chemistry

Point of zero charge

Point of zero charge is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Point of zero charge rather than just read about it. In short: The point of zero charge (pzc) is generally described as the pH at which the net electrical charge of the particle surface (i.e. adsorbent's surface) is equal to zero. This concept has been introduced in the studies dealing with colloidal flocculation to explain why pH is affecting the phenomenon.

Key takeaways

Point of zero charge belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Point of zero charge to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Point of zero charge from memory before moving on to harder problems.

Reference excerpt

The point of zero charge (pzc) is generally described as the pH at which the net electrical charge of the particle surface (i.e. adsorbent's surface) is equal to zero. This concept has been introduced in the studies dealing with colloidal flocculation to explain why pH is affecting the phenomenon. A related concept in electrochemistry is the electrode potential 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. 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 titrations and several titration methods have 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 value for which the net surface charge of adsorbent 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 (AgI) crystals can be determined by the concentration of iodide 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 [I–] = pI−).

Relation of pzc to isoelectric point The pzc is the same as the isoelectric point (iep) if there is no adsorption of other ions than the potential determining H+/OH− at the surface. This is often the case for pure ("pristine surface") oxides 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 titrations of colloidal dispersions while monitoring the electrophoretic mobility 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 ionic strength). Once satisfactory curves 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-electrolyte interface is generally charged. If the electrode is polarizable, then its surface charge depends on the electrode potential. IUPAC 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 in a given electrolyte. IUPAC also defines the potential difference with respect to the potential of zero charge as:

Epzc = E − Eσ=0 where:

Epzc is the electrode potential difference with respect to the point of zero charge, Eσ=0 E is the potential of the same electrode against a defined reference electrode in volts Eσ=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 (that is, the particles remain stationary in an electric field), minimum stability (exhibits maximum coagulation or flocculation rate), maximum solubility of the solid phase, maximum viscosity of the dispersion, and other peculiarities.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Point of zero charge

Start with the simplest possible case. Write down what Point of zero charge claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Point of zero charge before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Point of zero charge ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Point of zero charge

In research: Point of zero charge appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Point of zero charge in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Point of zero charge is common in secondary-school and first-year university syllabi. It links to neighbouring topics Colloidal chemistry, Physical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Point of zero charge outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Point of zero charge in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Point of zero charge means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Point of zero charge out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Point of zero charge in simple terms?

Why does Point of zero charge matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Point of zero charge?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Point of zero charge.