pH Meter

[Sitaram Barhate]

My sample readings on different pH instrument are showing variation of around 0.3 whereas my standards in the same range showing readings within ±0.03
I have tried on different makes of pH meter and electrodes although I am getting this variation in readings of only samples not for standards.
Why this variation is there for only samples( Injectable product)

[HW Mueller]

Did you calibrate the different pH meters and different electrodes with your standard(s)?

[rhaefe]

I am not an expert in pH measurements but are the ionic strenths of your samples much lower than the standards?
Very low ionic strengths can cause rather large variations in pH readings. If this is the case addition of a neutral electrolyte (like sodium nitrate or sodium bromide) might help.

[Sitaram Barhate]

Thanks for reply