The pH of clay suspensions in the field and laboratory, and methods of measurement of their pH

doi:10.1016/0169-1317(90)90018-K

Applied Clay Science

Volume 5, Issue 2, August 1990, Pages 123-133

https://doi.org/10.1016/0169-1317(90)90018-K Get rights and content

Abstract

The pH of water standing in pools in deposits of kaolin-family clays tends to be low, i.e. pH 4 to 5. Streams draining such clay-containing areas commonly also yield pH values below 7, whereas streams flowing over primary, and clay-parent, silicate rocks typically yield a pH of 7 or higher - hence testing the pH of streams may aid in clay-prospecting in igneous country-rock areas.

Laboratory-prepared suspensions of clays, simulative of natural clay sols, are acid within the kaolin family but may be alkaline with Na-smectite. Measurements of pH made on the visually clear supernatants of clay sols using colorimetric test paper and glass-electrode methods indicate that the colorimetric method yields the most representative results; apparently the multitude of clay-surface properties and charge effects from clay in even the visually clear sol override and mask the specific electrical properties by which an electrometric method measures the pH value of a clean ionic-molecular solution.

References (13)

D.P. Brezinski
Influence of colloidal charge on response of pH and reference electrodes: the suspension effect
Talanta
(1983)
K.I. Cheng
pH glass electrode and its mechanism
I. Feldman
Use and abuse of pH measurements
Anal. Chem.
(1956)
W.H. Grant
Abrasion pH, an index of chemical weathering
Clays Clay Miner.
(1969)
W.D. Keller
Acidity-alkalinity of surface drainage waters as related to underlying silicate rocks
J. Sediment. Petrol.
(1960)
W.D. Keller
The sedimentology of flint clay
J. Sediment. Petrol.
(1981)

There are more references available in the full text version of this article.

Cited by (17)

