Dear Nguyen

Non-stable pH readings can come from many causes. The most common are the bad use of combined glass-electrodes. I recommend you to check if:

1) The salt-bridge is immersed into the solution or/and,

2) The orifice for internal solution flowing is open

If any of these problems appear, the problem must be in the electrode membrane or in the potenciometer. I would recommend you to test a new electrode, first.

Nguyen,

Many factors are involved here.  Is it the probe or the preparation/measurement environment?

You didn't specify the time range of the drift or the actual buffer/acid/base being used. Temperature plays a big role in pH stability. If an acid or base is just prepared it may be warm - as it cools the pH will also change. As temperature increases the disassociation constant  also increases. Weak acids and bases will be the most affected. 

The temperature change can also affect the probe directly. 

There is also the nature of what is being measured. Bases as -OH will change to weaker carbonates when exposed to carbon dioxide in the air. As the base becomes weaker the pH will drop.

If the probe remains stable in a standardizing solution I would turn my attention to the preparation.

Many thanks all!

I think it is due to the electrode glass. I calibriated it at two pH points (6.86 and 4.01) and at constant room temperature. However, after calibrating it at pH 4.01, the pH value decreased to about 3.8. 

I measured the pH of kaolin suspensions with different salt concentrations, so what should be noted when measuring the pH of these suspensions?

Thank in advance!

Nguyen,

I work with amorphous silica and slight pH drift is not uncommon as silica/alumina becomes more dissolved and  hydroxylated in aqueous solutions above a pH >~ 2.1. As -OH is "pulled" from the solution the relative hydrogen ion concentration will rise to drop pH. The higher the pH the quicker the hydroxylation will occur . pH 7.0  to pH ~8.5 will give fastest results. Above 8.5 structural dissolution can be a problem.

If you are dealing with kaolin clay I'm sure you will see the same. It's hard to take a pH on something that slowly can re- equilibrate over time. I'd record the theoretical concentration, time in suspension, pH, and temperature. I'd do this immediately before taking the material to the next process stage.

You may have to set a time limit for the measurement along with a temperature and a drift range.  I've seen some people suspend materials overnight  with stirring and then take the pH. As an alternative you might be able to heat the material in solution and let it cool to speed up the kinetics of hydroxylation/dissolution. The temperature needs to be constant or at least compensated by the probe.

I have also found these materials tend to plug the frits of the probe over time. Minimizing probe exposure until needed is a good idea. I'd also rinse the probe well (after removal) inside and out as well as replace the electrode solution if it continues to drift excessively. Silicic and aluminic acids are very hard on the probe frits and some metal based filler solutions.

http://www.clays.org/journal/archive/volume%2028/28-6-412.pdf