MINISTRY of PUBLIC HEALTH of UKRAINE ODESSA NATIONAL MEDICAL UNIVERSITY Department of Clinical Chemistry and Laboratory Diagnostics Medical chemistry Information block for first-year students of medical faculty Part 2. Acid-base equilibrium in biological fluids «Elements of quantitative analysis. The quantitativecomposition of the solutions. Methods for calculating the concentration of solutions Test questions: 1. Solutions and their composition. 2. Physical and chemical theory of solutions 3. Ways to express the composition of the solution. 3. Solubility. Factors affecting solubility. 4. Concentration. Types of concentration. Ways to express concentrations. 5. The role of solutions in nature, living organisms, science and technology. The biological role of solutions. 1 6. Hydrates, crystalline hydrates, crystallization water. 7. Medical solutions. Doses Calculation of the dosage of the drug. Theoretical basis 1.1. Composition of solutions. Solutions are of great practical importance. Many chemical and biochemical reactions proceed in solutions. Solutions are homogeneous systems of variable composition, consisting of two or more substances (components). A true solution is a homogeneous system formed by at least two components. The composition of the solution can be changed continuously within certain limits. The true solution differs from colloidal solutions and mechanical mixtures (suspensions, emulsions, aerosols). Colloidal solutions are multicomponent heterogeneous systems. The homogeneity of the solutions makes them similar to chemical compounds and distinguishes them from mechanical mixtures. Solutions can be in three states of aggregation: gaseous, solid and liquid. In solid solutions, particles of one substance are randomly distributed among particles of another solid. For example, hydrogen is well soluble in some metals (platinum, palladium), and this is a solid solution. Mixtures of gases (e.g. air) are not called solutions. The fact is that an important property of solutions is a noticeable interaction between solvent particles and dissolved substances, and in gases such interaction is practically absent. In liquid solutions, the solvent and the solute are distinguished. A solvent is a substance that is in the same state of aggregation as the solution. If the aggregate state of the substances that make up the solution is the same (for example, alcohol and water), then the solvent is a substance that is present in excess compared to other components. The remaining components, which are in solution in a smaller amount, are called solutes. According to modern theory, solutions are liquid dissociated systems formed by particles of solvent and solute and those uncertain compounds that form between them. 2 Dissolution is a physicochemical process. When dissolving, heat is always released or absorbed, and a change in volume occurs. This indicates a chemical interaction between the solute and the solvent. As a result of this interaction, solvates or hydrates (solvent- water) are formed. On the other hand, solutions do not obey the law of constancy of composition; they, like mixtures, can be divided by physical methods into their constituent parts. Most hydrates and solvates are weak compounds that decompose readily. However, hydrates can also be strong; they can be easily isolated during crystallization. Examples of crystalline hydrates: CuSO4·5H2O, CaSO4·2H2O, BaCl2·2H2O. Such medicinal substances also include glucose, terpinghydrate, magnesium sulfate, copper sulfate, alum, codeine, etc., which are crystalline hydrates with different contents of crystallization water. When water is removed from crystalline hydrates, they change the appearance and individual properties (copper sulfate, gypsum, crystalline soda, etc.). In the hydrated state, individual ions of salt dissolved in water also remain, which is crucial for many properties of salt solutions. In chemical practice, the most important are liquid solutions. Qualitatively, by the ratio of the dissolved substance and the solvent, the solutions are divided into: concentrated – solutions in which there is a lot of dissolved substance; diluted – solutions in which there is little solute. But these concepts are conditional, relative and indefinite. The following classification of solutions is more defined.: saturated solution is a solution in which at a given temperature the substance no longer dissolves, i.e. in this case the solubility product (SP) is equal to the product of ion concentrations in degrees equal to stoichiometric coefficients; unsaturated solution is a solution in which at a given temperature there is less soluble substance than in a saturated solution, i.e. in this case the solubility product (SP) is less than the product of ion concentrations in degrees equal to stoichiometric coefficients ; 3 supersaturated solution is a solution in which at a given temperature in the dissolved state there is more substance than in its saturated solution under the same conditions, ie an in this case the solubility product (SP) is bigger than the product of ion concentrations in degrees equal to stoichiometric coefficients ; . Oversaturated solutions are very unstable. They are able to exist only in the absence of a solid phase of a dissolved substance. Crystallization can be caused by the addition of crystalline, shaking, friction with a stick on the walls of the vessel. 1.2. Solubility Solubility is the ability of a substance to dissolve in a particular solvent. Solubility is a spontaneous physicochemical process. It occurs due to the diffusion of molecules or ions from a region with a higher concentration to a region with a lower concentration. As a result, the substance is evenly distributed throughout the solution. Solubility is a bidirectional process: a solid goes into solution, and a dissolved substance goes into a solid. Therefore, both dissolution and crystallization occur simultaneously. These processes proceed with the same speeds over time - dynamic equilibrium sets in. In this case, the concentration of the soluble substance remains constant without changing the conditions. This condition is called the saturation state, and the solution is called saturated. The ability of various substances to dissolve in a particular solvent is called solubility. A measure of solubility is the princiconcentration of a saturated solution at a given temperature and pressure. The dynamic equilibrium Le Chatelier`s principle is applicable to the dissolution process. Solubility depends on temperature, external pressure, nature of the solute and solvent. If more than 10 g of a substance is dissolved in 100 g of water, then such a substance is called highly soluble. If less than 1 g of the substance is dissolved, the substance is sparingly soluble. A substance is considered practically insoluble if less than 0.01 g of the substance passes in solution. Absolutely insoluble substances do not exist. Even when we pour water into a glass vessel, a very small part of the glass molecules inevitably passes into solution. 4 Depending on the content of the solute and the ratio of the rates of dissolution and crystallization processes, saturated, unsaturated and supersaturated solutions are distinguished. In general form for any sparindly soluble electrolyte: y+ x- (AxBy )cryst = xA + yB . If T= const, [AxBy]=const, we get the following expression: y+ x x- y Keq*[AxBy]= [A ] *[B ] = SPAxBy In a saturated salt solution, the product of the concentrations of its ions (activities) in powers equal to stoichiometric coefficients is a constant value at a given temperature, called the product of solubility (SP). If the stoichiometric product of ion concentrations is greater than SP, then the solution is supersaturated and a precipitate forms. If the stoichiometric product of ion concentrations is less than SP, then the solution is unsaturated and a precipitate will dissolve. If the stoichiometric product of ion concentrations, SP are equal, then the solution is saturated. Solubility expressed by the mass of a substance that can dissolve in 100 g of water at a given temperature is also called the solubility coefficient. The mutual solubility of liquids or solids in liquids depends on a number of properties of these substances: chemical nature, value and structure of particles, electric charge (in the case of ions), dipole moment and others. Unlimited solubility is when two liquids dissolve one into the other in any ratio. Limited solubility is one liquid that dissolves in another in a certain concentration. With limited mutual solubility of the two liquids A and B, each of them dissolves. However, after settling, two layers are formed, which are arranged one above the other in the order of decreasing density. Information on the solubility of drugs is given in pharmacopeia articles and reference tables. 5 1.3 Сoncentration of solution The main characteristic of the solution is its concentration. In chemistry, the concentration of a solution is the quantity of a solute that is contained in a particular quantity of solvent or solution. Knowing the concentration of solutes is important in controlling the stoichiometry of reactants for solution reactions. Chemists use many different methods to define concentrations, some of which are described in this section. Quantitatively, the concentration of the solution is expressed in many ways. Six methods for expressing the concentration of solutions are most commonly used.: mass fraction.............................................................................