A comprehensive study on modified-pillared clays as an adsorbent in wastewater treatment processes
2021, Process Safety and Environmental Protection
Pillared clays are special kinds of modified clays with significant and permanent porosity, produced by embedding columns or pillars between the layers of clay. However, these materials are very well-known for their catalytic properties; their adsorptive characteristics in wastewaters towards different classic and growing contaminants need to be studied comprehensively. The present work is performed to introduce different methods for clays' pillaring, the main steps of the process, comparison of various pillaring agents, etc. During recent years, several investigations have been carried out to find a way for pillared clays' modification (by introducing active chemical functional groups, doping, heat treatment, etc.) to achieve selective adsorbents towards specific pollutants. Therefore, the main modification techniques including surfactant modification, metal doping, etc. are introduced and classified in the present work. Furthermore, adsorptive applications of (modified) pillared clays towards organic, pharmaceutical, and metal ions pollutants, the relationship between physicochemical properties and adsorption performance of pillared clays, adsorption mechanisms, and quantitative comparison of pillared clays' ability for pollutants removal are the other issues that have been considered in the present work.
Purification of natural clays
2013, Developments in Clay Science
Citation Excerpt :
The pH must not decrease below 4.5; control with a glass electrode is not always reliable because the electrode often does not indicate correct pH values in the presence of clays. Indicator strips are more reliable (Keller and Matlack, 1990). Recommended is an acetate buffer solution of pH = 4.8 (2 M sodium acetate + 2 M acetic acid) instead of hydrochloric acid.
Raw clays and clay minerals are worldwide abundant but are never pure, except vermiculites or micas. This is very damaging for their applications in different areas. The presence of impurities complicates the full scientific characterization of clay minerals. Relatively simple techniques are used in industrial processing of clays (see Chapter 4.2 in Volume B). This chapter reports the basic principles of purification of clays and describes reliable methods and recommended procedures to obtain relatively pure samples of clay minerals. The complex purification process consists of the decomposition of carbonates, dissolution of (hydr)oxides, oxidation of organic material, dissolution of silica, dialysis, and fractionation. Also reported are a simplified purification method and a pilot purification technique.
Chapter 4 Synthetic Clay Minerals and Purification of Natural Clays
2006, Developments in Clay Science
Citation Excerpt :
The pH must not decrease below 4.5, control with a glass electrode is not always reliable because the electrode often does not indicate correct pH values in the presence of clays. Indicator strips are more reliable (Keller and Matlack, 1990). Recommended is an acetate buffer solution of pH=4.8 (2 M sodium acetate+2 M acetic acid) instead of hydrochloric acid.
This chapter describes synthetic clay minerals and purification of natural clays. A complete purification cycle is time consuming. However, not all the purification steps are always necessary; steps can be omitted depending on the purpose of the investigation. In contrast to soil clays, clays from geological deposits (such as bentonites, and kaolins) commonly contain only small amounts of organic materials. As such, oxidation with H₂O₂ or peroxodisulphate may not be required and the simplified procedure may be satisfactory. Similarly, silica does not always act as a cementing agent, and it can be removed without chemical treatment. Even small amounts of carbonates, (hydr)oxides, and organic materials can strongly influence the colloidal and rheological properties of clay pastes, slips, and dispersions. The chemical reactions involved in clay mineral purification may also cause changes to the clay mineral structure. In selecting a given purification procedure, the effect of different reactions must be duly considered.
The negative charge of nanoparticles
2006, Microchemical Journal
Recent development of non-faradaic potentiometry
2002, Microchemical Journal
Non-faradaic potentiometry has been plagued by a great many fundamental errors and a lack of conceptualization. Of greatest concern is the second Nernst equation hiatus. Potentiometry may be generally classified as faradaic and non-faradaic. The former deals with the redox reactions using the Nernst equation to explain the potential origin. The latter deals with the non-redox reactions using the Boltzmann and modified Boltzmann equations to explain the origin of electrode potential. Redox faradaic potentiometry has been well described in the textbooks. However, non-faradaic potentiometry has been almost completely neglected in the literature. Many well-known electrodes, such as the pH glass electrode, common reference electrodes, and ion selective electrodes (ISE) have been mistakenly interpreted as redox reactions or ion exchange reactions. New theories and experimental results show their mechanisms to be non-faradaic in nature. Furthermore, the reaction mechanisms for ISE have been confused in textbooks with redox reactions and the Nernst equation. The ISE potentials originating from adsorption of ions or charged particles based on surface charge density will be explained using the double and counterion triple layers concept. The new counterion triple layer concept may be applied to the potential development of sensors. The reason for a new concept, theory, or mechanism is to better explain the phenomena. Examples will be given of how our new concept explains the capacitor, counterion triple layer, surface adsorbed layers interactions, and the interface structure. We will also discuss the new sensor development based on the new adsorption concept. For the first time a new type of Ag/AgCl reference electrode for non-faradaic potentiometry will be presented, one without a liquid junction and with a Pt wire instead of a salt bridge. They will help open up a new horizon for electrochemical sensor research and may be used under unusual conditions, such as high temperature and high pressure, stirring, etc.
Effect of pH on rheological properties of purified sodium bentonite suspensions
1999, Journal of Colloid and Interface Science
The effect of pH on the rheological behavior of three purified sodium bentonite (HP, GP, and WP) suspensions is studied. Each clay is studied as a function of pH, at a chosen constant concentration. These concentrations correspond to the lower concentrations at which the yield stress can be detected. They are 12.5, 8.5, and 5.5% w/w, respectively, for the three clay suspensions. The natural pH of these samples measured by a pH meter or by a colorimetric test paper are usually neutral or weakly basic. To obtain a large range of pH values, HCl or NaOH solutions are used in the preparation of the suspensions. From the natural pH and when pH values become more basic, the yield stress measured, using the vane method, decreases. It reaches a minimum before it increases sharply, at very high basic medium. When the pH decreases, the yield stress increases and reaches a maximum for pH in weakly acidic media before decreasing again, in a very highly acidic medium where the structure of the clay is probably attacked. The pH where the maximum appears depends on the nature of the clay. At the same time, the conductivity and the electrophoretic mobility are measured, at the different pH values, on the supernatants obtained after centrifugation of the previous suspensions. The isoelectric points of the edges of the three studied purified sodium bentonites, which are of a great importance for the understanding of the interactions between the clay particles, have been determined. In summary, a correlation between rheological and electrokinetic results, is shown. And moreover, different types of interactions between clay particles are deduced, depending on the pH value of the medium. At acidic medium, there is a dominance of the attractive forces between particles. At high basic medium, interparticle interactions lead to a card-house-like structure based on edge to edge, edge to face, and face to face repulsion, instead of attraction.

View all citing articles on Scopus

^∗1: Present address: School of Geophysical Science, Georgia Institute of Technology, Atlanta, GA 30332 (U.S.A.).

View full text

The pH of clay suspensions in the field and laboratory, and methods of measurement of their pH

Abstract

Talanta

pH glass electrode and its mechanism

Use and abuse of pH measurements

Anal. Chem.

Abrasion pH, an index of chemical weathering

Clays Clay Miner.

Acidity-alkalinity of surface drainage waters as related to underlying silicate rocks

J. Sediment. Petrol.

The sedimentology of flint clay

J. Sediment. Petrol.