Osmotic pressure

Osmotic pressure is the pressure on the solution, separated from the pure solvent membrane is permeable only to solvent molecules (semi-permeable membrane), which stops osmosis. Osmosis is called spontaneous penetration (diffusion) of the solvent molecules through a semipermeable membrane in a mortar or mortar with a lower concentration in a solution with a higher concentration.
Measuring osmotic pressure produced with the assistance of osmometers. The scheme is simple osmometer shown.

osmometer
Scheme of osmometer: 1 - water, 2 - plastic bag (semi-permeable); 3 - solution; 4 - glass tube; h - the height of the column of liquid (measure osmotic pressure).

As a semi-permeable membranes are used film of cellophane, collodion and other
Osmotic pressure of the diluted solutions of non-electrolytes at a constant temperature proportional molar concentration of the solution, and at a constant concentration is the absolute temperature. Solutions with equal osmotic pressure, is called isotonic. A solution with a high osmotic pressure is called hypertension, and with less - hypotonic.
Osmosis and osmotic pressure plays a big role in the exchange of water between cells and their environment. The osmotic pressure of blood in norm on average equal to 7,7 ATM and is determined by the total concentration of all substances dissolved in the plasma. Part of the osmotic pressure of blood determined by the concentration of plasma proteins and is equal to the normal 0,03 - 0,04 ATM, called oncotic pressure. Oncotic pressure plays a significant role in the distribution of water between the blood and lymph.
Cm. also Dialysis, Isotonic solutions. Electrolytes.

Osmotic pressure is the external pressure on the solution, separated from the pure solvent semi-permeable membrane, which stops osmosis. Osmosis is called unilateral diffusion of solvent solution through separating them with a semi-permeable membrane (parchment, animal bubble film of collodion, cellophane). Membrane such permeable, solvent, but do not let the dissolved substance. Osmosis and see if, when a semi-permeable membrane separates two solutions with different concentrations, and the solvent moves through the membrane from the less concentrated solution of sodium, more concentrated. The osmotic pressure of a solution is determined by the concentration of kinetically active particles (molecules, ions, colloidal particles).
Measurement O. D. produced using devices called osmometers. The scheme is simple osmometer presented in Fig. Filled with the test solution vessel 1, the bottom of which is a semipermeable membrane, immersed in a vessel with 2 pure solvent. Due to osmosis solvent will move into the vessel 1 up until the excess hydrostatic pressure measured by a column of fluid height h, reaches the value, when to Roy osmosis stops. Thus between the solution and the solvent is set osmotic balance, characterized by equality speed of passage of molecules of the solvent through a semipermeable membrane in a solution and molecules of a solution in a solvent. Excess hydrostatic pressure of the column of liquid height h is a measure O. D. solution. The definition O. D. solutions often produce indirect way, for example by measuring the temperature drops freezing solutions (see Geometria). This method is widely used to determine O. D. blood plasma of blood, lymph, urine.
Osmotic pressure isolated cells was measured by the method of plasmalite. To do this, the investigated cells are placed in solutions with different concentrations of any of the dissolved substance for which the cell membrane is impermeable. Solutions with O. D. more than O. D. the contents of the cells (hypertonic solutions), cause wrinkling cells (plasmas) due to release of water from the cells, solutions
with O. D. lower than O. D. the contents of the cell (hypotonic solutions), cause swelling of the cells in the transition of water from solutions in a cage. The solution O. D. equal O. D. content of the cells is isotonic (see Izotoniceski solution), does not change the volume of the cell. If the concentration of such a solution, O. D. the contents of the cell is calculated by equation (1).
O. D. diluted solutions of non-electrolytes should the laws established for the gas pressure, and can be calculated but the equation of van't Hoff:
p=сRT, (1)
where p is the osmotic pressure, with the concentration of the solution (in moles per 1 liter of solution), T is the temperature on an absolute scale, R is constant (0,08205 l·ATM/deg·mol).
O. D. electrolyte solution more O. D. solution nonelectrolyte the same molar concentration. This is due to the dissociation of molecules of dissolved electrolyte ions, resulting in increases in the concentration of kinetically active particles in the solution. O. D. for dilute solutions of electrolytes is calculated by the equation:

where i is isotonic ratio shows how many times O. D. electrolyte solution more O. D. solution nonelectrolyte the same molar concentration.
Total O. D. human blood in norm 7-8 ATM. Part of O. D. blood caused it contains high-molecular substances (mainly blood plasma proteins), called oncotic, or colloid-osmotic pressure of the blood that is in norm is 0,03-0,04 ATM. Despite its small size, oncotic pressure plays an important role in regulation of water exchange between the bloodstream and tissues. Measurement O. D. is widely used to determine molecular weight biologically important high-molecular substances, such as proteins. Osmosis and osmotic pressure plays a big role in the processes osmoregulation, i.e. maintaining the osmotic concentration of dissolved substances in body fluids at a certain level. With the introduction of various kinds of liquids in the blood and in the intercellular space least disturbance in the body called isotonic solutions, i.e. solutions, O. D. which is O. D. body fluids. Cm. also Permeability.