pH sensors accuracy

[kerberus13]

Hello everyone,

I am new in the forum and I am looking forward to learn a lot from here.

My native language is spanish, so correct me, and ask me to rephrase if needed.

 

Here´s the question:

I got two different pH sensors that are calibrated with the same calibration solutions. However, when measuring the same solution the difference in measures is approximately 0,2. for example:

 

-Phosphate buffer measured with pH sensor 1: 4,74

-Same Phosphate buffer measured with pH sensor 2: 4,90

 

Is this difference a big deal?

Why is it caused?

can I fix it?

many thanks

[Sensei]

Each measuring device has some tolerance. Not just pH sensor, but all devices.

Tolerance should be mentioned on the box, manual, or on-line specification.

From what I see on-line, pH sensors have often +-0.2 pH accuracy.

f.e.

https://www.campbellsci.com/cs526-specifications

"Accuracy: ±0.2 pH over 10° to 40°C"

 

Check manual/specification.

Edited June 2, 2015 by Sensei

[CharonY]

It will depend on your application, but e.g. for molecular biological work an accuracy of ± 0.2 would be rather unusable. But assuming they are out of spec, you should check the quality of the probes (have they dried out, do they need cleaning, have they been stored properly etc.) and also check whether the temperature sensor (if present) is working properly.

[kerberus13]

It will depend on your application, but e.g. for molecular biological work an accuracy of ± 0.2 would be rather unusable. But assuming they are out of spec, you should check the quality of the probes (have they dried out, do they need cleaning, have they been stored properly etc.) and also check whether the temperature sensor (if present) is working properly.

 

how about in cell culture and fermentation?

[CharonY]

For most applications in this area you need a good control of pH, i.e. a higher accuracy probe. As Sensei mentioned, check the manuals of your system, and you can also measure repeatedly the pH of reference solutions to see which is more off.

[John Cuthber]

Most pH sensors are rather sensitive to temperature as well as pH so make sure that you are measuring the pH at the same temp.

Also, as long as the sensors are stable you can produce a calibration table; you just need some solutions with known pH.*

Depending on your resources, that might be easier than getting a better meter.

 

 

* that's not a "catch 22".

http://delloyd.50megs.com/moreinfo/buffers2.html

 

Not only does the sensor change with temperature, so does the pH of a solution. The pH of pure water is near 7 at room temp, but near 6 when it's boiling so the difference between a pH at 20C where you calibrate the probe and 37C in a fermenter might be more than 0.1 pH units.

Edited June 2, 2015 by John Cuthber