Acids and alkalis are two extreme positions of the same scale: their properties (completely opposite) are determined by the same quantity - the concentration of hydrogen ions (H + ). However, this number itself 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, 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 medium and an alkaline medium are determined by the concentration of H + hydrogen ions and that the higher their concentration, the more acidic the solution (and vice versa, the lower the H + concentration, the more alkaline the higher the concentration of opposite OH- ions), was known science for a long time. However, it was only in 1909 that the Danish chemist Serensen first published studies in which he used the concept of a hydrogen index - PH, which was subsequently replaced by pH.
Acidity calculation
When calculating the hydrogen index, it is assumed that water molecules in solution, although in very small quantities, still dissociate into ions. This reaction is called autoprotolysis of water:
H 2 O -> H + + OH -
The reaction is reversible, therefore, an equilibrium constant is determined for it (showing what, on average, the concentrations of each component are established). The constant value for standard conditions is given here - temperature 22 ° .
Below in square brackets are the molar concentrations of these components. The molar concentration of water in water is approximately 55 mol / liter, i.e. a second order magnitude. Consequently, the product of the concentrations of H + and OH - ions is about 10 -14 . This quantity 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 medium.
Next, you need to distract from the water and consider a solution of some acid or alkali. Take acetic acid, for example. The ionic product of water will remain the same, but the balance between the H + and OH - ions will shift towards the former: hydrogen ions will come from partially dissociated acetic acid, and the "excess" hydroxide ions will go into undissociated water molecules. Thus, the concentration of hydrogen ions will be higher and the pH will be lower (do not forget that the logarithm is taken with a minus sign). Accordingly, the acidic and alkaline environments are pH related. And related as follows. The lower the pH value, the more acidic the medium.
Acidic Properties
An acidic medium is 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 value from 1 to 14, in fact, solutions with a pH of less than one (and even less than zero) and more 14 exist. For example, in concentrated solutions of strong acids (sulfuric, hydrochloric), the pH can reach -2.
The solubility of certain substances may depend on whether we have an acidic or alkaline environment. For example, take metal hydroxides. Solubility is determined by the value of the solubility product, which in structure is the same as the ionic 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 “tear 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 digestive tract of a person has an acidic environment: the pH of the gastric juice ranges from 1 to 2. Deviation from these values to a smaller or larger side can be a sign of various diseases.
Alkaline Properties
In an alkaline medium, the hydrogen index takes values in excess of 7. For convenience, in environments with a high concentration of hydroxide ions, the basicity index pOH is used instead of the hydrogen acidity pH. 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 is the equality pH + pOH = 14. Therefore, pOH = 14 - pH. Thus, for all statements that are true for pH, the converse is true for basicity pOH. If the pH of the alkaline medium is large by definition, then its pOH is obviously small, and the stronger the alkali solution, the lower the pOH value.
A logical paradox has just appeared in this sentence, confusing many arguments about acidity: a small acidity indicator indicates a high acidity of the medium, and vice versa: large pH values correspond to low acidity. This paradox appears because the logarithm is taken with a minus sign, and the acidity scale is inverted.
The practical definition of acidity
To determine the acidity of the medium, so-called indicators are used. Usually, these are rather complicated organic molecules that change color depending on the pH of the medium. The indicator changes color in a very narrow pH range: it is used in acid-base titration to achieve accurate results: titration is stopped as soon as the indicator changes color.
The most well-known indicators are methyl orange (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 and alkaline environments, different indicators are used depending on which region their transition interval lies in.
There are also universal indicators - they change their color gradually from red to deep purple during the transition from strongly acidic to very alkaline environments. In fact, universal indicators are a mixture of ordinary indicators.
For a more accurate determination of acidity use a device - a pH meter (potentiometer, the method, respectively, is called potentiometry). Its principle of operation is based on measuring the emf in a circuit whose element is a solution with a measured pH. The potential of the electrode immersed in the solution is sensitive to the concentration of hydrogen ions in the solution - hence the change in the EMF, based on which the real pH is calculated.
Acidity of various environments in everyday life
Acidity 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 salts of fatty acids. In water, they dissociate: the fatty acid residue - very long - has a negative charge on one side and a long non-polar chain of carbon atoms on its other side. The end of the molecule, on which the charge participates in hydration, gathers water molecules around itself. The second end joins other non-polar things, for example, fat molecules. As a result, micelles are formed - balls in which “tails” stick out with a negative charge, and “tails” and particles of fat and dirt are hidden inside. The surface is washed away from grease and dirt due to the fact that the detergent binds all the grease and dirt into such micelles.
Acidity and health
It has already been mentioned that pH is of great importance to the human body. In addition to the digestive tract, it is important to control the acidity indicator in other parts of the body: blood, saliva, skin - for many biological processes, acidic and alkaline environments are of great importance. Their definition allows you to assess the state of the body.
Now gaining popularity pH tests - the so-called rapid tests for checking acidity. They are ordinary strips of universal indicator paper.