Increasing the hardness test resolution


Junior Poster
Mar 30, 2008
A few days ago, when measured hardness of water - KH and GH, I hit into the idea of a substantial resolution increase, without any additional indicator use.

The original instructions for my test looks like this:
take 5ml of the water and then add a drop of indicator everything stirring. If, after adding a drop the color change the number of drops is equal the hardness of water (in german degrees). In this guide is also written that we can increase the accuracy of the test by doubling tested water (10ml) and the result we obtain by dividing the number of drops by two.

My idea is this:
1. Follow closely by the above instructions to determine water hardness.
2. Treat what you take as an upper limit of hardness and then
add to the test tube by drop the tested water stirring thoroughly. This
time we are waiting for the withdrawal of the colors. Remember the number of
drops given to the transition.
3. Hardness are counting on the basis of the formula:
dH = number_of_indicator_drops * 5ml / (5ml + number_of_added_water / 20)
assuming that 20 drops equals to a 1ml volume and the primary tested water volume is 5ml.
4. Obtained by this measurement resolution is approximately 1% of
number_of_indicator_drops so 0.1dH for 10dH or 0.01dH for 1DH.
It follows to the accuracy of applying 1 drop = 0.05 ml

At the base of this reasoning is the assumption that reaction occuring is reversible in color change.


Tom Barr

Staff member
Jan 23, 2005
Well, you based it on the assumption that 1 ml and one drop conversions are accurate, so they really are not that accurate though.........particularly user to user.

It's a good idea if you have some equipment and can measure microliters of solutions.

The Lamotte test kit gets to 4ppm, or about 1/4 a degree.
The Hach digital titrator, about .1ppm(maybe if you are lucky).
I think 1/4 a degree is fine for most applications for aquariums.

You can easily get that with this + a 20mls sample vs a 5 ml sample volume.
1/2 a degree is easier with a 10ml sample volume etc.

This has it's upper limits though.

Still, there's a few ways to do better.

The Lamotte kit is normally to 4 ppm, so doubling it will yeild 2ppm and you might get to 1ppm okay. That's 17.86X less than a typical kit and method.

Other colormeteric meters can go much lower.

Tom Barr


Junior Poster
Mar 30, 2008
Yes, mesuring microlitres is the best but it is also easy to check how many user drops contain 1ml.
Now user can calculate hardness:
dH = 5ml * number_of_indicator_drops / (5ml + number_of_added_water / how_many_water_drops_in_1ml)

What I like in my method:

- it is repeatable (with my tests for now)
- increases resolution a lot, even if it is not very close to teoretical 1% of first measured dH
- it is a part of normal test procedure so you can stop if you don't need more resolution
- user don't use any additional indicator drop - it is cheap

I don't like that:

- calculations are neccessary
- to increase real accuracy user must calibrate test

Piotr Sroczyński