On a scale from 0 to 14, how much would we know about pH levels and the pH chart? The main question that comes to our mind   What is the pH of Coca-Cola and what does Coca-Cola’s pH (or the pH of a solution in general) measure?  The article here talks about all what we need to know about pH measurement.

What does the pH of Coca-Cola measure?

A pH value refers to the concentration of hydrogen ions (H+) in a solution. The strength depends on the quantity of H+ dissociated in the solution. Strong solutions, like hydrochloric acid, release many; they dissociate almost completely. In weak solutions, like pure water, they dissociate only a little.

The hydrogen ions combine with water molecules to form hydronium ions (H3O+):

H2O + H2O ⇔ H3O+ + OH

The H3O+ concentration matches the H+ concentration, so we can say:

H2O ⇔ H+ + OH

As for the pH value, we can work with this formula:

(pH = -log [H+]), e.g. – log 10-7 mol [H+] = 7 pH

pH levels
Courtesy of Braukaiser

What is Coca-Cola’s pH?

As sophisticated as the human sense of taste is, it can’t accurately judge pH. What would we consider more sour, a lemon or a Coca-Cola? Naturally, we would say lemon and guess that it has a pH lower than the pH of Coca-Cola.

The sugar in the Coke fools us into thinking Coca-Cola is not acidic but it doesn’t actually affect pH levels. This means that we can’t really tell Coca-Cola’s pH just by tasting it. Lemon has a pH of around 2.5, but the pH of Coca-Cola hits 2.3 because of all the phosphoric acid coke contains. And this is only one of the reasons why the world needs accurate pH measurement.

When it comes to alkaline drinks, we won’t find many. Most fruit juices skew acidic, and even milk has a pH close to neutral. One drink that sits on the right side of the pH scale is green tea. However, alkaline fruits and vegetables can help balance our body’s pH. Speaking of pH in the body, we have an important liquid in our bodies which should have its pH controlled – blood. The blood pH should sit somewhere between 7.35 and 7.45.


pH chart
Courtesy of Seneye

Considering that the pH in drinking water is a regulated resource in most countries, perfectly neutral water would have a pH of 7.0. However, tap water pH commonly rises above 7.0. A value below 7.0 could harm the people drinking it, not only from the acidity but also from metals dissolved from pipes damaged by the acid. On the other hand, a pH above 8.50 could harm us as well. Besides the effect on our bodies, it would weaken disinfection agents in water.

Other applications of pH Measurement

We can find pH measurement in many processes, from making cheese and producing antibiotics to creating fertilizers and mining gold. In fact, we have a video explaining how companies use pH measurement in gold production.

Measuring the pH of a solution – the pH electrode

A pH electrode with one measuring cell and one reference cell is the best sensor to date for finding the pH of a solution. When we immerse the electrode in a liquid, an electrical potential proportional to the liquid’s pH value builds between the two cells. Measuring this potential gives us the basis for all modern pH measurements, and the same goes for measuring the pH of coke.

The pH measuring half-cell

This half-cell has a glass membrane that, when exposed to water, forms a thin swelling layer that creates an ion exchange. We call this reaction hydrolysis. The H+ concentration in the layer changes with the pH value of the solution. This charging and discharging process continues until it creates an H+ equilibrium between layer and solution.

In the case of strongly hydrolyzed glass – due to high temperatures or alkaline applications, for example – the thickness of the swelling layer increases and the charging process takes longer. The same process occurs inside the membrane, except the H+ activity stays the same due to a fixed internal buffer.

The glass both separates the inner and outer swelling layers and connects them. The different surface potentials lead to a potential difference between the inside and the outside. To measure this difference requires zero current and a high-resistance millivolt meter.

pH measuring half-cell
Courtesy of Visaya.solutions

Depending on where and how you want to measure, different types of electrodes have different types of glass. Laboratories usually want fast responses and will need some kind of glass with low impedance. But this kind of glass wouldn’t last in a harsh environment.

The pH reference half-cell

The reference electrode must create a stable electrical baseline over time for its measurement. This is definitely the most vulnerable and complicated part of a pH electrode’s measuring chain as far as stability. Unlike the pH-sensitive glass membrane, the reference electrode comes in direct contact with the medium via a diaphragm, gap, or hole.

Therefore, the electrode needs a contact point made of a chemical-resistant material. Most reference electrodes today use silver wire coated with silver chloride, which connects with the solution using an electrolyte (salt solution).

pH reference half-cell
Courtesy of Visaya.solutions

This electrolyte, based on potassium, chloride, and silver ions, gives the reference system a stable environment. It also provides an electrical bridge with the solution over the diaphragm. The electrolyte can be a liquid, a half-liquid gel, or a solid polymer.

What is a pH meter?

A pH meter is a device that we connect to a pH electrode to convert its signal into a pH value. Usually, it has a display, where we can read the value, and some kind of output signal. As we’ll learn later, the temperature can influence the measurement and the electrode. Therefore we’ll also find on the pH meter a connection where you can add a temperature probe. In most cases, we can also read the temperature on the display.

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