You will remember that every third sample from the “two lakes and the river” was kept. The samples were stored in the dark, but at room temperature. Earlier samples from the Weerwater and several other lakes were kept as well. Later the same was done while “sampling along the dyke”, also keeping every third sample.
For all samples, the pH dropped gradually during the days after and even during the hours after sampling, although the difference is quite small for only a couple of hours (usually only 0.01 or 0.02). Yet it was a reason to measure immediately, at the source location.
The decrease of the pH has to be related to biological activity, e.g. bacteria or algae (although we don’t know for sure). Adding cyanide could stop the process, but would change the pH at the same time. I don’t have cyanide and I don’t want to have it either, so that’s not an option. Heating would stop biological processes, but could also influence the pH and the same goes for cooling, although that could be a valid option. We’ll investigate it in the future.
For now it’s enough to observe this drop again and again. However, the change is not the same for every sample!
The (third) samples were measured two times (for the later sets in two cycles, but the standard deviation is always small: 0.01 – 0.02 and the drop is mostly ten times higher) with a check of the buffer’s pH before and after (usually no need for calibration). The results are presented in the graphs below, with some space for the days without a measurement.
Be aware that the scale of the Y-axis is slightly different from previous graphs.
The “Weerwater” is repeated for comparison (see previous post on this subject). This time the graph got the same Y-scale as the others, empty positions were added for days without measurement to make the time-scale more realistic and the demineralised water reference was removed.
The rate of change is not predictable. For one sample it can be 0.2 and for another one 0.6 or even more! Sometimes the pH seems to go up again, but I think that’s because of homogeneity or the sample. I later experiments the sample was stirred while measuring and then the results are more stable.
What really strikes me is that when I was obtaining the Weerwater samples and the days after, the temperature was really high: about 27oC The other samples were taken in a period with a temperature of about 10oC lower, around 17oC. Yet the change of the pH doesn’t seem to be very different. I wanted to show the pH-change against the original pH in a graph. Some approaches were too complex, but with a trick it worked out: a lot of values were available for day 5 and the ones missing had day 4 and day 6 values. I interpolated the latter (taking the average) and used them as day 5 values to get a complete graph. The result is clear: the more the initial pH differs from neutrality, the faster it is moving towards neutrality (although hardly even going beyond 8 within this period).