What is pH? In professional literature, we encounter the concept expressed with the letters pH and the accompanying numbers from 0 do 14.
I suppose, that in the near future it will be necessary to determine the pH in a modern cheese factory, for example in order to follow the course of fermentation in cheeses, or selecting an appropriate mixture of cheese mass intended for remelting by selecting emulsifying salts for it, etc.. Anyway, instruments for determining the pH, the so-called. pehametry, are getting cheaper and easier to use, and it is even simpler and cheaper to determine the pH using the so-called. indicators.
If we take equal amounts of 1-normal hydrochloric acid and such acetic acid and then titrate each one separately 1 normal sodium hydroxide solution, we will find, that we will use as much lye to completely neutralize the hydrochloric acid, as to the neutralization of acetic acid. They are therefore acids of equal value, or as is usually said, of equal power. But when we dilute them with equal amounts of water and try, we will say it, that hydrochloric acid is strong or sharp, while acetic acid is weak or mild. Where does this difference come from??
We already know from elementary chemistry, that acids, bases and salts dissociate in solution of water, i.e.. decay into atoms or groups of atoms, some of them have a positive electric charge ( + ), other negative (—). The atoms or groups so dissociated are called ions, that is, wanderers. They will be directed towards the electrodes, when we pass an electric current through the solution, in which they are located. Ions with a positive charge are called cations ( + ), because they travel to the cathode, and negatively charged ones are called anions (—). These go towards the anode. This is also the case in dissociated solution:
But there is such a big difference between the two acids, that in hydrochloric acid, the decomposition into ions is immediately approx 91%, while in acetic acid only 2%. We will also notice the difference in the behavior of these acids, when we pass electric current through each of them. Hydrochloric acid conducts electricity much better and shows a greater swing on the current meter than acetic acid. This means a stronger dissociation into ions in hydrochloric acid than in acetic acid. It is this difference in the amount of free hydrogen ions that makes, that we call hydrochloric acid strong and acetic acid weak. Since chemical reactions are the most common reactions between ions, therefore, in hydrochloric acid, it can act immediately 91% free ions, while in acetic acid only 2%. The rest of the ions will be formed from the resource only under the influence of some stimulus, which is e.g.. lye added during titration or dilution with water.
Acidity, which we found during titration regardless of the presence of already free and then liberating H + ions, is called potential acidity. In contrast, acidity, which results from the presence of free H + ions only at a given moment, is referred to as actual acidity or also R ea l n, that is, active or real, because in fact only this acidity as a result of free H + ions is active. For example, when fermenting cheese, we want to find out, what is the acid medium that affects the life of bacteria. We are then concerned with the issue, what is the active acidity, not a question, how much acid from ionized and non-ionized is in the entire resource.
How to understand the pH symbol usually used as a measure of the concentration of H + ions? The analysis also uses the term "grammolecule” or "mol”. So 1 mole of HCl is 1 + 35,5 = 36,5 g. For water, the product of the concentrations of both ions is always constant. Denoting this product with the letter K we get: K = [H+] X X [(OH—)]. Slash brackets denote both ions in their concentration (i.e.. content) ionic.
Found, that this constant K is in water 10-14. It follows from the above, that from one water molecule it is always formed with one hydrogen ion H + and one hydroxide ion (OH)— , that is, their concentrations are always equal and amount to each other 10-7.
The reaction of such a solution is called neutral. Since the atomic weight of hydrogen is 1, therefore 1 gram atom of hydrogen - 1 g, therefore, the concentration of hydrogen ions in 1 liter of neutral water is 1 X 10-7. Dissociation in water is negligible, but still measurable.
The hydrogen ions originally found in the water will increase quantitatively, i.e.. their concentration will be greater, when we add acid to water, e.g.. HCl. Then instead of H + - 10—7 we will get, e.g.. values 10-4 or 10-3, that is, fractions with fewer zeros in the denominator lower negative exponent, the greater its value, the greater the ion concentration.
As we have seen before, the product of hydrogen and hydroxide ions is always constant. So if the content of hydrogen ions increases, at the same time, the content of the hydroxyl groups is reduced to the same extent. Thus, when in the above-mentioned example the concentration of H + ions increased to 10-3, at the same time, the concentration of hydroxide ions will drop to 10-11, because 10-3 x 10-11 = 10-14. However, if we add bases to a solution, e.g.. NaOH, then particles of sodium hydroxide break down into Na ions7” i (OH)—, that is, the concentration of ions increases (OH)-, at the same time, the concentration of H + ions decreases.
The environment within the concentration limits of hydrogen ions from 10-7 do 10° = 1, and as basic - at concentrations of hydrogen ions from 10-7 do 10-14.
H+ Neutral reaction (OH)—
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
very acidic weakly acidic weakly alkaline strongly alkaline.
Since operating with negative powers such as. H+ = 10-6 is inconvenient, therefore Sorensen created the symbol pH, taking the negative logarithm of the corresponding values i.e.. just in with the inverse sign exponent of the power (—log H+ = pH). In the above example, pH = is obtained 6.
Colorimetric method. Ion concentration can also be determined with many color-shifting reagents. Such a reagent is e.g.. alizaryna i fenoloftaleina.