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Chemically clear water partially dissociates on ions:

Н2О ↔ H⁺ + OH^¯

Thereby forming a H + ion rapidly hydrated (attaches to itself a water molecule) to form hydronium ion H₃O⁺. However, for simplicity instead of hydronium talk about hydrogen ion. The law of mass action applied to the dissociation of water of any aqueous solution expressed in this equation:

(2.1)

where, K – a constant of dissociation of water or its solution.

The constant of dissociation of water at 22 °C is equal 1,8 °C10-16, that is degree of dissociation of water is very small. Therefore concentration of not dissociated molecules [H2O] can be considered a constant. At its expression in moths on 1 l it turns out:

(2.2)

From the formula (2.1):

(2.3)

Thus, water and any aqueous solutions at a constant temperature, product of the concentrations of hydrogen and hydroxyl ions is a constant value, called the ion product of water. If the equation (2.3) instead of K substitute its value equal 1,8×10^-16, the product ion at 22 °C we have:

Кw= 55,56×1,8×10^-16 = 10^-14 (2.4)

At dissociation of water the identical quantity of ions of hydrogen and a hydroxyl is formed; therefore, concentration of both ions in the distilled water at 22 °C will be:

(2.5)

i. e. 0.1 microgram per 1 liter.

Since the concentration of hydrogen ion and hydroxyl ion-linked product, they are of the conjugate. Therefore, any increase in the concentration of H + is suitable entails a decrease in the concentration of OH and vice versa. The concentration of one of the ions can be determined by knowing the concentration of the other. From the formula (2.5)

(2.6)

Consequently, acidity, alkalinity, and the degree of oxidation and the alkalinity of the solution depends on the presence and concentrations of H + ions. If concentration of H+ of ions is equal to concentration OH^- ions, solution is neutral. In case [H+]>[OH-],solution is acidic, in case[H+]< [OH-] – alkaline.

For the characteristic of acidity or alkalinity of solution, it is enough to calculate concentration of one ion. Usually (as it is accepted the international convention) define concentration of H+ (g×equiv/l). As concentration of ions of hydrogen with what deal in practice, is small, for convenience at records and calculations reaction of solution express a symbol рН.

рН = – lg[H+], (2.7)

i. e. pH value – is the negative logarithm of hydrogen ion concentration. The concept of pH scale was first introduced by Danish chemist Soren Peder Lauritz Sorensen at the Carlsberg Laboratory in the year 1909.The reaction among all the solutions from highly acidic to highly alkaline can be expressed in a single pH scale ranging from 0 to 14.

For example, pH = 0 means that [H +] = 100 = 1, i.e. in 1 liter of solution containing 1 g of hydrogen ions in 1L.

Determining the reaction of the environment should not be forgotten that pH of the solution significantly affectedby temperature, which depends on the degree of dissociation of water.

Each ion plays a role in the biological and chemical processes. The special position among other ions take up hydrogen ions. They are essential for enzymatic activity. Each enzyme has an optimum of the action at a certain pH when the enzymatic reaction rate is at a maximum. Sometimes, the optimum pH is expressed very strongly, and the action of the enzyme appears only in a narrow range of pH values. Enzymes are proteins have an electric charge, so their structure is dependent on the pH. The concentration of hydrogen ions has a huge impact for the vital activity of organisms, the functional activities of individual animals and tissues of higher animals and humans. Diphtheria microbe better developed at a pH in the range7.3-7.6; microbe Escherichia coli – at pH 6-7; active nitrogen fixation by soil microbes observed at pH = 7.2. Death of warm-blooded animals there comes a shift of blood pH at 0.03-0.04 units (normal blood pH 7,35±0,02). Reaction urine (urine pH) normal diet mixed with acidic or neutral (pH range 5.0-7.0). The response should be determined in fresh urine. Predominance of the animal protein in the diet gives a shift toward acidic, and the prevalence of plant food – towards the alkaline reaction. By fevers, diabetes, starvation, kidney failure, etc. urine has very acidic reaction. Alkaline urine is observed in cystitis and pielitah, hematuria, after vomiting and diarrhea, with resorption of exudates, when taking soda, mineral water.

Under the influence of hydrogen ions may change the basic physical and chemical properties of substances and solutions: solubility, surface tension, viscosity, stability, osmotic pressure, swelling, and others. All this is the cause of frequent determination of the concentration of hydrogen ions in chemistry, biology, medicine, agriculture and technology.

Figure 2.1. Scheme of measuring the pH of the solution

1 – hollow ball of electronic glass; 2 – glass electrode; 3 – internal contact electrode; 4 – auxiliary electrode; 5 – Electrolytic key; 6 – a porous wall; 7 – pH meter

Under the influence of hydrogen ions the main physical and chemical properties of substances and solutions can change: solubility, superficial tension, viscosity, stability, osmotic pressure, swelling, etc. All told is also the reason of frequent determination of concentration of hydrogen ions in chemistry, biology, medicine, agriculture and equipment.

