The pH meter keep saying 1 M sodium phosphate buffer pH 7.0 is pH 6.5?

Oh Jeonghun @Oh-Jeonghun

02 August 2025 1 7K Report

Our lab use thermoscientific pH meter orion 3-star.

I made 1 M sodium phosphate buffer following NEB protocols:

Solution A: Dissolve 138.0 g NaH2PO4–H2O in 1 liter dH2O (pH 7.0).

Solution B: Dissolve 142.0 g Na2HPO4 in 1 liter dH2O (pH 7.0).

Mix 423 ml Solution A with 577 ml Solution B.

Autoclave and store at room temperature.

However on step 3, the pH meter keep saying that the pH is 6.5-6.6. Calibration went well.

I double-checked the mass and the stock while making.

I asked this phenomenon to chatGPT and it says that glass electrode pH meter often says it's pH 6.5 although the actual pH is 7.0.

Is that right? Are there anyone who experienced this issues like me?

Mark Krause

You could be seeing the effect of sodium on the electrode. I wouldn't think so but it may be. I'd dilute it 10x and try it again to see if you get the same result. If you don't then you are probably seeing sodium interference.

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