The method is revised. You may find ample information at

http://books.google.com/books?id=iP7LWpn5oJ0C&pg=SL2-PA250&lpg=SL2-PA250&dq=method+determine+the+point+of+zero+charge+of+soils&source=bl&ots=rWMEK9X2UD&sig=ngZD6Pnwu7YbwwsomVfBkhKrVzM&hl=en&sa=X&ei=uLSBVO6zJoi1UcPlgNAP&ved=0CFgQ6AEwCA#v=onepage&q=method%20determine%20the%20point%20of%20zero%20charge%20of%20soils&f=false

Murtaza, G., A. Ghafoor, M. Qadir and S. Muhammed. 2000. Charge characteristics of soils as affected by pH and neutral electrolytes. J. Indian Soc. Soil Sci. 48:433-437.

ICP Mass spec analyses of the mineral/elemental values in the soil will do all that without wet-chemistry and the possibility of mistakes and evolutionary compounds.

By having the actual elemental values the computer program then calculates the cross-valences of the elements and can be used to extrapolate values accordingly. You merely plug in the "c" unknown with the basis of reference and you will have all the possible scenarios associated with the elements found in that soil.

Your method of titration, over a course of time, elicits movement in ions and electrons from one compound to the next and creates new compounds and differing valences, not found in the soil, that affect the calculations. Both HCL and NaOH will form molecules that continue to grow as the valences across the entire gradient change over a period of time.

I guarantee that you are having problems with your resultants when analyzed over a course of time. Yes?

If you need more help on this contact me.

You might like to have a look at the following papers:

Meijer E L, Buurman P 1987 Salt effect in a multicomponent variable charge system: curve of zero salt effect, registered in a pH-stat Journal of Soil Science 38, 239–244.

Fernandez Marcos M L, Meijer E L, Buurman P 1990 Variable charge in natural multi-component systems: line instead of point of zero charge, blocking effects. Proc. 14th Int. Congr. Soil science, Kyoto, Japan. 01/1990; 2(Commission 2):2 pp.

Fernandez Marcos M L, Buurman P, Meijer E L 1998 Role of organic matter and sesquioxides on variable charge of three soils from Galicia, Spain Communications in Soil Science and Plant Analysis 29, 2441-2457.

 It is shown in them that soils containing high amounts of both organic matter and free iron oxides are characterized by a so called Curve of Zero Salt Effect (CZSE) rather than a Point of Zero Salt Effect (PZSE) or point of zero net charge (PZNC). Maybe this is the case of your soils.

Probably you are seeing the effects of incumbent acid-neutralising capacity (ANC), hence the acid-base chemistry reactions are shifting your titration curves.  You could run back titrations (eg. adjust down to pH 4, then up to pH 12, and down again).  At the least this will confirm whether ANC is the issue.  However, you may be changing your mineralogy and structure by doing this.  Perhaps what you are seeing is simply the reality that soils, being complex, multi-mineral systems + organics do not have one IEP or PZC, but are constantly interacting internally to balance internal and external charge.  So if you dissolve some minerals or precipitate others, the PZC changes.  Also, surface complexation can change the PZC, eg. nickel attached to iron can shift the PZC by 2 pH units.  Basically, your material is changing, so the net bulk PZC changes also.

We have successfully used both Electroacoustic Spectroscopy and Dynamic Light Scattering techniques to directly measure the zeta-potential and surface charge, respectively of soils, as well as inorganic multi-mineral systems. Also by titrating in electrolyte solutions.  These techniques measure the electrostatic charge directly, rather than surmising from acid-base titrations.  There may still be issues with ANC meaning that we see the effects of excess H and OH reactions, rather than the reactions of the mineral surfaces, which again may be addressed by first removing the incumbent ANC.

Article Electroacoustic Isoelectric Point Determinations of Bauxite ...

Please see method in these below papers. You might find them useful

- DETERMINATION OF THE POINT-OF-ZERO CHARGE OF MANGANESE OXIDES WITH DIFFERENT METHODS INCLUDING AN IMPROVED SALT TITRATION METHOD

- Comparison of salt titration and potentiometric titration methods for the determination of PZC