Chapter XV. The effects of heating water on pH

I'm taking a break from taxonomy to provide an update on the experiment I described in Chapter XIII.  After writing "This should kill 99% of bacteria etc. but I think leave the water essentially unchanged" it occurred to me that I better look into what the likely effects of boiling water before adding it to an aquarium might be. There is very little information I could find, especially relating to fish keeping. But the always dependable Skeptical Aquarist has an article on using boiling to soften water. The linked article predicts that the water will be depleted in dissolved oxygen and may have lost some of its buffering capacity. I decided to test what the effect of heating my tap water (TW) was. I heated 15 liters of TW to 70 Celsius in a stainless steel stockpot. I measured the pH before heating and after. I then added an air stone and left it for one week, taking daily pH readings. As I described in Chapter XII, leaving my TW for a week results in a slight reduction in pH. I assume this is the result of the aerobic respiration of organisms present (see here for a great article on bio-acidification). If my heated TW had lost buffering capacity then I might expect to see a more dramatic reduction in pH than in the AW as a population of microbes developed. I went for 70C because I didn't wan't the water to bubble and drive off too much dissolved gas but I should still kill most microbes when the time came to repeat  the process for real on the AW.

TW 0h

Heated TW 70 C + 16h

Heated TW 70 C + 6 days

As you can see, either heating the TW reduced its pH or leaving it overnight has (I should have bought two stockpots, filled them both with water and only heated one of them :). There doesn't seem to have been any further drop in pH during the week so no evidence of a reduction in buffering capacity. The drop in pH is about the same as I see in the AW. Given that this heated TW must have been low in microbial life and has dropped in pH within 16 hours, it does make me wonder if bio-acidification is responsible for the reduction in pH I see in the AW. Maybe its a chemical process. To test this I measured the pH of a sample of tap water straight from the tap and filled a saucepan, left it for 30 minutes and measured the pH.

TW 0h

TW 30m

There's a clear difference and the 30 minute sample looks to be about the same pH as the heated TW after six days. I would guess the process is chemical as 30 minutes seems too fast for a biological process. My explanation would be that water for domestic use is pumped around under pressure. When I fill a container with it the water equilibrates to atmospheric pressure and gaseous exchange takes place. In my case this seems to result in a slight drop in pH (which may suggest that CO2 is being absorbed). The pH then seems to remain stable. I haven't been able to find much information on what the likely effect on pH of storing water would be but here is a link where they measure an increase in pH over 12 days. In the discussion they say "The observed increases in pH during storage could be due to the activities of the resident flora and or their death, which results in the release of inorganic substances such as ammonia". This doesn't seem to be happening with my water. My question is: what were the processes in my aquarium that had previously conspired to maintain the tank pH 7.6 or above while the water I was adding naturally drops to a pH below that after adjusting to atmospheric pressure? As I mentioned in Chapter XIII, I had never measured tank pH lower than 7.6 in the entire two years and three months of cyanobacteria. Anyway, I was satisfied that heating my water to 70C doesn't have a dramatic effect on its pH so I repeated the process on AW and used it for a water change today.

AW 6 days

AW 6days 12 hours after 70 C

It took 30 minutes to heat up the 15 litres of AW and eight hours to cool from 70 C - 20 C. To achieve this rate of cooling I left the stockpot outside on a cold day. When I got back from work I added a heater and an airstone for an hour or so to add some oxygen and warm it up to tank temperature (23 C). As you can see, heating the AW doesn't seem to have affected its pH as predicted. After the water change the fish seemed fine. Now I play the waiting game.