The Drop Checker – How to & the Science behind it

1. Preparation of Indicator Solution:

The correct indicator solution is a combination of KH Standard / 4 degree KH (4 dKH) and pH indicator (Bromothymol Blue).

With drop checker in hand, turn upside down and fill the reservoir of the drop checker approximately half full (or to level manufacturer recommends) with 4 dKH.  A syringe is an ideal tool to use for this.  Then, add approximately 2 – 4 drops of pH indicator (Bromothymol Blue) until a strong, TRANSPARENT blue color is achieved.

The Science:

pH and pH IndicatorpH is a measure of the acidity or alkalinity of a solution. pH indicator is a chemical detector that causes the color of the solution to change depending on pH.

KH and KH StandardKH (Carbonate Hardness) is the measurement in degrees of carbonate and bicarbonate in the water.  KH Standard is a solution with a known KH, a standard, with no other buffers other than carbonate and bicarbonates.

To measure CO2 in the aquarium, you need to have an accurate measurement of KH and pH.  With a known value of KH / carbonate hardness in the indicator solution, the color of the solution will turn green when the optimal concentration of 30 ppm of CO2 is achieved in the aquarium water.  A yellow or blue color indicates too much or too little CO2 in the water, respectively.


2. Place in Aquarium and wait to observe color change:

With the correct side up, place the drop checker in the aquarium, in a visible location.  After approximately one hour, you should notice a change in the color of the indicator solution.  After about two (2) hours you should be able to observe the color and take an accurate measurement of the concentration of CO2 in the aquarium.  For increased accuracy it is recommended to observe the color against a solid white background.

BLUE ->  too little CO2 / low CO2 levels  ->  increase the rate of CO2

GREEN ->  proper CO2 levels  ->  no action required

YELLOW ->  too much CO2 / high CO2 levels  ->  decrease the rate of CO2

The Science:

The drop checker is a reservoir designed to contain an indicator solution and an airspace or air bubble.  When submerged in an aquarium, the airspace eliminates contact between the indicator solution and the aquarium water.

When CO2 is injected into the aquarium, the CO2 will out gas from the aquarium water into the airspace of the drop checker.  As CO2 enters the air, it is absorbed into the indicator solution.  The absorption of CO2 is the result of the gas seeking a point of equilibrium, or equal distribution of CO2 in the solution and the air; the basic principle behind the function of the drop checker.  As CO2 gas is absorbed into the indicator solution, it lowers the pH of the KH standard, which in turn changes the color of the pH indicator.  This condition of equilibrium allows the drop checker, a simple glass reservoir, to provide a highly accurate measurement of carbon dioxide (CO2) in the planted aquarium.

This process is reversed when the injection of CO2 is decreased or zeroed, and the indicator solution contains more CO2 than the aquarium water.  The CO2 will outgas from the indicator solution to seek equilibrium with the airspace and the aquarium water.  This out gassing of CO2 will raise the pH of the solution and change the color of the pH indicator.

In summary, never underestimate the power of the drop checker as an accurate tool for the measurement of CO2 (when used correctly).  Look forward to a video to follow.

One thought on “The Drop Checker – How to & the Science behind it

  1. Great write-up, Orlando. Thanks for sharing it in the last newsletter.

    Your more advanced or obsessive customers may like this calculator, which will let them DIY a reference solution for any CO2 target while using one (your example) or two (a theoretical idea that works very well in application) of the great drop checkers offered by GLA!


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