General characteristics of nonelectrolytes solutions. Water solutions of nonelectrolytes. Acid-base characteristics of solutions, страница 2

In biomedical researches the measurement of boiling point increase is used seldom because it often leads to destruction of biological structures – protein melting, ferment inactivation etc..  It is also important to remember that atmosphere pressure has a great influence of boiling point.  Besides, freezing temperatures of water solutions can be measured more precisely by a greater value of absolute size of  water cryoscopic constant. (KH2O = 1,86>EH2O = 0,51).  There are values for some biological liquids.

Table 2

Freezing point decline of some biological liquids comparing to water

Liquid name	∆tf, °C
Blood	0,56
Tissue juice	0,6-0,8
Cerebrospinal fluid	0,56
Saliva	0,19-0,49
Gastric juices	0,49-0,65
Perspiration	0,13-0,6
Urea	1,12-2,3
Bile	0,51-0,61
Liver juices	0,81-0,98
Milk	0,55-0,57

1.3 THE PHENOMENON OF DIFFUSION AND OSMOSIS

          Solutions like gas mixtures have got the ability for diffusion.  Spontaneous particles distribution of one soluble substance in another is called diffusion.

In solutions such transference happens owing to chaotic movement of molecules – entropy growth. Diffusion is observed both at mixing of various substances and at contact of solutions of various concentrations.  As a result of diffusion the concentration of solution becomes equal in the whole volume of water. If we divide solutions of various concentration by a thin membrane permeable only for molecules of dissolvent then we notice the picture of dissolvent transition through the membrane from a diluted dissolvent to a more concentrated dissolvent.  Membranes which let molecules of dissolvent (water) through but stop molecules of soluble substance are called semipermeable membranes.  Cellophane, parchment, cell cover etc. have got the characteristics of semipermeable membranes.

One sided  transition of dissolvent molecules through semipermeable membrane is called osmosis.  The phenomenon of osmosis is observed with the help of special device osmometer. They fill in the solution into a glass tube with a semipermeable membrane on the end of it. Then the tube is being sunk into the a vessel with pure water.  Owing to establishment of thermodynamic balance water will go through the membrane from the vessel to the tube and go up on the rube.  Liquid increase in the tube produce hydrostatic pressure in it which is growing gradually.  A number of water molecules which go from the tube to the vessel is growing with hydrostatic pressure increase.  Eventually there comes dynamic balance when speeds of water moving in one and other side become identical.  From this moment osmosis finishes and liquid in the tube will balance the motive force of osmosis – osmotic pressure.  Be measuring hydrostatic pressure of water column in the tube we can define osmotic pressure:

P=h*ρ

Where P – osmotic pressure;

            h – height of liquid increase;

            ρ – density of a solution.

Thus, OSMOTIC PRESSURE IS THE PRESSURE WHICH SHOULD BE APPLIED TO A SOLUTION TO BALANCE IT WITH A PURE DISSOLVENT SEPERATED WITH SEMIPERMEABLE MEMBRANE FROM IT.

Osmotic pressure at constant temperature  is in direct proportion to a concentration of soluble substance and at a constant concentration is proportionate to absolute temperature.

Despite of the various mechanism of origination of osmotic and gas pressure, between them there is a certain analogy.  Van’t Hoff pointed that osmotic pressure of solutions submits to laws of Boyle-Mariotte, Gay-Lussac, and Avogadro.

Osmotic pressure is equal to a pressure which soluble substance being in a gas condition would put upon if it would have a volume  (at the same temperature) equal to a volume of solution.  Equation for description of osmotic pressure for nonelectrolytes solutions, offered by van’t Hoff (1884), can be presented like this:

P=Cos*R*T

Where P – osmotic pressure, atm or kPa;

           Cos – osmolar concentration, mol/L;

           R – universal gas constant, equal to 0,0082 (L*atm)/(mol*K) or 8,3 (L*kPa)/(mol*K);

           T – temperature, K.

Osmolar (integral) concentration (osmolarity) – is the total mol quantity of  all kinetic, able to particles’ independent moving in 1L (1000 ml) of the solution, irrespective of the nature, the form and the size. Osmolar concentration is being calculated by the following formulas.

For solutions containing only one substance.

Cos=i*c

                    P=(i*m/M*V)*R*T

Where: m- weight of soluble substance, gram;

           V – volume of solution, L;

           M – molar weight of soluble substance, gram/mol;

           I – isotonic factor (for nonelectrolytes i=1).

If the solution contains few soluble components then just as the pressure of gas mixture is equal to the sum of their pressures,  the total osmotic pressure will be the following:

                                                                                     i=n

Ptotal = i1*R*T*C1+ i2*R*T*C2+…+ in*R*T*Cn= R*T*Σij*Cj=RTCos

                                                                                               j=1

                    i=n

where Cos = Σij*Cj

          j=1

From van’t Hoff equation follows that solutions of various substances  with identical osmolar concentration at the same temperature produce identical osmotic pressure.  Such solutions are called isotonic or isoosmotic.  For example, osmolar solutions of glucose and urea  at 0°C have the same osmotic pressure equal to 22,4 atm (2268,7 kPa).