The below mentioned article provides a short note on Osmosis:- 1. Definition of Osmosis 2. Experiment of Osmosis 3. Application 4. Electro-endosmosis.
Definition of Osmosis:
Osmosis is defined as the spontaneous flow of water into a solution, or, from a more dilute to a more concentrated solution when the two solutions are separated from each other by a semi-permeable membrane.
Osmosis occurs in the direction opposite to that in which diffusion occurs. The animal membranes are not completely “semi- permeable”. The artificial membrane, Cu2Fe(CN)6, is the most selective. This is prepared by keeping copper crystals in the pores and dissolved in K4Fe(CN)6 solution.
Experiment of Osmosis:
In Fig. 2.4 below, the semipermeable membrane is fitted at the mouth of the thistle funnel. The thistle funnel is provided with a sucrose solution up to a mark and inverted over water in a trough.
After some time it has been found that the level of sucrose solution is increased in the thistle funnel owing to the entry of water through the semi-permeable membrane.
The flow of water will be prevented when both the pressure (inside and outside) are equal. The excess pressure can be measured by a mercury manometer.
Application of Osmosis:
1. The purgative action of Epsom (MgSO4 7H2O) or Glauber’s (Na2SO4 10H2O) salts is an osmotic phenomenon. A strong solution of a salt in the intestine prevents absorption of water or withdrawal of water from the body causing dilution of the intestinal contents.
2. The pain caused by contact of sugar with exposed nerves of teeth is due to the osmotic withdrawal of water from the exposed area by strong sugar solution.
3. The pain experienced by the application of salt on the cut of the skin follows the same osmotic phenomenon as in No. 2.
4. Water or salts (chiefly NaCl) are excreted by the kidney to keep the blood isotonic with the cells. Hence, osmosis is of great importance in the process of urine secretion.
5. The clinical application of osmotic force is the injection of hypertonic solution of magnesium sulphate to reduce the volume of the brain or lower the pressure of cerebrospinal fluid.
6. Hemolysis is caused by the dilution of RBC by the osmotic phenomenon.
If a potential develops across the ends of the capillary when a liquid is forced through it, it is not surprising to find that the liquid flows through the capillary when a potential is applied across it.
The movement of a liquid through a capillary under the influence of an electric potential is termed electro-endosmosis.
Membranes prepared from collodion, tissue membranes etc. may be considered structures containing great numbers of pores.
The walls of the pores of those and most other membranes are negatively charged, and the water in the pore is positively charged.
When such a membrane is placed in water between electrodes so that the liquid is separated into two compartments, water flows from the compartment containing the positive pole through the membrane into the compartment containing the negative pole.
This process continues until the level of water in the negative electrode compartment is sufficiently above that in the positive electrode compartment to balance the rates of water movement through the membrane in opposite direction. Membranes with positively charged pole walls give electro-endosmosis in the reverse direction because the water is negatively charged and mobile.
The addition of acid “hydrogen ion (H+)” causes negative pole walls to become less negative and positive pole walls more positive. Alkali (OH−) has the reverse effect. Salts also tend to change the pole charge relating to the liquid, the change depending upon the selective adsorption of salt ions on the pole walls.
The electro-endosmotic effect is changed accordingly as the relative charges of pole walls and mobile liquid are changed.