Outgassing rate of O2 from water

[wongal]

At my school we have decided to try and construct a water barometer, which will work the same way a mercury barometer does, but will be 10+ meters tall. Once set up, the "vacuum" part of the barometer (above water level) will be filled with water vapor, and we can correct for the vapor pressure readily enough. However, of more concern are other gasses, particularly oxygen, outgassing into the "vacuum" at the top of the barometer. We will address this at first by using degassed water, but eventually, since the bottom of the water column must be open to the air, the water will dissolve oxygen (and, presumably, other gasses), some of which will sooner or later make it up into the "vacuum". My question is this: is there any way to calculate the rate at which oxygen will make its way from the air outside, up the water column, and into the "vacuum"? I have no sense as to whether it would take minutes, days, or years for enough oxygen to be transferred to affect our atmospheric pressure measurements. Any ideas for retarding this process?

Thanks!

 

[Borek]

Cover the water with some foil (like trash bag).

Add some oxygen scavenger to water.

 

[dshacks]

Dissolved gasses will effect the accuracy of the device, or rather will change the amplitude of change. It seems to me that the evaporation of water into the vacuum space will have a significantly greater and more rapid effect on the over all operation. Adjustment for the water vapor will be a continuous process unless your environment is carefully and closely controlled (not normally the case in educational environments).

You should also note that both temperature and pH will have a direct effect on the solubility of the atmospheric gasses on the "exposed", as well as the vacuum side of the system. The solubility of gasses into "degassed" water and the rate at which they dissolve is also a function of temperature and pH, as well as is the amount of surface area exposed.

I would suggest a light mineral oil in place of the water. To avoid most of these issues. Unless you just love hassling with atmospheric gas perturbations.

 

[Borek]

Light oil will mean much higher column, 10 meters is already high.

But perhaps light oil poured on the water (instead of the foil) is an easier idea.

 

[alphy]

I am happy to hear about your project to construct a water barometer. It is an interesting experiment and I am sure you will learn a lot from it. Outgassing of oxygen from water is a common phenomenon and it is important to consider its effect on your barometer.

To answer your question, the rate at which oxygen will make its way from the air outside, up the water column, and into the "vacuum" can be calculated using the Henry's Law. This law states that the amount of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas in the surrounding atmosphere. In simple terms, the higher the partial pressure of oxygen in the atmosphere, the faster it will dissolve into the water.

However, there are other factors that can affect the rate of outgassing, such as temperature, agitation, and the surface area of water exposed to the atmosphere. It is difficult to provide an exact calculation without knowing all the variables in your experiment.

To retard the process of outgassing, you can try using degassed water and keeping the temperature of the water constant. You can also minimize agitation and reduce the surface area of the water exposed to the atmosphere by using a smaller opening at the bottom of the water column. Additionally, using a degassing chamber before filling the barometer may help to remove excess gases from the water.

Overall, it is important to carefully consider the outgassing of oxygen and other gases in your experiment and take appropriate measures to minimize its effect on your results. Good luck with your project!