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Topic: Does the order of buffers matter when standardizing a pH meter?  (Read 5861 times)

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Offline Babcock_Hall

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Does the order of buffers matter when standardizing a pH meter?
« on: January 08, 2015, 05:08:13 PM »
Let's assume for simplicity that one is doing a two-point calibration at 25 °C.  Is it preferable to start with the pH 7 standard then go to the pH 4 standard buffer?  Or is the opposite order equally effective?  If I am not mistaken, pH 7 is the isopotential point for most electrodes.

I have asked several people with more experience in electrochemistry than I have, and the consensus was:  If an electrode is not performing well, you will see it right away with the pH 7 buffer, which will not be near zero millivolts.  However, the order should not matter otherwise.  I have also consulted some product literature, and one source of information (Orion) said that usually one starts with the pH 7 buffer.  This link also suggests starting with the pH 7 buffer:  http://www.ph-meter.info/pH-electrode-calibration

Offline Arkcon

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Re: Does the order of buffers matter when standardizing a pH meter?
« Reply #1 on: January 08, 2015, 05:31:42 PM »
Our Hanna Instruments also starts at pH 7.0, then asks for 10.0, then 4.0., 1.68 and finally 13.0.  (The instrument accepts 5 calibration points.)  Once we have a slope, I re-verify the pH of each buffer.  I tend to go 13.0, 10, 7, 4, and 1.68 when I'm just reading them for verification.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Archy12345

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Re: Does the order of buffers matter when standardizing a pH meter?
« Reply #2 on: January 12, 2015, 09:58:27 PM »
The order does matter.
7.0 is first so that an "origin" is defined. Then, you use additional buffers to establish the slope of the pH line for the meter. If you are working in only acidic conditions, you only need to use 4.0. If you are only working in basic solutions, 10.0 or 11.0 whichever you have. If you are working in a range of acidic and basic solutions, it's probably a good idea to use the acidic buffer first as basic solutions tend to stick to the SiO2 structure of the glass.

Offline UKInstrumentation

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Re: Does the order of buffers matter when standardizing a pH meter?
« Reply #3 on: January 13, 2015, 10:27:42 AM »
It does matter - effectively in a 7.00 pH buffer the mV generated should be zero - so normally the zero point calibration should be carried out first. This can help ascertain if there are any stray voltages or RF interference.

Normally for industrial equipment the instrument will require a two point calibration (Normally 4 & 9 or 4 & 10) to ensure that you're seeing a linear relationship across the pH decades (even though pH is logarithmic) although it's often wise to measure the pH close to the process you're measuring or controlling. Temperature has an effect too - normally the ATC will trend toward 0mV or 7.0pH as the temperature increases due to the increased activity of the hydrogen ions at increased temperature (unless you're measuring above 13.00pH in which case you have a whole new bag of problems).

I wouldn't recommend anyone uses a pH meter which has a single point calibration and I would also strongly recommend that the buffer solutions you use are traceable to NIST standards or the pH meter is calibrated inline with your local equivalent of UKAS.

Of course the secret to accurate pH measurement is all in the electrode…

 

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