Poor Man's CO2 Indicator

Inspired by the Dupla Diving-Bell CO2 indicator and by Gary Bishops's
DIY CO2 controller, and of course by my Canadian $ paycheck, I put
together a CO2 indicator for my tank that required essentially no
cash outlay.

Essentially, the indicator consists of a vial of distilled water with
a known concentration of NaHCO3 and a few drops of bromthymol blue
indicator solution from an inexpensive pH test kit.  The vial is connected to
the tank water by an air gap as shown:
    |  ______ |
    |  |    | |
----|  |----| |-------- tank water line
    |--|    |_|
    |  |    | | <- indicator solution
    |  |    |_|

At equilibrium, the CO2 concentration in the vial equals the CO2
concentration in the tank.  The CO2 concentration, in turn, controls
the pH and therefore the color of the solution in the vial.  Once the
pH has been determined from a color chart, the CO2 concentration can
be computed or looked up in a table.  Many variations on this design
are possible, but resist the temptation to move the indicator solution
out of the tank; if there's a temperature difference between the vial
and the tank water, distillation will occur.

The tricky part in all of this is the tiny amount of NaHCO3 needed to
make the indicator solution.  To get a pH of 7.0 at 15ppm CO2, 0.119g
NaHCO3 must be added to 1.0l distilled water.  Accurately measuring
out 0.119g of anything requires a very good scale and a lot of care to
avoid contamination.  Of course, once you've made up a batch of
solution, you can give most of it away to your friends; since my vial
only holds 4ml, 249 of you can get some free from me (the catch is
that you have to pick it up yourself).  Three drops of bromthymol blue
are about right for 4ml solution.

The relationship between pH and ppm CO2 for this system is:

  ppm CO2 = 10^(8.18 - pH)

Or, in convenient tabular form:

  pH   ppm CO2

  7.4     6
  7.2    10  
  7.0    15
  6.8    24
  6.6    38
  6.4    60

The major problem with this indicator is its very slow response time
(hours).  I use mine to check the CO2 concentration no more than once
or twice a day.  A wider vial would increase the surface area of the
indicator solution and improve the response time a bit.  Getting a
good color match can also be a problem with cheap pH test kits.

Many thanks to Paul Sears for doing the math and mailing me exactly
0.119g lab-grade NaHCO3.  He also pointed out the problem of distillation
in my original design.
Kevin Conlin   kcconlin at cae_ca   "We're Canadians.  We HAVE to be cheap"
Finger as332 at freenet_carleton.ca for PGP public key.
It's sunny and -10C in Montreal today.