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post #1 of 8 (permalink) Old 03-11-2010, 12:05 AM Thread Starter
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why 4kdh?

from what ive seen in a quick search was people said you needed this as tank water would have contaminates that would mess your solution.

but what if you used pure ionized water without adding baking soda or such to raise the kdh?

maybe, how i think a drop checker works, and how it actually works is different.



Now what im assuming is they read the off-gas of the tank water into the indicator solution which has the bromthymol blue in it to allow you to "read" it.

for this reading to take place im assuming the kh of the indicator solution fluctuates causing this color change.

so if the kh of the indicator is going to change with use regardless couldn't you use any pure (contaminate free) source of water to go in along with the bromthymol blue.

with this thinking i get to a point, where I think they use 4 kdh so you know what color it will look like if it is the right co2 level. (ie. for a new guy) so you have something to go by as it changes. If you've seen the colors its going to change you don't need this starter level of KH to begin with.

again all out of my head i may be WAY off


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post #2 of 8 (permalink) Old 03-11-2010, 12:56 AM
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You're right -- it has to do with targeting a specific CO2 level. The generally accepted "target" value of CO2 concentration is 30ppm. Since CO2 changes the acidity of a solution, and bromthymol blue is sensitive to a narrow range of pH, this works to our advantage. By shooting for 30ppm CO2, the pH of water is such that bromthymol blue needs to be in a 4dKH solution to get the easily-seen green color. If a different indicator were used, it's likely the solution concentration would change as well.
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post #3 of 8 (permalink) Old 03-11-2010, 01:16 AM
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Quote:
Now what im assuming is they read the off-gas of the tank water into the indicator solution which has the bromthymol blue in it to allow you to "read" it.
That part is correct

Quote:
for this reading to take place im assuming the kh of the indicator solution fluctuates causing this color change.

so if the kh of the indicator is going to change with use regardless couldn't you use any pure (contaminate free) source of water to go in along with the bromthymol blue.
That part is incorrect. The KH of the indicator solution stays constant and the acidity of the indicator solution changes due to the diffusion of co2. The change in acidity is what changes the color of the bromthymol blue, and at the given KH of 4 that is commonly use, we know that the solution will be a nice green color when the co2 level is approx 30 ppm.


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post #4 of 8 (permalink) Old 03-11-2010, 01:18 AM
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A drop checker works using the ph-kh relationship chart.

http://csd.net/~cgadd/aqua/art_plant_co2chart.htm

The reason 4 dkh is used is because at 30ppm CO2 reads at a nice green. If you were to use a lower kh solution, say 2, it would change the color that you see.

This is done as a standard. Technically, you could use anything, so long as you know the precise kh and know that it is only carbonate hardness and not effected by tanic acids, among other things.
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post #5 of 8 (permalink) Old 03-11-2010, 01:19 AM
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Originally Posted by Stasiu View Post
That part is correct



That part is incorrect. The KH of the indicator solution stays constant and the acidity of the indicator solution changes due to the diffusion of co2. The change in acidity is what changes the color of the bromthymol blue, and at the given KH of 4 that is commonly use, we know that the solution will be a nice green color when the co2 level is approx 30 ppm.
To elaborate, the CO2 changes the pH which changes to color of the brom blue.
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post #6 of 8 (permalink) Old 03-11-2010, 04:21 AM
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When the solution is green the pH is about 6.6. If the KH is 4 dKH, the ppm of CO2 will be about 30 ppm. If the KH is 2 dKH, the ppm of CO2 will be about 15 ppm. If the KH is 8 dKH, the ppm of CO2 will be about 60 ppm. https://www.plantedtank.net/forums/fe...co2-chart.html So, you can choose whatever KH you want to match the CO2 concentration you want to have. But, since the easiest color to judge, with bromothymol blue, is green, it will work best to use 6.6 as the target pH. That is how the 4 dKH number was selected.

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post #7 of 8 (permalink) Old 03-11-2010, 06:03 PM Thread Starter
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good to know I was only seeing the one side of the equation. i might play with a higher kdh on my nano since there are no animals in it just plants but i keep my diy C02 higher to fight the algae, that tank so easily gets even with EI.


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post #8 of 8 (permalink) Old 03-11-2010, 07:29 PM
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You can still use the 4 dKH water just increase CO2 until the drop checker turns yellow or ignore it altogether.

A drop checker is used to check the relative level of CO2, they are not that accurate, if there is no livestock in the tank you can crank the CO2 without worrying about gassing your fish, when using a drop checker staying in the green zone not only means approximately 30 ppm CO2 but a safe level for livestock too...
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