We recently requested a professional review for our book “Osmosis: The Molecular Theory”. That was again an interesting experience. We have had the opportunity to find reviewers who are incompetent or dishonest. Sometimes it is difficult to spot the difference from a short conversation.
So, we will review osmosis history once again. Osmosis was discovered by an innovative priest who wanted to find out why is urine was yellow. In 1748, wine was a common beverage since water supplies were limited and/or polluted. What happened to the purple color of the wine? He added a membrane from a pig’s bladder to the bottom of a tube, then filled the tube with wine. The membrane end of the tube was inserted in water. Hydrostatic pressure from the column of wine would cause flow through the membrane. Nollet would determine if the membrane was filtering the color from the wine.
It was a simply defined experiment to find what happened to the color of the wine. The results were puzzling, however. Pure water flowed up through the membrane into the wine. And a substantial pressure applied to the wine was needed to stop the flow of water into the wine. We note that osmosis flow through the membrane is slow. It must have taken Nollet many experiments to discover this. And in 1748, many areas of scientific knowledge were yet to be discovered and, wine becomes a sacred element in the sacrament of Eucharist.
During the following 150 years and many osmosis experiments, sugar water was substituted for wine and manmade membranes were developed. Others were discovering the thermodynamic behavior of a gas. Van’t Hoff compared the behavior of a perfect gas ( pressure of the gas is directly related to the number of gas molecules in a fixed volume) to the osmosis experiment data. For low concentrations of sugar in sugar water, the pressure required to stop the flow of water through the membrane (osmotic pressure) was directly related to the concentration of sugar in the sugar water. This was the first thermodynamic verification of the osmosis experiment. This was significant.
At this time, it was understood that flow was related to pressure difference. If osmotic pressure on the sugar water was needed to stop flow through a membrane, then there must surely be a similar and equal pressure on the water side of the membrane. It was also discovered that increasing pressure on the sugar water side, would cause water flow from the sugar water into the pure water. This is called reverse osmosis and is being used today to extract pure water from polluted water.
Many scientists questioned van’t Hoff’s theory from the time it was first introduced. Some even suggested that vapor pressure and vapor were important for osmosis. Although this was considered, osmotic pressure was much greater than the vapor pressure.
We came to the discussion more than 100 years after van’ Hoff’s excellent work. We had the opportunity to design and build controlled atmosphere storage facilities for DeKalb Genetics Hybrid seed corn. We used a simple arrangement of thermostats and humidistats to control the equipment required to keep design conditions in the seed storage warehouses. After we switched to a digital control system, we looked to simplify the programming and found that water vapor pressure was an excellent choice for a control variable.
Later, we modeled a seed interaction with its environment using vapor pressure as the driving force for moisture transfer through the seed hull. With this model, we introduced a theoretical understanding of several rules of thumb that were in used in the seed industry. After we asked several colleagues to review our efforts, we quickly found that our analysis did not use the accepted theory of osmosis for moisture transfer through a membrane.
So, we studied osmosis.