The amount of dissolved oxygen in water can be measured directly with instruments or by titration. To determine the quantity of dissolved nitrogen in water-, the total gas pressure Must first be determined. This total gas pressure can be measured with an instrument called a Saturometer. The amount of dissolved oxygen and water vapor pressure, after it is converted to mm of pressure, can then be subtracted from the total gas pressure and the pressure of dissolved nitrogen results This is not a simple subtraction, but a rather involved calculation is used, including considerations for temperature and pressure. Thus a thermometer and a barometer are necessary to furnish the information needed for the solution.

Gas levels in water can reflect the recent history of the water. Water pumped from well or obtained from a spring frequently has higher levels of nitrogen than surface water. a cold surface to warmer areas in the soil Springs often have supersaturated levels of nitrogen year around but & especially in the spring of the year during and after snow melt. The very weight of the water in the water table will increase the solubility of nitrogen due to higher pressure even though the solubility of nitrogen decreases due to increase in temperature.

Snow melts at 32 degrees F, nitrogen equilibrium (100%) is 23 ppm. Note: this is much colder than the standard 68 degrees F. As the water percolates into the ground with an increasing column of water above it from more melting snow the pressure increases. It also warms as it goes deeper to about 50 degrees. Nitrogen equilibrium at 50 degrees is about 18 PPM and the water is then at 128% nitrogen saturation ! Theoretically this is possible but rarely encountered. Nitrogen supersaturations of 103% to 107% are quite common with supersaturations of 115% to 120% are not unusual, especially in the spring with well water. As an experiment, seal a container that is about half full of saturated water at 68 degrees F. If at standard temperature and pressure, this water now contains 9.1 PPM dissolved oxygen and 14.9 PPM dissolved nitrogen. Now remove all the air in the container and replace it with pure oxygen. It will take weeks for the contents of the container to reach equilibrium if it just sits there the process can speeded up by shaking the container on a paint shaker for 10 to 15 minutes. If the atmosphere in the container is now analyzed and we find it to be 20% nitrogen and 80% oxygen and about 3.7 pa dissolved nitrogen and 36.4 PPM dissolved oxygen (DO) in the water. It is just this process that explains why we cannot get 100% of oxygen dissolved in water when we introduce pure oxygen into water. Increased pressure will reduce the amount of free gas but there will always be some residual gas coming out of the water.

Reprinted from July/August 1993 AQUACULTURE MAGAZINE

VMG INDUSTRIES, INC., Rock Roskowski and Bruce Marshall