Diagram of zeta potential and slipping planeV2

The point of zero charge (pzc) is generally described as the pH at which the net

electrical charge Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

of the particle surface (i.e.

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 ...

's surface) is equal to zero. This concept has been introduced in the studies dealing with

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...

flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...

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

electrode potential An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can c ...

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 Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid– gas interfaces, solid– vacuum interfaces, and liquid– gas interfaces. It includes the ...

. For example, in the field of environmental science, it determines how easily a substrate is able to

adsorb 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 ...

potentially harmful ions. It also has countless applications in technology of

s, e.g., flotation of minerals. Therefore, the pzc value has been examined in many application of

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 a ...

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 research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...

s and several titration methods have been developed. Related values associated with the

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

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 value for which the net surface charge of

is equal to zero. This concept has been introduced by an increase of interest in the pH of the solution during adsorption experiments. The reason 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 a lower pH, hydrogen ions (protons, H⁺) would be more adsorbed than other cations (adsorbate) so that the other cations would be less adsorbed than 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 pH increases. From the view of the adsorbent, if the pH of the solution is below the pzc value, the surface charge of the adsorbent would become 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 electrical charge on the surface of

silver iodide Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a grey colouration. The silver contamination arises because some samp ...

(AgI) crystals can be determined by the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

iodide An iodide ion is I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency ...

ions present in the solution above the crystals. Then, the pzc value of the AgI surface will be described by a function of the concentration of I⁻ in the solution (or by the negative decimal logarithm of this concentration, -log₁₀ ^–= ''p''I⁻).

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 electric charge, electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). Howe ...

(iep) if there is no

of other ions than the . This is often the case for pure ("pristine surface")

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

s in suspension 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 is the motion of charged dispersed particles or dissolved charged molecules relative to a fluid under the influence of a spatially uniform electric field. As a rule, these are zwitterions with a positive or negative net ch ...

of the particles and the pH of the suspension. Several titrations are required to distinguish pzc from iep, using different supporting electrolytes (including varying the

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

ionic strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such a ...

). Once satisfactory curves are obtained (acid/base amount—pH, and pH—

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.is a scientific term for Electrokinetic phenomena, electrokinetic Electric ...

), 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

initialism An acronym is a type of abbreviation consisting of a phrase whose only pronounced elements are the initial letters or initial sounds of words inside that phrase. Acronyms are often spelled with the initial letter of each word in all caps wi ...

s, 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 a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

-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 A reference electrode is an electrode that has a stable and well-known electrode potential. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. ...

) 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

. 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 A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

exhibits zero

(that is, the particles remain stationary in an

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

), minimum stability (exhibits maximum coagulation or

rate), maximum

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

of the solid phase, maximum

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

of the dispersion, and other peculiarities.

Application in environmental geochemistry

In the field of environmental science,

is involved in many techniques that can eliminate pollutants and governs the concentration of chemicals in soils and/or atmosphere. When studying pollutant degradation or a sorption process, it is important to examine the pzc value related to adsorption. For example, natural and organic substrates including wood ash, sawdust, etc. are 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. It was discovered by Carl Wilhelm Scheele, who called it " phlogisticated acid of niter". Nitrous ac ...

, which controls the atmosphere's

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

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