The pH of our lake is not as expected!

One has to start somewhere and since we live very close to a lake that’s what I preferred.

This lake is the central lake of Almere, called “Weerwater”, meaning “Water again”. It actually describes the history of the lake, because at first the former Zuyderzee (litterally Southern Sea) was closed off by a dam and eventually became a sweet water lake. Then about half of it was drained and became new land (polder) and on this land the town of Almere was built. To get enough sand for the construction works, a huge pit was dug to serve as a sand quarry. After completion of the construction works the pit gradually filled itself with water and was left as a lake, so it was water again after the draining. Now this hole in the sea bottom is the lake “Weerwater”. The bottom of the – rather shallow – lake is covered with water plants and those have to be mown now and then to allow the boats to cross the lake without destroying their propellers. It is about three times the size of the reservoir in Central Park New York.

Of course a sample at a single location would not do, so I selected several points around the lake to take my samples and went on a ride with my bike. Currently the tour is about 12 km. May 7, 2022 was a nice warm day with a temperature of about 25 oC

Sample points lake “Weerwater” Almere, with GPS coordinates

After collecting the samples, I started the pH measurements. Of course I had a hypothesis and it was that areas with more decaying organic material would be slightly more acidic than others. Because of the CO2 in the air I expected the water to be slightly acidic anyway, although not the 5.75 measured in my demineralised water after being shaking for a couple of seconds.

You already know I bought this nice pH meter and had to learn to work with it. At first I calibrated it with the three buffer fluids meant for this purpose, but it seemed like the meter had a “drift” (systematic error), because afterwards the buffer values seemed to be different (after every calibration). Measuring during a couple of days I noticed that the drift only applied to the values shortly after calibration. Then the deviation remains stable, at least for several days. The electrode was rinsed thoroughly after every measurement, especially when buffers where involved because they are capable of changing the pH of non-buffered fluids strongly.

Bottles used to store water samples

The bottles were plastic ones (150 ml), originally holding fruit shots, but thoroughly cleansed with hot and cold tap water and detergent rinsed several times with tap water and demineralised water, which after a while always seems to have a pH below 6 (slightly acidic). As a young boy, over fifty years ago now, I noticed that an inspector taking samples to determine milk quality at a farm, wrote with a pencil on an opaque area on the glass. Now I made a similar opaque area at the bottle’s surface using sand paper. No labels needed!

During sampling at the lake, the bottles were rinsed with lake water once, before taking the sample. Then the bottle was closed and kept at room temperature. The first series of measurements were done only a couple of hours after the collection of the first sample, but additional measurements were performed the days after. The additional hypothesis was that samples with more (microscopic – the fluids were all clear) organic material would tend to get a lower pH (more acidic).

To make a long story short: my original hypotheses about acidity went out of the window. The water was rather alkaline! I’m sure about that, because in between I measured demineralised water and after the measurements I determined the pH of the buffers again (showing more or less the same systematic error again and again). Although the pH meter is sold with a precision of 0.01, I think we have to accept an error of about 0.2 for the accuracy, which is all right. Especially in the alkaline area there is a slight tendency to exaggerate the alkalinity, meaning the measured pH is about 0.2 higher than what it should be according to the calibration buffer. Below the averages of the first three measurements (performed an hour after taking the last sample) are shown (the names identify the location – Dutch names).

Boat Maastrichtkwartier.8,16
Harbour Stedenwijk.8,02
Pier near Schipperplein8,30
Lumièrepark beginning8,27
(point inaccessible)
Fantasie beach8,29
Average pH of three measurements for all Weerwater samples

Below there is an explanation about the systematic error, but the conclusion is that we should take off 0.2 from the measured values to have a more reliable outcome. However, the water will remain alkaline anyway! The values around the lake differ slightly, but in general they are around 8!

Now the question is: how did the water get a pH above 7 (alkaline) instead of the expected value below 7 (acidic)? My new hypothesis was that this could be the result of the construction of the Floriade – the large world exposition for horticulture – or other constructions with concrete going on. That would mean other lakes in Almere would have a lower pH. I will tell you about those measurements another time, after explaining what happened with the pH during the days after collection.

For those who want more detail about the precision and accuracy of the measurements: Learning to know the new pH meter, I calibrated several times and noted down what the measured values of the buffers were after a while. It looked like there was a bit of a drift, but measuring after several days without calibrating, it turns out it’s a rather stable systematic error. To show this systematic error of the buffer measurements, I present the average and standard deviation below. Although there is a 95% confidentiality interval, the deviation from the buffer’s real value is much more interesting. I’ll spare you the details, but we have to correct the measured values by 0.2 towards neutral, reducing the alkalinity. The buffer 4 value is not very stable, but that’s because the focus is not at this range, making it less reliable. The electrode also needs some time to switch several pH points and during the first measurements and I probably didn’t wait long enough in the acidic region. After all the pH meter is very precise and stable, but not completely accurate at the moment. It may be better to wait some time before pressing the calibration button. Still learning!

 Buffer 6.86Buffer 4.00Buffer 9.18
Measured (average)6,754,089,36
Standard dev.0,080,390,21
Lower 95% Conf.Int.6,603,298,95
Upper 95% Conf.Int.6,914,869,77
Precise values, but not completely accurate as there is a systematic error

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