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I'm still thinking about the headspace calculations. The papers on them get intense.You should do some figurin’ to see if your results are within the bounds of possibility. Here are some questions that might be useful?
What is the concentration of CO2 in the system when the pH drops from X to 7.55?
Would you expect the headspace above distilled water and tank water to be approximately the same? Show work
What concentration of CO2 in seawater would be in equilibrium with 1800 ppm CO2 in the headspace?
If all the carbon in the vodka and vinegar is converted to CO2 and did not leave the tank, what would be the concentration in the system?
How much oxygen is consumed in converting all the added ethanol and vinegar to CO2?
First, lets talk about how much Carbon I added and what that means for CO2 and O2 in the system.
12mL vodka - at 8x the carbon of vinegar - equivalent to about 100mL vinegar + 60mL actual vinegar, so about 160mL vinegar.
160mL (160g) * 5% acid = 8g of acetic acid.
[and I'm gonna ignore the direct acid effect because I didn't measure it, and focus on the CO2 effect which I do have data on.]
8g acetic acid / 60g molar mass = 0.133 moles Acetic acid (or acetate)
CH3COO- + 2O2 -> 2CO2 + H2O + OH-
2 CO2's for each acetate = 0.266 moles CO2 produced and 0.266 moles O2 consumed.
My system is ~265L so 0.266Mole/265L = 1millimole/L of CO2 produced and O2 consumed. We'll come back to CO2. on the Oxygen front, At 32g/mole = that's 32 mg/L of O2 consumed.
Maximum O2 level in seawater is about 7mg/L (see borneman), so that's enough Carbon to eat all the oxygen in my system 4.5 times. So that's the task for the aeration / photosynthesis.
Now, how does the CO2 production of 1millimole/L compare to the CO2 necessary to cause the pH shifts I observed?
This page from colorado state is amazing. Never seen it before. Calculate ocean pCO2 and pH from DIC, alkalinity, and temperature
Based on my (very low) alk, the observed shift of pH ~0.45 from mid 7.6ish to just over 8.0 in maximum aerated vs less aerated tank water corresponds to a total dissolved inorganic carbon (DIC = CO2 + HCO3 + etc.) change from 2178 to 1972 micromoles, a change of about 200 micromole/L of CO2 equivalents. And the vinegar I added could generate 1 millimole/L = 1000micromole/L of CO2. So I added enough Carbon to generate 5x the CO2 in the water that I actually observed via pH shift - again - saved by aeration & photosynthesis.
My thought here is misguided. Any CO2 released from that process is overwhelmed by the CO2 from added carbon, so that's what would be observed.The bacteria consuming gunk ought to be producing some CO2.
That page with the awesome seawater calculator also allows you to see what ppm CO2 would be in equilibrium with those parameters.
It said that based on my Alk (2.27meq) and the pH shifts I put in, the 8.05 pH water would be in equill with 388ppm CO2 and the 7.6 pH water with 1290ppm CO2. My measured CO2 values for air in the bottles with those water samples were 680ppm and 1480ppm. Not the greatest agreement, but the calculator shows a 900ppm CO2 diff and I measured an 800ppm CO2 diff. So perhaps mostly reasonable.