We use cookies. By browsing our site you agree to our use of cookies. Find out more

FAQ Login
The Fish Tank Forum is no longer allowing posting of any kind or new members to register.
All visitors can still join the group discussion which has been moved to Facebook.
Fish Tank Forum will be remaining here for reference purposes only.

I just want to say thank you to everyone who took part in the forum over the years and I hope to see you all on our Facebook group.

Drop Checker's

Articles on general fish keeping

Drop Checker's

Postby Jack Middleton » Thu Jul 07, 2011 7:40 pm

The drop checker was invented by Takeshi Amano, he wanted a way of accurately measuring CO2 concentrations and invented the following method:

Basics and how they work:

Drop Checkers are small pieces of glassware used to measure concentrations of CO2 within high-tech planted aquaria, they work on the principle of diffusion, as CO2 is dissolved into the water column by the means of an atomizer, atomic diffuser, CO2 ladder etc, carbonic acid is formed which reduces the pH, the drop checker is filled with a pH indicator (Bromothymol Blue) and 4 DKH water, we can then deduce the CO2 concentration by measuring the pH and KH of the solution within the drop checker. The air pocket in the drop checker is lower in CO2 concentration than the water column surrounding it, so CO2 will diffuse into the air pocket from the water column to create an equilibrium, the same happens with the small amount of solution within the bell of the drop checker, carbonic acids are then created within the bell thus reducing the pH of the solution and ultimately changing the colour of the solution.

Drop checkers should always be filled with pure water that has had bicarbonate or carbonate added to increase the KH to 4 degrees, and then a few drops of bromothymol blue should be added, because when a solution has a KH of 4 and a pH of 6.6, there is a concentration of 30Mg/L of CO2 dissolved within it, we know the PH is 6.6 when the solution turns to a lime green colour, this is why using pure water with carbonates added is imperative, to ensure that the only chemicals dissolved in the solution are carbonates and that there are no acids interfering with the PH results.


You can buy 4DKH water but it is just as easy to make your own, you'll require a 1l volumetric flask and a set of scales with an accuracy of 0.01 grams. Add 1.2 grams of Sodium Hydrogen Carbonate and then add RO water up to the 1l line on the volumetric flask (The bottom of the meniscus is 1l) this solution will have a KH of 40 degrees, then take 100ml's of this solution and empty the remainder from the volumetric flask and rinse with RO water. Then add the 100ml's of Sodium Hydrogen Carbonate solution back into the volumetric flask and make it up to the 1l line with RO water again, you now have 1l of 4DKH water. (You could always use a 500ml volumetric flask and half the mass of Sodium Hydrogen Carbonate etc but the tolerance for accuracy will be doubled to about + 10%.)

Bromothymol blue is quite a common liquid too, any standard hobby test kit with a range of 6-7.6 on the pH scale will be Bromothymol blue, I know API kits are popular and the re-agent in their standard pH kit is indeed Bromothymol blue.

Drop checkers are readily available, there are lot for sale on ebay from Honk Kong, the nicest drop checkers I have seen without a doubt are made by Cal Aqua Labs, and are sold directly from them or are available from aqua essentials. Some kits also come with Bromothymol blue which is sometimes called PH indicator instead, if you do buy a kit throw away the instructions as they're 99% of the time all wrong, follow this guide and you wont go wrong.

Setting up your drop checker:
Using a pipette add a quantity of 4DKH water to the bell in the end of the drop checker so that it is half way up the bell and has the largest surface area possible, if you're using a square shaped drop checker then obviously this is irrelevant. Then add 2-3 drops of Bromothymol blue, add more if you feel that it could be a little darker, the solution will be blue to begin with. Then upturn the drop checker ensuring no solution escapes and place it in the aquarium so that there is a large air pocket in the neck of the drop checker, and adhere it to one of the walls of the aquarium using a suction cup.

There is a 1-2 hour lapse in colour change, so what you're actually seeing when the drop checker is in the tank is what the CO2 concentration was approximately 1-2 hours ago.

As I mentioned before, you're aiming for lime green, here is a sort of colour scale but for a more accurate idea look at the scale on your PH kit, you want the lime green colour at 6.6 on the pH indicator scale.

Too little CO2
Just right
Too much CO2

Replace the solution weekly when you do a water change and if you need to clean the drop checker soak it in a chlorine bleach solution (1 part bleach to 5 parts water) and leave for a few hours, then soak in a strong solution of water with a 10X over dose of dechlorinator in it, or you could use sodium thiosulphate to bind the chlorine, then rinse the drop checker with RO water.

This is how your drop checker should be orientated:
Ignore the colour of the solution in the image, it had only been in the aquarium for 30 minutes after a solution change.
Jack Middleton
Posts: 396
Joined: Wed Jun 08, 2011 3:42 pm

Re: Drop Checker's

Postby malakye » Thu Jul 07, 2011 8:16 pm

another great article, thank you so very much Jack.
User avatar
Site Admin
Site Admin
Posts: 11469
Joined: Wed Feb 18, 2009 11:03 pm
Location: Scotland

Return to Articles

Who is online

Users browsing this forum: No registered users and 6 guests

Join our facebook group