The value of the hydrogen index plays an important role for many chemical and biological transformations occurring both in laboratories and in production, as well as in living organisms and the environment. The amount of hydrogen ions not only affects the result of any reaction, but also the possibility of its occurrence. To maintain a given pH value, buffer solutions are used. Their task is to maintain this level when diluting solutions or adding acids and alkalis to them.
The pH of water is one of the signs of water quality for various purposes. In nature, the development of plants, the aggressiveness of the environment on metal and concrete structures, depend on it. It should be remembered that the hydrogen indicator changes the toxicity of pollutants for organisms living in rivers, lakes, ponds.
PH value
This parameter characterizes the content of Η + ions in solutions. It is denoted by pH. Mathematically, the hydrogen index equals the inverse decimal logarithm of the concentration Η + ( + , mol / L): Η = −lgC + . The amount of + ions in water is determined by the dissociation of 2 molecules, which occurs according to the expression: 2 <–> + + - .
Despite the fact that water is not accepted as an electrolyte, it is a slightly dissociating substance. For it, we can write the dissociation constant: K d = ( + · - ) / 2 . At t = 22 ° C, its value is 1.8ˑ10 -16 .
This figure is so small that Η + and OH - ions in water could be neglected. But in the chemistry of solutions, the value of the hydrogen index is applicable to create a pH scale. Consider its meanings.
PH scale
With its help, you can quantify the acidity of a solution.
| P value | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
| Environmental quality | sour | neutral | alkaline |
The hydrogen indicator of the medium is easy to calculate. You just need to know the concentration of hydrogen cations and use the formula: C n + = 10 n , where n is the pH value with the opposite sign. For example, the concentration of + in the solution is + = 10 –5 mol / L. That is, n = –5, and pH = 5.
PH values of some media and solutions
Everything in the human environment has its own specific pH values. This helps various body systems to more easily cope with their tasks. As you know, for pure neutral water, the hydrogen index is 7. However, human skin has a slightly acid reaction. Their pH = 5.5. In part, this fact affects the appearance of dry skin with frequent contact with water. The following are pH indicators for some substances.
| Substance | pΗ |
| Battery Electrolyte | <1.0 |
| Gastric juice | 1,0—2,0 |
| Lemon juice | 2.0 |
| Table vinegar | 2,4 |
| Cola | 3.0 |
| Apple juice | 3.0 |
| Coffee | 5,0 |
| Shampoos | 5.5 |
| Black tea | 5.5 |
| Human skin | 5.5 |
| Acid rain | <5.6 |
| Saliva | 6.5 |
| Milk | 6.7 |
| Water | 7.0 |
| Blood | 7.36 |
| Sea water | 8.0 |
| Solid soap | 9.5 |
| Bleach (bleach) | 12.5 |
Types of Solutions
Aqueous solutions, as already indicated above, can have a neutral, acidic or alkaline reaction of the medium. The fact that the acidity of the solution is due to the presence of H + ions, and alkalinity due to OH- ions, does not mean that they do not contain others. In acidic environments, it is possible to detect an excess of hydrogen ions, and in alkaline - an excess of hydroxide ions.
In neutral solutions, the hydrogen index is 7. This means that the concentration of H + cations in them is 10 –7 mol / L, but the content of hydroxide anions is also 10 –7 mol / L. In other words, in neutral solutions there is no excess of Η + or Η- ions.
Ionic product of water
Why can pH range from 1 to 14? To answer this question, it is worth returning to the expression of the dissociation constant. By converting it, we can write K d · C H2O = C H + · C OH- . The value of Cd is known, and the concentration of water molecules can be easily calculated. Considering water as a solution of 2 in 2 , one can find out its molar concentration, making up the proportion: 18 g 2 - 1 mol, 1000 g 2 - mol. Hence x = 1000/18 = 55.6 mol / l. This constant is designated K w and is called the ionic product of water.
Next, we multiply the value of K d by the found value: 55.6 · 1.8ˑ10 –16 = Η + · Η– ; 10 –14 = Η + · Η– . That is, we can write: w = Η + · Η– = 10 –14 .
This value allowed us to conclude that pΗ + pOΗ = 14, which is the answer to the above question.
Acidic environment
All strong acids in water dissociate irreversibly. So, hydrochloric acid completely decomposes into Η + cations and Cl - chloride anions: ΗCl = Η + + Cl - . If 1ˑ10 -2 mol ΗCl was added to water of 1 liter, then the concentration of Η + ions will also be equal to 1 . 10 -2 mol. That is, for such a solution, the hydrogen index is 2.
Weak acids dissociate reversibly, that is, as in the case of water, part of the oppositely charged ions are again combined into acid molecules. For example, carbonic acid decomposes into the following ions: Η 2 CO 3 <–> Η + + ΗCO 3 - . Not only do not all molecules dissociate, but also disintegrated again form a single whole. Therefore, to find the hydrogen acid exponent of acids, the dissociation constant is used.
In addition, the strength of the acid can be indirectly estimated by the pH of the solution: the larger it is, the lower the pΗ value.
Alkaline environment
Upon dissolution of the bases in water, their dissociation begins with the appearance of hydroxide anions. They interact with H + ions, which are present in neutral clean water. This leads to a decrease in their concentration, that is, to an increase in pH.
For example: NaOΗ = Na + + OΗ - ; Η + + OΗ - = Η 2 O.
