What is the pH level of deionized water and how does it affect its uses?
Perfectly pure water has a pH of 7 at 25°C (77°F), and deionized water should be close to that. In real life it usually reads slightly acidic, often around 5.5 to 7, because it absorbs carbon dioxide from the air, and pH meters read it unreliably because it has almost no ions. A reading of 8 or higher often points to contamination, a dirty electrode or exhausted resin.
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In this article
What is the pH of deionized water?
Theory says 7.0 at 25°C. Practice says a range: freshly made deionized (DI) water often measures about 5.5 to 7, and it drifts lower within minutes in an open container. Water in equilibrium with normal outdoor air sits near pH 5.6 to 5.7. Distillation, reverse osmosis and ion exchange can all produce water in this range.
Why is deionized water acidic?
Air contains carbon dioxide. It dissolves in water and forms a little carbonic acid, which lowers pH. Water with no other ions has almost no buffering to resist this, so a small amount of CO2 shifts the pH noticeably. Water that is slightly acidic in this way is not harmful by itself.
Why is the pH of my deionized water high?
If you see 8 or more, check these causes:
- The pH electrode is dirty, old or not calibrated with fresh buffers.
- Pure water has very low conductivity, so many standard electrodes give noisy, drifting readings. Instrument makers suggest an electrode and procedure designed for low-ionic-strength samples, such as an ionic strength adjuster, and keeping the sample away from air.
- The container or tubing leaches ions, for example a dirty bottle.
- The resin is exhausted and releasing ions, or the system is poorly rinsed.
Does temperature change the pH?
Yes. The neutral point of water moves with temperature, so hot pure water reads below 7 and is still neutral. Boiled or hot DI water is not an acid for that reason.
Does deionized water change the pH of a solution?
Adding DI water to an acid dilutes it and raises its pH toward 7 (it does not neutralize it fully), and adding it to a base lowers pH toward 7. It does not hold pH steady the way a buffer does. Its alkalinity is close to zero.
What are the uses of deionized water?
- Rinsing a pH electrode and preparing lab solutions and buffers
- Steam irons, humidifiers and topping up some lead-acid batteries, where minerals leave scale or deposits (follow the product manual)
- Aquariums, but minerals must be added back for fish
- Cleaning and final rinsing of glass and equipment
It is not meant to replace drinking water, because it contains none of the minerals found in ordinary water. Do not drink lab-grade water from a system that is not designed for drinking.
For pH paper, compare the color to the chart that came with that paper. Colors differ by brand.
People also ask
Is deionized water acidic?
Often slightly, because it absorbs CO2 from the air. Freshly made it should be about neutral, and a common reading is about 5.5 to 7.
Why is the pH of my deionised water 8?
Likely a dirty or poorly calibrated electrode, an electrode not suited to low-conductivity water, a leaching container or exhausted resin.
Can deionized water neutralize an acid?
It dilutes an acid and raises the pH toward 7, but it does not neutralize it like a base would.
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