Diagram of zeta potential and slipping planeV2

The point of zero charge (pzc) is generally described as the pH at which the net charge of total particle surface (i.e. absorbent's surface) is equal to zero, which concept has been introduced in the studies dealt with colloidal

flocculation Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from ...

to explain pH affecting the phenomenon. A related concept in electrochemistry is the

electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ( ...

at the point of zero charge. Generally, the pzc in electrochemistry is the value of the negative decimal logarithm of the activity of the potential-determining ion in the bulk fluid. IUPAC Gold Book The pzc is of fundamental importance in surface science. For example, in the field of environmental science, it determines how easily a substrate is able to adsorb potentially harmful ions. It also has countless applications in technology of colloids, e.g., flotation of minerals. Therefore, the pzc value has been examined in many application of adsorption to the environmental science. The pzc value is typically obtained by

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

s and several

method has been developed. Related values associated with the soil characteristics exist along with the pzc value, including zero point of charge (zpc), point of zero net charge (pznc), etc.

Term definition of point of zero charge

The point of zero charge is the pH for which the net surface charge of

adsorbent Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...

is equal to zero. This concept has been introduced by an increase of interest in the pH of the solution during adsorption. The reason why pH has attracted much attention is that the adsorption of some substances is very dependent on pH. The pzc value is determined by the characteristics of an adsorbent. For example, the surface charge of adsorbent is described by the ion that lies on the surface of the particle (adsorbent) structure like image. At the lower pH, hydrogen ions (protons, H⁺) would be adsorbed more than other cations (adsorbate) so that the other cations would be less adsorbed in the case of the negatively charged particle. On the other hand, if the surface is positively charged and pH is increased, anions will be less adsorbed as hydroxide ions are increased. From the view of the adsorbent, if the pH is below the pzc value, the surface charge of adsorbent would be positive so that the anions can be adsorbed. Conversely, if the pH is above the pzc value, the surface charge would be negative so that the cations can be adsorbed. For example, the charge on the surface of silver iodide crystals may be determined by the concentration of iodide ions in the solution above the crystals. Then, the pzc value of the AgI surface will be described by the concentration of I⁻ in the solution (or negative decimal logarithm of this concentration, pI⁻).

Relation of pzc to isoelectric point

The pzc is the same as the

isoelectric point The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also u ...

(iep) if there is no

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

of other ions than the potential determining H⁺/OH⁻ at the surface. This is often the case for pure ("pristine surface") oxides in water. In the presence of specific adsorption, pzc and isoelectric point generally have different values.

Method of experimental determination

The pzc is typically obtained by acid-base

s of colloidal dispersions while monitoring the

electrophoretic mobility Electrophoresis, from Ancient Greek ἤλεκτρον (ḗlektron, "amber") and φόρησις (phórēsis, "the act of bearing"), is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric fi ...

of the particles and the pH of the suspension. Several titrations are required to distinguish pzc from iep, using different electrolytes (including varying the electrolyte ionic strength). Once satisfactory graphs are obtained (acid/base amount—pH, and pH—zeta potential), the pzc is established as the common intersection point (cip) of the lines. Therefore, pzc is also sometimes referred to as cip.

Related abbreviations

Besides pzc, iep, and cip, there are also numerous other terms used in the literature, usually expressed as initialisms, with identical or (confusingly) near-identical meaning: zero point of charge (zpc), point of zero net charge (pznc), point of zero net proton charge (pznpc), pristine point of zero charge (ppzc), point of zero salt effect (pzse), zero point of titration (zpt) of colloidal dispersion, and isoelectric point of the solid (ieps) and point of zero surface tension (pzst or pzs).

Application in electrochemistry

In electrochemistry, the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

-electrolyte interface is generally charged. If the electrode is polarizable, then its surface charge depends on the

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

defines the ''potential at the point of zero charge'' as the potential of an electrode (against a defined reference electrode) at which one of the charges defined is zero. The potential of zero charge is used for determination of the

absolute electrode potential Absolute electrode potential, in electrochemistry, according to an IUPAC definition, is the electrode potential of a metal measured with respect to a universal reference system (without any additional metal–solution interface). Definition Acco ...

in a given electrolyte. IUPAC also defines the ''potential difference with respect to the potential of zero charge'' as: : where: * ''E''_pzc is the electrode potential difference with respect to the point of zero charge, ''E''_σ=0 * ''E'' is the potential of the same electrode against a defined reference electrode in volts * ''E''_σ=0 is the potential of the same electrode when the surface charge is zero, in the absence of specific adsorption other than that of the solvent, against the reference electrode as used above, in volts The structure of electrolyte at the electrode surface can also depend on the surface charge, with a change around the pzc potential. For example, on a platinum electrode, water molecules have been reported to be weakly hydrogen-bonded with "oxygen-up" orientation on negatively charged surfaces, and strongly hydrogen-bonded with nearly flat orientation at positively charged surfaces. At pzc, the colloidal system exhibits zero

zeta potential Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...

(that is, the particles remain stationary in an electric field), minimum stability (exhibits maximum coagulation or

rate), maximum

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...

of the solid phase, maximum

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

of the dispersion, and other peculiarities.

Application in environmental geochemistry

In the field of environmental science, adsorption is involved in many parts of technologies that can eliminate pollutants and governs the concentration of chemicals in soils and/or atmosphere. When studying pollutant degradation or the geochemical process, the pzc value related to adsorption has been examined. For example, natural and organic substrates including wood ash, sawdust, etc. are to be used as an adsorbent by eliminating harmful heavy metals like arsenic, cobalt, mercury ion and so forth in contaminated neutral drainage (CND), which is a passive reactor that could possible metal adsorption with low-cost materials. Therefore, the pzc values of the organic substrates were evaluated to optimize the selection of materials in CND. Another example is that the emission of

nitrous acid Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in solution, in the gas phase and in the form of nitrite () salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagent ...

, which controls the atmosphere's

oxidative Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

capacity. Different soil pH leads to the different surface charges of minerals so the emission of nitrous acid would be varied, further impacting on the biological cycle involved in the nitrous acid species.

References

{{reflist Physical chemistry Colloidal chemistry