# calibrate at 7ph first then 4?



## Rasmusm (20 Jan 2012)

Hi

I'd like to ask if its an golden rule to first calibrate at 4.01ph and then 7.01?
My profilux ask for 4ph water first when i calibrate my electrode. But i read somewhere that if calibrating with 4+7water always do 7 then 4. If calibrating with 7+10 water do 7 then 10.

Reason I ask, my tank operate at 6.70ph and calibrating with 7.01 first will give a more acurate pH value?

Thanks 
Regards Rasmus


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## dw1305 (20 Jan 2012)

Hi all,
Depends upon the pH of the liquid that you are going to test, if it is in between pH 4 - pH7 you  use pH4 followed by pH7, if the pH is above pH8 you use pH7 and pH10 buffers.

You start with pH4 (1 x 10-4 H+ ions). Due to the log10 nature of the pH scale you should be able to calibrate the meter exactly to pH4, pH7 is more problematic as you have equal numbers of OH- and H+ ions (1 x 10-7 H+ & OH- ions) and pH is the ratio of H+:OH- ions. Because of this factr if you can't get the pH to read exactly pH7, in the pH7 buffer, it isn't as important. You need to blot the electrode between readings and give it a rinse with some DI or RO water.

Dependent upon the meter you may have to manually set the temperature and then adjust the meter to pH4 & pH7 with a small grub screw. More expensive meters tend to have automatic temperature compensation and buffering, or at least a dial you can turn for buffering.

cheers Darrel


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## Rasmusm (20 Jan 2012)

Thanks alot for your answer


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