Author: Dr. Michael Chase, 16th May 2019
This article describes the method and PRELIMINARY outcome of a relatively low cost “laboratory” (kitchen) experiment on the response of seawater pH to increases in atmospheric CO2.
When additional CO2 is added to the air above seawater the chemistry of the water is expected to change, and there is much focus on the pH measure of acidity. I find that the change in pH when equilibrium is established is readily detectable with a low cost portable pH meter with 0.01 resolution, but the measured size of the effect appears to be significantly smaller than reported in the recent scientific literature. This article deals mostly with the experimental method and results.
A schematic plan of the experimental apparatus is shown in the following figure. A 2-litre “airtight” food storage jar contains seawater and air, and portable (battery powered) instruments to measure seawater pH and atmospheric CO2 level, with alterations to the latter by injection of pulses of additional CO2. The water is mixed continuously by a magnetic stirrer.
The actual experimental system used is shown in the following photo, the pH meter is white, and the CO2 meter is grey, with its air intake visible just above the water surface. Food storage technology provided the 2-litre airtight container, the plastic “table” that holds the CO2 meter just above the water surface, and “Cling Film” to protect the CO2 meter from getting wet, with a hole cut for the air intake. Details of the meters used, and of the magnetic stirrer, are given at the end of the article.
The original plan was to use a small portable electric fan to mix-up the air, but it was found that the batteries ran-out after about an hour, insufficient time to achieve pH equilibrium, which is found to take around 10 hours.
CO2 GENERATION & INJECTION
CO2 was generated by adding vinegar to bicarbonate of soda in a beaker (with splash guard), injected via an unused ear syringe, a plastic syringe or eye drop dispenser could be substituted:
In a draft-free room the CO2 generated in the beaker can be thought of as a relatively dense fluid which remains there, and which can be drawn into a suitable device and injected into the 2-litre glass container. The aim is to get around 1000 ppm of CO2 in the 2-litre glass container, which is 1 part in a thousand, i.e. 2 ml of CO2 at atmospheric pressure, which can be easily generated and captured.
Seawater samples of around 450 ml each time were obtained from the shoreline near Weymouth in Dorset, on the South coast of England. The shoreline had some seaweed, which may influence the pH. Each sample was stored in a sealed 500 ml plastic bottle in the “laboratory” overnight to warm it to the laboratory air temperature.
Each run of the experiment lasts around 12 hours, which is roughly the observed time required for pH equilibrium to be achieved, with data recording at 15-minute intervals. The runs start mid-morning and run to around 10 pm, a period in which the room temperature remains fairly constant.
The following figure shows early results for 3 runs, one with no injected CO2 (uppermost data), one with injected CO2 such that around 820 ppm was measured (shown in blue with a constant +0.2 shift in all pH values), and one where the final CO2 level was around 1500 ppm:
If the recent growth rate of around 2.4 ppm per year of atmospheric CO2 is maintained then the level at the end of the 21st century will be around 600 ppm, with a change in seawater pH of around 0.15, which is considerably lower than figures quoted in the scientific literature. Note that the change of 0.15 in pH applies to seawater of 8.8 pH, which is at the high end of the observed range. Hydrogen ion concentration at a pH of 7.8 is ten times higher than for a pH of 8.8, so the change of pH in 7.8 pH seawater may be even lower than 0.15.
· pH meter: Hanna Instruments HI 98108
· CO2 meter: Kane-alert-CO2
· Magnetic Stirrer: Hanna Instruments HI 190M
Note: Water in a sealed container gives 100% humidity, and condensation may eventually damage the electrics of the meters, everything is thoroughly dried at the end of each run.
Readers can follow developments, and make comments, at the following website: