What is the pH of RO water with zero TDS?

[HawkeyeDJ]

So I've invested in a Hanna pH meter that apparently requires calibration to perform accurate readings. The 7.0 calibration fluid seems a bit expensive to me.

As I recall from my few chemistry and biology courses, the pH of pure water is 7.0.

Is RO water with zero TDS pure? If so, shouldn't it read 7.0? And if it does, what is the rationale for the calibration fluid over RO Water?

What am I missing?

 

[bushdoc]

pH of pure water is 7.0, but RODI water may contain some dissolved CO2, apparently some wells contain increased amount of Dissolved CO2.
CO2 presence would cause RODI water to be slightly acidic.
Quality distilled water would also have pH of 7.0
Perhaps using calibration solution is better option.
Maybe Randy would help you better.

 

[dwest]

So I've invested in a Hanna pH meter that apparently requires calibration to perform accurate readings. The 7.0 calibration fluid seems a bit expensive to me.

As I recall from my few chemistry and biology courses, the pH of pure water is 7.0.

Is RO water with zero TDS pure? If so, shouldn't it read 7.0? And if it does, what is the rationale for the calibration fluid over RO Water?

What am I missing?

Individual pH packets are about $1

Pinpoint PH Fluid 7.0 | Premium Aquatics

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[clownfish81]

So I've invested in a Hanna pH meter that apparently requires calibration to perform accurate readings. The 7.0 calibration fluid seems a bit expensive to me.

As I recall from my few chemistry and biology courses, the pH of pure water is 7.0.

Is RO water with zero TDS pure? If so, shouldn't it read 7.0? And if it does, what is the rationale for the calibration fluid over RO Water?

What am I missing?

Pure water is 7.0 but there maybe dissolved CO2 which can change PH

You should just buy ph packets

Milwaukee pH Calibration Fluid - Single Use Packet

Milwaukee Calibration Solution sold in single use packets are great for calibrating your pH probe or pen. Many pH probes will start to loose their calibration in as short as a month or two depending on your probe. We suggest calibrating probes every 30-60 days for the most reliable readings...

www.bulkreefsupply.com

$1 for 3 different PH to calibrate
more accurate

 

[KStatefan]

As with most chemistry topics in this hobby @Randy Holmes-Farley has an article on it. scroll to the end for the PH part

"2. The pH of highly purified water is not accurately measured by test kits, or by pH meters. There are several different reasons for this, including the fact that highly purified water has very little buffering capacity, so its pH is easily changed. Even the acidity or basicity of a pH test kit’s indicator dye is enough to alter pure water’s measured pH. As for PH meters, the probes themselves do not function well in the very low ionic strength of pure freshwater, and trace impurities on them can swing the pH around quite a bit."