You get the data sheet from your filler supplier and you see a pH value reported on it. But how can a solid substance have a pH, and how can it be measured? We know that we can measure the pH of an aqueous solution by using a pH meter or litmus paper, but what do you do to measure the pH of a mineral such as kaolin?
Measuring the pH of a solid
The answer is actually pretty simple. If you look up how to measure the pH of a various solid substances, you will find that the general method is to contact a specific amount of your solid with a specific volume of distilled water and then you measure the pH of the water after contact with the solid. This is the same way we measure the pH of kaolin and probably the same way other suppliers measure the pH of their fillers.
The pH of Serina kaolin is 4 – 5, which is not all that surprising given that the Cape Peninsula is famous for its acidic soils which host the famous fynbos plants like proteas, ericas and restios.
Thus this kaolin is on the acidic side of neutral which can be a benefit to certain customers. For example, in packaging adhesives, our clients report that the acidic nature of this filler helps with the bond strength of the adhesive, as well giving the normal benefits of improving crush resistance and viscosity. It also means you may not need to add extra acid to the formulation.
Other customers add Serina kaolin to a very alkaline environment, such as a paint or a soap slurry, with no detrimental effects. The overwhelming alkaline environment quickly neutralizes the kaolin's slight acidity and the kaolin gets on with the job that it was put there to do.
pH is a measure of [H+] ions in solution
It is interesting to look into why kaolin will release H+ ions into solution in the first place. The charges on a kaolin platelet look like this:
So a kaolin platelet generally has a slight negative charge overall or, put another way, kaolin has a certain cation exchange capacity(12.75 cmol/kg, in fact - we measured it recently). Of course there would be ions adsorbed onto the surface so that the overall charge is neutral. One of these ions would be the H+ cation, so when the kaolin platelet comes into contact with distilled water, these H+ cations will dissociate from the platelet and hence you would measure a pH of 4 – 5 with the pH meter.
Kaolin is a good adsorbent
The cation exchange capacity of kaolin is the reason it is used, believe it or not, in human and animal diarrhea remedies. It picks up some of the nasty compounds giving you diarrhea on its way through your body and out again (kaolin does have some fascinating uses, doesn't it?).
Kaolin is also widely used as a pesticide carrier since the active ingredient adsorbs onto the kaolin surface. The kaolin also bulks up the product and makes it easier to apply as a dusting powder or in a spray, and thus a uniform distribution of the active ingredient can be achieved.
To find out more about pH, check out this website: http://en.wikipedia.org/wiki/PH