH electrode by S I Analytics with a meter such as TRUEscience, or the Myron Ultrameter 6PIIfc.   If you have any questions about this, or any other water testing questions, just leave a comment below;