Indoor Air Quality: The Effects Of Carbon Dioxide On pH

AvatarBy Robert Jordan 19 years agoNo Comments

One of the things that confounds the beginning aquarist and those not schooled in the science of chemistry is the problem of a falling pH that usually occurs overnight. A change in Ph of 0.10 is considered a normal occurrence. It is normal because as changes occur in the atmosphere and oceans, all living things have learned to adapt to these changes. This article will deal with just one of the environmental conditions that will affect pH. One of these is the increase of carbon dioxide in well sealed homes that do not have a fresh air intake installed on the heating and air conditioning systems. This compounded by the lack of an exhaust fan to pull out stale air. I am also hoping to stir up some interest in Indoor Air Quality for all aquarist’s who have never given it a thought that their every day habits contribute to a downward trend in water quality. Later in this article, I will give some web sites that you can download information on indoor and
outdoor air pollution. When a residence is not provided with proper ventilation, an increase of CO2 will occur and this mostly happens at night when all windows are secured and the occupants retire for the night. In the indoor environment, the main contributors of C02 are human beings and the use of fuel burning appliances. Human beings are responsible for most of it.

The amount of C02 that it takes to lower pH is indicated in data presented below. The data will be interesting to those who want to improve the air quality and to maintain a stable pH. The outdoor C02 concentration is about 340 ppm. But the concentration indoors can easily reach over 1200 ppm, which is enough to effect the pH downward 0.30 to 0.40. I performed 3 tests to determine just how much C02 is needed to bring down pH, when the alkalinity is 3.5 meq/L. Prior to my first test, I opened all the windows and interior doors to balance the indoor concentration with the outdoor C02 concentration. I did this with the help of a carbon dioxide monitor that has a continuous digital readout. I placed this monitor in the fish room. My presence in the fish room added 40 ppm to the reading over outside air concentration when the test started. Here are the results:

Table 1: TEST #1
Test start timepHCO2 concentrationTest end timepHCO2 concentration

As you see, with low C02 concentration, there was only a 1/100 drop in pH.

In test #2, I balanced the air again. After which I then closed all windows and two interior doors that lead to the fish room. The room is 13 by 15 by 7 feet high. With no ventilation coming into the room and myself as the only occupant of the house, I stayed in the room for 3-1/2 hours.


Table 2: TEST #2
Test start timepHCO2 concentration

In this test, there was a significant drop in pH of 0.18 with a high concentration of C02. If there was a crowd of people in the room under the same test conditions, the decrease in pH would not have taken 3-1/2 hours. Some aquarist’s have already noticed this effect.

The final 6:00 am test readings indicate two things: one is that the pH cannot recover as quickly as it dropped because, as is well known by experienced aquarists, C02 can easily enter seawater but takes longer to be blown off by aeration or water movement. And the other is that it took 6-1/2 hours after opening the two interior doors to vent the excess C02 in the rest of the house to bring it down to 540 ppm.

In test #3, 1 left the room immediately and did not enter until the test was over. As in test #2 the air was balanced with the outdoor concentration.

Table 3: TEST #3
Test start timepHCO2 concentrationTest end timepHCO2 concentration

As you see, this test indicates a normal drop in pH when the C02 levels are kept in the range of 600 to 660 ppm. For economic reasons, this is the range I keep during the heating and cooling seasons. At other times in the year, my pH moves in a range of 0.05 because I usually have doors and windows opened. Now the question of alkalinity comes up. I did not want to change it lower or higher as my tank is running fine. But I believe that if the alkalinity was raised, then it would take a higher concentration of C02 to effect the pH. So if you have a falling pH, you can determine if excess C02 is the cause by following this procedure and comparing your results with the difference between my starting pH and ending pH and its final C02 concentration which should give you a better then ballpark figure for your CO2 concentration. For instance, if your test procedure indicates a drop of 0.10 pH, then looking at test #3 shows a drop of
0.11 with a ending concentration of 660 ppm. This would be your approximate C02 levels in the fish room. Here is the test procedure you should follow:

In the evening, take a sample gallon of tank water and with a strong aerator, place it in a sheltered location outdoors — this is important — and let it aerate for at least 9 hours. But, make sure you take a pH reading the next morning, before the lights come on and before too much sunlight has a chance to effect your readings on your sample gallon. This is because algae will start photosynthesis when the test water is exposed to enough light and this in turn will start to raise the sample water’s pH.

To make it easier for you to interpret your test reading, here are some numbers to go by:

pH SwingApproximate CO2 Concentration
At a difference in pH of 0.05 or lessC02 it is about 340 to 380 ppm
At a difference in pH of 0.10C02 it is about 600 to 650 ppm
At a difference in pH of 0.20C02 it is about 800 to 1000 ppm
At a difference in pH of 0.30C02 it is about 1100 ppm or higher

Now that we see how CO2 affects the pH, what can you do to control it? As mentioned earlier, you have to install a fresh air intake into your heating and air conditioning mechanism, and or add an exhaust fan or open nearby windows.

For those of you who may be interested in the instrument that was used to measure C02 concentrations, it is manufactured by the “Vaisala” company, and to see the specifications and more information, you can go to: The model number is GMW21D. This is designed for shelf or wall mount and is the only model in this series that has a digital readout. These models also have a built in transmitter for high end users who have recorders in their laboratories. The cost of the monitor at the retail outlet is $505.00. Cole Parmer is one mail order company where this item can be purchased. There are four choices of ranges that the monitor can be calibrated for at the factory. These are 0 – 2000 ppm, 0 – 5000 ppm, 0 – 10,000 ppm and 0 – 20,000 ppm. The one that I use is calibrated to 0 – 2000 ppm and is suited for our purpose. So if ordering one, tell the order taker you want the 0 – 2000 calibrated one. The instrument is
designed to operate at 24 vac/vdc. It does not include a power supply. You can purchase one at any electronics store for about $12.00. I am using a 24 volt ac adapter from Radio Shack, part #11328010, 120 vac to 24 vac. There is a recommended interval of 5 years between factory calibrations.


Here are two web sites to see more information on Indoor and outdoor Air Pollution:

I would like to point out that Indoor Air Quality is the homeowner’s responsibility and the Environmental Protection Agency can only advise on maximum exposure levels by setting standards. It is up to the homeowner to abide and take precautions on the many products we use around the home that will contribute to the over-all pollution levels found in the home environment. I hope this article stirred your interest. The EPA has performed studies in private homes and has found levels of toxic chemicals that are five to 10 times higher then found in outdoor concentrations. Now what solution can you think of that could possibly absorb pollutants – the answer is, Air-Water- Interface. It is my belief that the subject of Indoor Air Quality will be the next new topic where questions of water quality are concerned.

If anyone has questions on this article, I can be contacted at or on CompuServe’s Aquaria/Fish forum.

  Advanced Aquarist

 Robert Jordan

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