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post #1 of (permalink) Old 05-09-2011, 03:19 AM Thread Starter
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~Explanation of how a drop checker works~

It has come to my attention that not everyone knows how a drop checker works, or why it should be used. Since this piece of equipment is normally a vital tool to dosing CO2, I thought I would explain in detail.

What is needed to use a drop checker:
glass drop checker
suction cup
Bromothymol Blue (pH drops)
standard reference soltuion

Fill the compartment with reference liquid about halfway, if it is a bulb type. Then put about 3 pH drops in, or however many it takes to get the perfect opacity of blue, not too clear, not too dark. Suction the drop checker to the inside wall of the tank, under water. Keep it right side up, so no tank water gets in the bulb. Wait a few hours and compare the color to your target color.

A reference solution comes in different dKH. The most common is 4dKH. If you are using 4dKH, this is how you determine your CO2 levels:

Blue - not enough CO2
Green - 30ppm CO2 (target level)
Yellow - too much CO2, dangerous for inhabitants

Some science behind it.
A drop checker is basically a pH test, but not for your tank water. It measures the pH of your drop checker solution, which indicates CO2 levels. How it works is there is an enclosed air chamber trapped between the tank water and the drop checker liquid. The CO2 in your tank will flow freely into the trapped air, and then intothe drop checker compartment. It will do this until the water in the tank and the water in the drop checker reach equilibrium. This means that there is equal concentrations of CO2 in both chambers of liquid. CO2 is the only variable effecting the pH in the drop checker, therefore pH in the drop checker is directly related to CO2 levels in both the tank.
Effect of CO2 and KH on pH:
The more CO2 dissolved in water, the lower the pH will be.
The lower your hardness, the lower your pH will be
There are several other miscellaneous parameters that could effect pH

A reference solution is a solution that has an exact hardness degree that is known. In addition, there are no other unknown water parameters that could affect the pH reading.

Every water parameter is constant, an known, except the CO2 level. The amount of CO2 is the only thing that is affecting the pH. Therefore, the pH reading in the drop checker solution correlates to an exact amount of CO2 .

For example, in a 4dKH reference solution of water and baking soda, a pH level of ~6.6 indicates 30ppm of CO2. A 6.6 pH with Bromothymol Blue turns this solution green, and therefore when the drop checker is green, you have 30ppm of CO2. In conclusion, a drop checker test is just a pH test, in water that has no other variables beside CO2.


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