# What is meant by hydrogen ion concentration and how is the pH scale determined

The term means just what it says, namely the concentration of hydrogen ions that are present in a solution. It is a scientific method of referring to the degree or intensity of acidity or alkalinity and is based on the fact that, in solution, the molecules of some substances split up and disperse throughout the liquid to a greater or lesser degree. The pH of a liquid tells not only whether the liquid is acid or alkaline, but also to what degree or extent.

The symbol pH stands for the 'potential of hydrogen'. The word 'ion' means traveller, so that the hydrogen ion concentration refers to the degree of dispersal of ions of hydrogen in a given solution. It also refers to the fact that such hydrogen atoms are in an active condition and are charged with positive electricity, commonly denoted as H+.

To understand this more, we must consider the state of affairs in pure water. This is neither acid nor alkaline, but it has been calculated that in neutral water one molecule in ten million ionises or splits up and disperses as one atom of hydrogen charged with positive electricity and one group of elements consisting of an atom of hydrogen and one of oxygen. This is known as a hydroxyl group and is charged with negative electricity (OH_). There is therefore complete neutrality, the positive charge on the hydrogen atom exactly neutralising the negative charge of the hydroxyl group.

In pure water, being neutral, only one part in ten million parts is in this state of ionisation. Mathemetically 1/10,000,000 can be expressed as 10~7, that is ten to the minus seventh power. Hence in stating the pH of a neutral liquid such as pure water scientists write pH7, omitting the minus sign as being superfluous.

Let us now consider what happens when we have a liquid in which there is a higher concentration of hydrogen ions, such as is the case when an acid is diluted with water. Here there may be one part ionised hydrogen in a million parts of the liquid. This can be expressed as 1/1,000,000, that is 10~6, and so is written pH6. This shows that the liquid is acid in character, but not very strongly so, for only one part in a million is actively acid or capable of reacting as an acid.

The greater the intensity of the acid the greater the concentration of hydrogen ions will be. The expressions pH5, pH4, pH3, pH2 and pH1 mean respectively that one part in 100,000, 10,000, 1000, 100 and 10 are in this condition. As the numeral beside the pH decreases so the intensity of the acid present in the liquid increases (see Fig. 31).

In the case of neutral solutions we still refer to the hydrogen ion concentration but the dispersed atoms charged with positive electricity may not necessarily be hydrogen. The groups of negatively charged elements, however, neutralise these and alkaline reactions are recorded. For example, in sodium hydroxide, a well-known alkali, the ionised molecule will split up into one atom of sodium and one hydroxyl group. The former carries a weak positive charge and the hydroxyl group a stronger negative one, cancelling out the weak acid tendency and substituting stronger alkaline tendencies. Thus stronger

 252 Baking problems solved Concentration of H+ ion PH 1 g per litre 1 g per 10 litres 1 g per 100 litres 2 3 Acid increasing in multiples of 10 8 1 g per 10 million litres Alkali increasing in multiples of 10 9 10 11 12 13 17 g OH~ per litre Fig. 31 pH scale. negative electrical charges can be recorded. As the pH numeral increases so alkalinity increases, and whenever the numeral is above 7 the substance or solution of substances is alkaline. The higher the figure above 7, the greater the strength or intensity of alkalinity will be. Examples of the value of this information to the practical baker and confectioner include the following: • The development of rope in bread. Rope spores cannot grow unless the dough or loaf is lacking in acidity. This degree of acidity is known and provided the pH of the baked product lies between 5.4 and 5.5, rope cannot grow in dough or bread. • In the manufacture of high-ratio cherry cake the low-viscosity character of the batter does prevent the cherries from sinking during baking. However, if an addition of tartaric acid is made to the batter to bring the acidity to pH 5.4 or less, then the gluten of the flour is strengthened and batter viscosity increases so that the cherries remain suspended during baking (see 10.3). High-protein, high-ratio cake flour responds even more readily to the use of tartaric acid.

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