Alastair said:
I was just pondering to myself about my drop checkers today as you do when it's all new to us, and was just thinking, if the air pocket inside a glass drop checker was much less, would this make the reaction time slightly quicker as there is less space for the gas to build up inside before reacting with the solution?? Hmmmm :?
Hi,
In order to grasp the mechanism of the dropchecker try imagining a dropchecker filled entirely with water and zero air. We would then be left with a water/water interface, wherein the tank water is in direct contact with the reagent water.
CO2 is 10,000 times less soluble in water than it is in air. So the movement of the gas from the tank water to the dropchecker water is inherently slow. CO2 easily out-gasses to atmosphere. That is why we have to keep pumping it into the tank for hours on end. 90% or more of the gas you inject escapes to atmosphere immediately. The air pocket inside the checker is therefore your "mini atmosphere" which facilitates escape of the gas from the tank water into the dropchecker. Therefore, you want more air in the checker. That air bubble acts as hoover, pulling CO2 from the tank water in the same way that the atmosphere above the tank pulls CO2 out of the tank's water. As the partial pressure of the CO2 in the bubble increases it then diffuses into the reagent water.
Likewise, if the CO2 in the tank water falls, then the partial pressure of the CO2 in the air space falls, and the higher partial pressure of the CO2 in the reagent water quickly vents into the air bubble, thereby facilitating it's movement back into the tank. The bubble therefore serves two purposes, it physically isolates the tank water from the reagent water so that only gases can move between the two waters and it acts as a hoover to move the gasses more easily between the two waters. As a result, it serves no purpose to minimize the volume of air in the checker. If you do this then you will only be making the checker even more slower to respond than it already is.
When you think about CO2, or any solute in a solvent, you should always think about the solutes pressure. 30 part per million (ppm) of CO2 in water is really a measurement of the
pressure of CO2 within the total pressure of the water. 15 ppm of Nitrate is really an indication of the pressure of those Nitrate molecules being exerted against the body of water in which it is dissolved. When the gas escapes from the water it's pressure within the solvent collapses in the same way that a balloon collapses when the air escapes. The only way to prevent the gas from escaping from the water is to have an equal or higher pressure of that gas above the water.
When you inject CO2 you are injecting it at a higher pressure than what is present in the atmosphere. That's why you regulator gauge reads something like 1.3 PSI, which means that the gas is entering the water at 1.3 PSI higher than atmospheric pressure. This pressure forces the gas into solution and raises the CO2 partial pressure in the water, but since the atmospheric pressure of CO2 is much lower just above the tank water, the CO2 pressure within the water escapes has a tendency to fall immediately.
Hope this helps.
Cheers,