I was wondering if anyone could help troubleshoot an experiment I'm working on.

I'm running an experiment to measure the microbial respiration of compost samples using a $\ce{NaOH}$ trap. Each sample is in a jar with 20 ml of 1M $\ce{NaOH}$ and is placed in an incubator for 24 hrs. After 24 hrs, 20ml of .5N BaCl2 and an indicator are added to the $\ce{NaOH}$ and it is then titrated with $\ce{HCl}$. I run a few blanks each time, with an $\ce{NaOH}$ trap in a jar without a compost sample. This is repeated over 4 days In theory, I should need less $\ce{HCl}$ to neutralize the $\ce{NaOH}$ which has been sitting with the compost samples then the $\ce{NaOH}$ just by itself. Sadly, after hours and hours of work in the lab, this has not been the case!

I have 45 different samples of different types of compost. Each time I run the test, about a third of the samples require more $\ce{HCl}$ to neutralize than the blank.

The blanks are all consistent in the amount of $\ce{HCl}$ needed, I ran three last night and one the night before. The test also seems to be picking up samples which are respiring heavily- when there are moldy samples, they require only a small amount of $\ce{HCl}$. I can't really decipher a trend with the ones that require more $\ce{HCl}$ to titrate than the blank other than that it's somewhat consistent. 41% of the samples which needed more $\ce{HCl}$ than the blank on day 2 also needed more on day 1, and of the day 2 "error samples", 88% were from a treatment which had an "error sample" on day 1.

I thought perhaps this had to do with ammonia volatilizing from some of the samples, but this doesn't necessarily make sense with the samples which are taking more $\ce{HCl}$ than the blank. I also noticed that on the samples which require more $\ce{HCl}$ to titrate than the blank, there seems to be more precipitate (barium carbonate?) that settles out after the titration.

Do you have any idea what could be happening or what I am doing incorrectly? It's definitely a long process to do this measurement for 48 replicates over 4 days - I'm hoping the data isn't totally worthless.


Edit: Pictures

enter image description here

On the left are samples which required less HCl to neutralize than the blank. (I forgot to add an extra 20ml of BaCl2 to the second from left, which I had added an additional 10 ml NaOH to, so that one is an error). In the middle is the blank and a sample which required the same amount of HCl to neutralize as the blank. Then on the right are samples which required more HCl than the blank to neutralize.

  • $\begingroup$ re: "Each sample is in a jar" I assume that the jar is sealed? If so the oxygen in the jar will be depleted quickly. Basically you need oxygen to make the CO2. $\endgroup$ – MaxW Feb 27 '17 at 22:25
  • $\begingroup$ Is the jar sealed? Have you tried stirring during the incubation? Have you computed the theoretical change associated with your experiment? $\endgroup$ – Zhe Feb 27 '17 at 22:59
  • $\begingroup$ Hmmmm interesting. The method says to "close the lid tightly" of the incubation vessel. Otherwise I believe the trap would pick up CO2 being respired by all the other samples. I'm expecting about 300 mg CO2 respired on average per day? Doing some rough back of the envelope calculations there would be about .016 moles O2 in the jar and with 300 mg CO2, that would be .006 moles used. If the CO2 respiration was a lot higher, that could definitely deplete all the O2. Even if all O2 was depleted in a sample, shouldn't that NaOH trap need less HCl to titrate than the blank? $\endgroup$ – Spodzol Feb 27 '17 at 23:12
  • $\begingroup$ Jar is sealed. I would worry about them not being air tight but my blanks all require the same HCl to titrate. I haven't stirred during the incubation. As for the theoretical change I believe it would go like this: CO2 + 2NaOH = Na2CO3 + H2O, then Na2CO3 + BaCl2= BaCO3 + 2NaCl, then the HCl + NaOH = NaCl + H20 $\endgroup$ – Spodzol Feb 27 '17 at 23:52
  • $\begingroup$ Filtering BaCO3 out? $\endgroup$ – MaxW Feb 27 '17 at 23:59

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