Oxygen saturation

Funny thing today...

Yesterday some students were doing photosynthesis experiments after school. Two of these experiments were to measure dissolved oxygen concentration (that is produced by photosynthetic activity, dah) under different light conditions (using filters) using a dissolved oxygen probe we have.

It couldnt have gone worst.

Nobody, yours truly included, had ever or knew how to operate the probe. And that is one bitch of a probe. Secondly, there was the issue of the sensor, which is supposed to be filled with KCl solution, was dry. The first 2 trials were irrelevant because i failed to notice the sensor was dry. Once I figured that out, i made some .1M KCl filler and filled it up. Alas the probe was giving out a reading. Then calibration was a problem because our sodium sulfite was somehow bad (sodium sulfite solutions won't let oxygen in, so they're 0% O2 in solution). So i made new solution. The probe calibrated alright and everything was good to go.

I set it all up in the biology lab. I placed the elodea plants in the distilled water, got a reading of 4.1mg/mL and started the test. 5 minutes into the test the laptop that was gathering the data runs out of battery and thus dies. After clearing up a bit I moved the experiment to my office. I set it all up again, warmed up the probe, calibrated and started the experiment with a reading of 4.5mg/mL. This was ok because i had expected that in the half hour it took to move the experiment (i also went for coffee hehe), more O2 would be dissolved, so no problem.

Here's the thing, concentration went up to 4.7mg/mL in the first 10 minutes of the experiment and stayed there... for the next 4 hours.

All i could say is... what the fuck.

After much thought, i realize that OF COURSE I'M AN IDIOT. Given the altitude of the city (~900-1000m above seal level), barometric pressure and temperature will have a significant effect on oxygen saturation. In this case, using a table i calculated saturation at around 5.1mg/mL. But given the change in biochemistry the water undergoes because the plant is in there, i think it's pretty safe to assume it'll be around 4.7 to 5. And 4.7 it was.

So I boiled 1L of dH2O for an hour and got it to drop the O2 concentration; it read 2.01mg/mL, which is not bad considering the substantial increase it'll have if saturation occurs at 4.7mg/mL.

I let the water cool downa and started the whole show again.

Funny how science works sometimes. The starting concentration of 2mg/mL is right now 1.4mg/mL, and the experiment's been running half an hour.

We'll see how this one works out.