Why is p in lowercase and H in uppercase in "pH value"?

pH value, also known as hydrogen ion concentration index, is a method of expressing hydrogen ion concentration.

It is the negative value of the common logarithm of the hydrogen ion concentration (activity) in aqueous solution, which is -lg[H+].

For the determination of hydrogen ion activity index, qualitative methods can be measured by using pH indicator and pH test paper, while quantitative pH measurement requires the use of a pH meter.

ionization of water

Water is an extremely weak electrolyte and can undergo weak ionization. Its ionization equation is: H2O+H2O⇌H3O++OH-

The abbreviation is H2O⇌H++OH-, which is an endothermic process.

Under normal circumstances (around 25°C, 298K), the closer the pH value is to 0, the more acidic the solution is, and the closer the pH value is to 14, the more alkaline the solution is. When the pH value is <7, the solution is acidic. When the pH value is >7, the solution is acidic. When the pH is 7, the solution is alkaline, and when the pH is 7, the solution is neutral.

pH represents the concentration of hydrogen ions in the solution, c (H+), and is used to determine the pH of the solution. However, in fact, the concentration of hydrogen ions in the solution cannot be measured, so pH≈-lg[c (H+) is generally defined operationally. )/(mol/L)], and lists the reference pH standard values.

Since pH is operationally defined, it is generally believed that pH has no units.

What is the abbreviation for pH value?

There is no doubt that the "H" in "pH" stands for hydrogen ion (H+), but the source of p is controversial.

One kind of p stands for the German "Potenz", which means strength, intensity. In addition, the French puissance also means "strength, strength";

The second term pH represents the Latin "pondus hydrogenii", which is "quantity of hydrogen (amount of hydrogen)". One is the Latin potentia hydrogenii (capacity of hydrogen), which corresponds to the English potential hydrogen.

The third theory is that p is just a symbol randomly chosen by the proposer Sorensen, because he also used q.

In today's chemistry world, p is added in front of a dimensionless quantity to represent the negative logarithm of the quantity, so p must be lowercase. H means hydrogen ion, because the element symbol of hydrogen is H, so it needs to be capitalized!

Source: Environmental Protection Platform