Acids and alkalis are two extreme positions of the same scale: their properties (completely opposite) are determined by the same value - the concentration of hydrogen ions (H+). However, in itself this number is very inconvenient: even in acidic environments, where the concentration of hydrogen ions is higher, this number is hundredths, thousandths of a unit. Therefore, for convenience, they use the decimal logarithm of this value, multiplied by minus one. It is customary to say that this is pH (potentia Hydrogen), or a hydrogen indicator.
The emergence of the concept
In general, the fact that an acidic environment and an alkaline environment are determined by the concentration of hydrogen ions H + and that the higher their concentration, the more acidic the solution (and vice versa, the lower the H + concentration, the more alkaline the environment and the higher the concentration of opposite OH ions -), has been known to science for a long time. However, it was not until 1909 that the Danish chemist Sørensen first published research in which he used the concept of a hydrogen index - PH, later replaced by pH.
Calculation of acidity
When calculating the pH index, it is assumed that water molecules in solution, albeit in very small quantities, still dissociate into ions. This reaction is called water autoprotolysis:
H2O H+ + OH-
The reaction is reversible, so an equilibrium constant is defined for it (showing the average concentrations of each component). Here is the value of the constant for standard conditions - temperature 22 °C.
Below in square brackets - molar concentrations of the indicated components. The molar concentration of water in water is approximately 55 mol/liter, which is a second order value. Therefore, the product of the concentrations of H+ and OH- ions is about 10-14. This value is called the ionic product of water.
In pure water, the concentrations of hydrogen ions and hydroxide ions are 10-7. Accordingly, the pH value of water will be approximately 7. This pH value is taken as a neutral environment.
Next, you need to look away from the water and consider a solution of some acid or alkali. Take, for example, acetic acid. The ionic product of water will remain the same, but the balance between the ions H+ and OH- will shift towards the former: hydrogen ions will come from partially dissociated acetic acid, and "extra" hydroxide ions will go into non-dissociated water molecules. Thus, the concentration of hydrogen ions will be higher and the pH will be lower (no needforget that the logarithm is taken with a minus sign). Accordingly, acidic and alkaline are related to pH. And they are connected in the following way. The lower the pH value, the more acidic the environment.
Acidic properties
Acidic environments are solutions with a pH less than 7. It should be noted that although the value of the ionic product of water at first glance limits the pH values \u200b\u200bin the range from 1 to 14, in fact, solutions with a pH less than one (and even less than zero) and greater than 14 exist. For example, in concentrated solutions of strong acids (sulphuric, hydrochloric) pH can reach -2.
The solubility of certain substances may depend on whether we have an acidic environment or an alkaline environment. For example, take metal hydroxides. Solubility is determined by the value of the solubility product, which is the same in structure as the ion product of water: multiplied concentrations. In the case of hydroxide, the solubility product includes the concentration of the metal ion and the concentration of hydroxide ions. In the case of an excess of hydrogen ions (in an acidic environment), they will more actively "pull out" hydroxide ions from the precipitate, thereby shifting the equilibrium towards the dissolved form, increasing the solubility of the precipitate.
It is also worth mentioning that the entire human digestive tract has an acidic environment: the pH of gastric juice ranges from 1 to 2. Deviation from these values up or down can be a sign of various diseases.
Properties of alkaline medium
Bin an alkaline environment, the pH value takes on values exceeding 7. For convenience, in environments with a high concentration of hydroxide ions, the pH indicator of acidity is replaced by the pH indicator of basicity pOH. It is easy to guess that it denotes a value equal to -lg[OH-] (negative decimal logarithm of the concentration of hydroxide ions). Directly from the ionic product of water follows the equality pH + pOH=14. Therefore pOH=14 - pH. Thus, for all statements that are true for the pH index, the opposite statements are true for the pOH basicity index. If the pH of an alkaline medium is large by definition, then its pOH is obviously small, and the stronger the alkali solution, the lower the pOH value.
This sentence has just introduced a logical paradox that confuses many discussions about acidity: low acidity indicates high acidity, and vice versa: high pH values correspond to low acidity. This paradox appears because the logarithm is taken with a minus sign, and the acidity scale is, as it were, inverted.
Practical definition of acidity
So-called indicators are used to determine the acidity of the medium. Usually these are rather complex organic molecules that change their color depending on the pH of the medium. The indicator changes color over a very narrow pH range: this is used in acid-base titrations to achieve accurate results: the titration is stopped as soon as the indicator changes color.
The most famous indicators are methylorange (transition interval in the region with a low pH), phenolphthalein (transition interval in the region with a high pH), litmus, thymol blue and others. In acidic environments and alkaline environments, different indicators are used depending on the area in which their transition interval lies.
There are also universal indicators - they change their color gradually from red to deep purple when moving from strongly acidic to strongly alkaline environments. In fact, universal indicators are a mixture of common ones.
For a more accurate determination of acidity, a device is used - a pH meter (potentiometer, the method, respectively, is called potentiometry). Its principle of operation is based on the measurement of EMF in a circuit, the element of which is a solution with a measured pH. The potential of an electrode immersed in a solution is sensitive to the concentration of hydrogen ions in the solution - hence the change in EMF, on the basis of which the real pH is calculated.
Acidity of various environments in everyday life
The acidity index is of great importance in everyday life. For example, weak acids - acetic, malic - are used as preservatives. Alkaline solutions are detergents, including soap. The simplest soap is sodium s alts of fatty acids. In water, they dissociate: the fatty acid residue - very long - on the one hand has a negative charge, and on the other hand - a long non-polar chain of carbon atoms. Thatthe end of the molecule, at which the charge participates in hydration, collects water molecules around it. The other end attaches to other non-polar things, like fat molecules. As a result, micelles are formed - balls, in which "tails" with a negative charge stick out, and "tails" and particles of fat and dirt are hidden inside. The surface is washed from grease and dirt due to the fact that the detergent binds all grease and dirt into such micelles.
Acidity and he alth
It has already been mentioned that pH is of great importance for the human body. In addition to the digestive tract, it is important to control the acidity index in other parts of the body: blood, saliva, skin - acidic and alkaline environments are of great importance for many biological processes. Their definition allows you to assess the state of the body.
Now pH tests are gaining popularity - the so-called express tests for checking acidity. They are regular strips of universal indicator paper.
