[cmquer]

The principle of electrometric pH is the determination of the activity of the hydrogen ions by potentiometric measurement using a glass pH indicating electrode coaxially joined to a reference electrode.

When I used buffer solutions of pH 7 and 10 as reference, I found the pH of my sample solution is 2.83, however, when I used buffer solutions of pH 7 and 4, I found the pH of my sample is 3.31.

Can anyone explain this? THANK YOU.

[Alpha-Omega]

1.  Are your standards good?
2. Are they both at thye same temperature?
3.  Are you using a stir bar while analyzing your sample?
4.  What are the four calibration points for each refeRENCE STANDARD
5.  You should be calibrating at pH 4 and pH 10...that should be your std bracket..

There are two different ways that temperature impacts pH measurement. The first involves actual chemical changes in the solution that you are measuring. Acids can, for example, become stronger or weaker as the temperature is changed. This is how calibration standards change their pH as a function of temperature (which is discussed in more detail in the calibration section below). If the solution has solids in contact with it (as is the case with saturated limewater in the presence of excess solids), the temperature can also impact how much acid or base is in solution impacting pH, and how much is just solid sitting on the bottom of the container. These effects are specific for every solution that you will encounter, and there is nothing general that one can or should do about this, except be aware that it happens.

The second impact of temperature is on the pH electrode itself. pH electrodes change their response in a very clear way as temperature is changes. They respond more strongly to pH changes at higher temperature than at lower ones. At 100 ºC, they change their output potential by 74 mV/pH unit, and at 0 ºC, they change by 54 mv/pH unit. Because pH meters are typically standardized at pH 7 (that is, zero mv = pH 7), the error from temperature differences gets greater and greater as the pH being measured gets further from 7. So it may be trivial when measuring something with a pH of 7.1, but very important when measuring something with a pH of 10 (or when calibrating with a pH 10 buffer).

There are usually three different ways of taking temperature into account. One is to make measurements close to the temperature at which you calibrated the meter (say, within a few degrees). The second is to "tell' the meter what the temperature is (digitally or with a dial). The third is that some meters have a temperature probe, usually called an ATC, which you stick into the measuring solution. This probe reports the temperature back to the meter, and the meter makes any necessary corrections (for this type of temperature effect).

As long as you use one of these three ways of dealing with temperature issues, you will get reasonably accurate readings.

[cmquer]

Firstly, thank you Alpha-Omega for your clear and detail explaination.

But actually what I want to know is, except the temperature, are the reference buffer solutions play roles in the differences of the measured pH of my sample?

The procedure is, first put the electrode into the pH 7 buffer, then to the pH 10 one and then to my sample. The second run is the same except changing the pH 10 buffer with the pH 4 one.

My consideration is the pH 7 and pH 4 buffer had some effects on the electrode and made it show different pH value for a same solution.

[Borek]

Calibration assumes perfect linear response of the electrode (ie electrode potential = k * pH). That's never the case, although good electrodes are close. Because the answer is never perfect, you should calibrate the meter with buffers as close to the measured point as possible.

Look at the attached picture. It tries to show how it works. Orange line is the potential given by the electrode in your solution. You read potential and you convert it to pH using calibration curve. If the electrode is perfect, calibration curve looks like the stright red line - no problems then, you read your pH at A point, where the red and orange linges cross. But in reality dependence between pH and potential can be anything, it may look as well as the blue line. If you calibrate electrode with buffers 4 and 7 - you will read pH at B point, where orange and steep green line cross (steep green line is your calibration curve determined by buffers 4 and 7). Your pH is close to the real one. But if you calibrate using buffers 7 and 10 and you read pH at C point you are completely off. Note, that using upper green calibration line in the range 7-10 (where the range is bracketed by calibration buffers) you are always reasonably close to the real pH.

There is no risk of pH 7 and pH 4 calibration buffers modifying the electrode properties, if any, high pH solutions can have some effect as they may dissolve the glass. Still, pH 10 buffer is a standard that won't hurt typical pH electrodes.

[Arkcon]

The convention I've stuck to is that you can't report pH unless you calibration std flanks your unknown's result.  So if you have to take similar pH's, you'll need to purchase some pH 1.0 standard.  It's pretty common.  Borek and Alpha-Omega tell you why.  My post is just a little industry conventions heads up.