Between a surface of glass and controlled solution there is a potential difference of E_x which size is defined by activity of hydrogen ions in solution and its temperature.

(2.8)

Where α_h – the activity of hydrogen ions in solution; Е₀ – glass electrode potential relative to a standard hydrogen electrode at α_h=1.

Contact electrodes are applied to creation of an electric chain at measurement: the internal contact electrode 3 which is carrying out electric contact with the solution filling internal part of a glass electrode, and an external contact electrode (a so-called auxiliary electrode) 4, the carrying-out electric contact with controlled solution.

For protection against influence of high temperatures (at measurement рН solutions which temperature is higher than air temperature) the auxiliary electrode is placed out of controlled solution, and communication with it is carried out by means of an electrolytic key 5 – the tube which is coming to an end with a stopper with glass fiber 6.

Solution of chloride potassium continuously filters through glass fiber of a stopper, preventing penetration from controlled solution into system of an electrode of 4 foreign ions which could change electrode potential size.

The electromotive force of the electrode system is equal to the algebraic sum of the potentials of the contact electrodes E_c and E_aux, potential occurring on the inner surface of the glass electrode and a defined pH internal solution E_int and potential arising on the outer surface of the glass electrode E_c. The values E_c,,E_aux and E_int does not depend on the composition of the control solution and only change when the temperature changes. The total electromotive force of the electrode depends linearly on the pH of the solution. By measuring the emf an electrode system with an electronic millivoltmeter 7, a scale which is graduated in units of pH, determine the pH-controlled solution.

Laboratory work № 3
pH of various solutions and biological liquids

Objective: Experimentally find pH values of various solutions and biological liquids.

Tasks:

1. Calibrate the device on standard solutions.

2 Measure рН of various solutions and biological liquids.

3. Draw graphs of pH values of various solutions, and biological and non-biological liquids.

4. Make conclusions about the observed phenomena and prepare a report.

Equipment and Materials: pH meter, nitric acid, alkali, solutions of proteins, gastric juice, urine, distilled water, vessels.

Procedure:

Rinse thoroughly with electrodes; the purity of the electrodes is judged by the pH of distilled water. Electrodes may be used for research in case, if pH value of distilled water close to 6,0-6,3.

Determine pH value of the following solutions: gastric juice diluted (5-fold) gastric juice protein solution, an acidic solution, an alkaline solution, tap water, distilled water. Measuring the pH of the sample solution should be performed at least in triplicate. Before replacing of the sample solution the electrodes should be thoroughly washed. Record these pH values in the table 2.3.1.

Table 2.3.1

Based on these data, you must specify what determines the specific pH of each solution.

Report design. The results bring to the table and to construct graphics (histograms). Define measurement error and to draw conclusions.

`Laboratory work № 4
Determination of pH of fermented milk products using a pH meter

Objective: pH of various solutions and of biological fluids.

Tasks:

1. Perform calibration standard solutions.

2. To measure the pH of various solutions and biological fluids.

4. Build a graphic pH of various solutions, and biological and non-biological fluids.

5. Make conclusions about the observed phenomena and prepare a report.

Equipment and materials: pH meter burette 1 and 50 ml, 8-10 cups 50 ml of 0.01N. sulfuric acid, distilled. water, yogurt (or any other dairy products)

Task 1. Determination pH of the solution with sulfuric acid of various concentrations

Procedure: you need to determine the pH of 0.01 n. sulfuric acid solution. Then prepare a dilution at two, four and eight times, and carry out measurements. Plot the pH of the test solution concentration.

Task 2. Determination pH of fermented milk products with different concentrations

Procedure: Measure the pH of the original dairy products. Then prepare a dilution at two, four and eight times, ten times, and carry out measurements. Plot the pH of the test solution concentration.

To explain the observed differences in the results and graphs.

Table 4.1.1

pH values

Chapter 2 «Physical-chemical methods» questions:

1. What is the pH of solutions and how to determine its value?

2. Why does the pH change?

3. What instruments allow us to estimate the pH

4. How big limits could change pH in living objects?

5. Explain, how ph-meter measure pH value.

6. What substances are buffering?

7. Describe the dissociation of the peptide.

8. Describe the dissociation of sulfuric acid, for some reason it does not have buffer properties?

9. Explain biological significance for constancy of pH within each cell, and in all body fluids.

10. Why diluted fermented milk product solution’s pH does not change much, while a dilution of sulfuric acid (or salt) in 10 times pH changes a lot?