In a solution of sodium hydroxide with a concentration of 1ˑ10 -2 mol / L, 1ˑ10 -2 mol / L of hydroxide anions appears. The concentration of Η + cations in such a solution will be 1ˑ10 -12 mol / L, and pΗ has a value of 12.
In all base solutions, the amount of H + cations is always less than 1ˑ10 -7 mol / L, and the hydrogen index is more than 7.
Determination of pH indicators
One of the easiest ways to approximately determine the solution Η is to use the strips of a universal indicator. By comparing their color with the indicator scale, which appears after dipping in the working solution, it is possible to estimate the concentration of ions Η + . A universal indicator is a mixture of several substances that changes its color sequentially from red to violet (as in a rainbow) with a decrease in acidity.
The main disadvantages of this method are the impossibility of determining the hydrogen index in colored or turbid solutions, as well as only a rough estimate of the concentration of Η + ions in the solution.
For an even more crude determination of the pH of the medium, various indicators are used. The most commonly used litmus, methyl orange, phenolphthalein and others. By changing their color, you can only find out whether the test composition is acidic, alkaline or neutral.
| Indicator | pΗ <7 | pΗ = 7 | p> 7 |
| Litmus | red | purple | blue |
| Phenolphthalein | colorless | colorless | raspberry |
| Methyl orange | pink | orange | yellow |
PH measurement with instruments
A much more accurate value of the concentration of Η + ions, and therefore of the pΗ solution, can be found using a pH meter. This method of analysis is called potentiometric. It is based on measuring the electrode potential and determining the relationship between its value and the concentration of the component in the test solution. The electrode potential arises due to the electrochemical process going on the metal-solution interface.
For measurements, a galvanic cell is made up of two half cells with electrodes, the potential of one of which is known in advance. Then measure the EMF. Most often, the determination of the hydrogen index in aqueous solutions is carried out using silver chloride and glass electrodes. The first of these is a reference electrode. The value of the potential of the second depends on the concentration of Η + ions in the solution.
The pH in laboratories is also determined colorimetrically. This method is based on the ability of two-color indicators to change their color or color intensity, depending on the content of hydrogen cations. The color that appears in the solution is compared with a standard scale, which is based on data on solutions with a known pH value.
Reasons to measure pH
They are as follows:
1. For the production of products with desired properties. During the manufacturing process, a deviation from the technological pH can cause disturbances leading to a change in the characteristics of the product. Such indicators may be taste or appearance.
2. To reduce the cost. In some industries, the yield of the product directly or indirectly depends on the pH of the reaction medium. Accordingly, the higher the yield of the reaction product, the lower its cost.
3. In order to protect labor or the environment. Since many compounds exhibit their harmful properties only at a certain pH, it is very important to control its value.
4. To meet product standards. In many regulatory documents that standardize the quality of goods, products, medicines, etc., there is a list of indicators to which they must comply. One of them is pH. Thus, its definition to some extent helps to protect the population from harmful substances.
5. To protect equipment. Most manufacturing equipment that comes in contact with chemicals is susceptible to corrosion. The rate of its development is very dependent on the pH values. In other words, pH measurement is important to protect production equipment from unnecessary damage.
6. For research purposes. The pH level is important for studying various biochemical processes. It is also measured for medical purposes to confirm a particular diagnosis.
Mathematical determination of pH
For the calculated determination of the pH of the solution, data on the molar concentration of the + or Η - anion cations are required. If they are known, then you can immediately use one of the formulas:
- pΗ = - lg [Η + ].
- pOΗ = −lg [OΗ - ].
- pΗ + pOΗ = 14.
The concentration of a particular ion in mol / l in a solution of electrolytes is easy to find by the equation:
C m ion = C m ˑαˑ⋅n, where:
C m ion and C m - molar concentrations of ion and electrolyte, respectively (mol / L).
α is the degree of dissociation.
n is the number of ions of the species in question, which is formed during the decay of only one electrolyte molecule.
The degree of dissociation of weak electrolytes can be determined according to the Ostwald dilution law: α = √ (K d / C m ).
Examples of solving problems
1. It is required to calculate the pH of a 0.001 N NaOH solution.
Solution: Since sodium hydroxide is a strong electrolyte, its dissociation in an aqueous solution is irreversible. It goes according to the equation: NaOΗ → Na + OΗ.
We use the formula C m ion = C m ˑαˑn. We assume that the degree of dissociation is equal to 1. When one NaOH molecule is destroyed, one OH- ion is formed, which means n = 1. C m by the condition of the problem is known and is equal to 0.001 or 10 -3 . Hence C OH− = 10 −3 ˑ1ˑ1 = 10 -3 .
The concentration of Η + ions can be determined from the relation K w = Η + · Η– = 10 –14 . Transforming the formula, we obtain + = w / Η– = 10 –14 / 10 –3 = 10 –11 . Next, we can calculate the hydrogen index: pΗ = -lg10 -11 = 11.
Answer: pH = 11.
2. It is required to calculate [Η + ] and [OH-] if in a given solution pH = 4.3.
Solution: It is easiest to first find the concentration of hydrogen cations: [Η + ] = 10 -pΗ = 10 -4.3 = 5ˑ10 -5 mol / L.
The concentration of hydroxide anions is conveniently found from the ratio of the ionic product of water: Η- = w / Η + = 10 –14 / 5ˑ10 -5 = 2ˑ10 –10 mol / L.
Answer: 5ˑ10 -5 mol / l and 2ˑ10 -10 mol / l.