Testing dKH: counting the drops - The Planted Tank Forum
Old 09-09-2016, 12:36 AM Thread Starter
Algae Grower

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Testing dKH: counting the drops

So the instructions in API GH and KH test kit say, "The KH value is determined by the number of drops of test solution that must be added to turn the water in the test tube bright yellow", and give a table that says 1 drop = 1dKH and so on.

What's been bothering me is--say, distilled water will turn yellow after one drop. But of course its KH is not 1 degree or 17.9 ppm, it's zero. So, doesn't that mean that dKH is the number of drops minus one? Or to put it another way, you first add a drop and then count how many drops after the first one it will take to change the color?
CaptHobbes is offline

Old 09-09-2016, 12:50 AM
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If it turns yellow at the first drop and capping the water and mixing it, then the KH is either 0 or 1. The test doesn't differentiate between the two. Results would be the same, regardless.

2 KH would be 1 drop = blue, 2 drops = yellow.

I get what you're saying, but don't overthink it! I was overthinking it too, after getting the GH and KH test kits! Was just too used to the other tests which are much simpler! No need to keep adding solution once you've put in the correct amount! And the amount doesn't change based on the water!
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Old 09-09-2016, 12:57 AM
Planted Member

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So what color "yellow" are we talking. I'm having issues with doing this as well.
mcdanielnc89 is offline

Old 09-09-2016, 03:05 AM
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1 drop in a 10ml sample =.5dKH if it is remotely blue.
1 drop in a 20ml sample =.25dKH and so on.
Blue hue is very hard to see in 50ml sample, this low really doesn't matter.

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Old 09-09-2016, 04:59 AM
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Just yellow. If you're not sure, look from the top of the tube to the bottom over a white background.

I've also gotten "green", so I figured that was probably a "half"!
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Old 09-09-2016, 05:18 AM
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The color change, though none of the colors are very vivid, is pretty distinct to me. Usually as soon as the color changes, you're done--though, yeah, like Zoidburg said, you'll want to make sure it's not in-between. Holding it over a paper towel or against a white wall usually works for me.
geisterwald is offline
Old 09-09-2016, 05:37 AM
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Maryland Guppy has a good point. I think.

To find out whether you have 0 KH or 1 KH, use 10 ml instead of 5 ml for the test! At 10 ml, 1 drop would be blue for 0, and 2 drops should be yellow for 1. At least, in theory.

Decided I would do a test to show what my tap water is like.... First tube, 1, has 1 drop of KH, second tube has 2 drops and so forth.

Last edited by Zoidburg; 09-09-2016 at 05:56 AM. Reason: fixed attachment
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Old 09-09-2016, 08:02 AM
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Quote:
Originally Posted by Zoidburg View Post
At least, in theory.
It really works.

Many have complained on the forum about API GH & KH test kit regarding no color change.
If there is nothing to measure what does one expect?

If you are reading a very hard water a 2.5ml sample can be used, every drop is 2dGH or KH.
This can save on reagents too if you test a lot of hard water.

After modifying all of the API test to some smaller or larger sample size the results have been fine.
Only test I don't trust fully is for NH3, occasionally I get false positives on 1.3ml / 2 drop tests.
For those testing many tanks etc.. or conducting experiments, this can really stretch the test kits to save a buck, why not?

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