Testing KH - API KH Test not correct?

[Sentral]

Hello chaps,

I'm having issues testing my KH...

Using the API KH test, following the instructions step my step and doing it a few times, I still don't get any reaction.

The instructions say keeping a drop until the orange reagent turns to green, I've got up to 30 drops and it just stays orange. Pretty sure the KH of water should be below this?!

The GH of the water being tested is 4.

Anyone have experience with KH? I'm quite confused

*Edit:

I've just read Jame's excellent thread about hardness, and found an article which goes into the test kits a bit more

Alkalinity using Test Kits
Of course, most reefkeepers measure alkalinity with a test kit, not with a pH titration. How does that work?

Well, in effect test kits do a pH endpoint titration. They all include pH indicating dyes (providing a color change) and an acid (frequently dilute sulfuric acid) to lower the pH. You typically add acid until the dyes turn color. Since these dyes are selected to have a color change in the pH = 4 to 5 range, what you get is a measurement of how much acid it takes to lower the pH to that range. This color change is used to approximate the endpoint of the titration.

Interestingly, many test kits use more than one pH indicating dye. Using more than one dye at the same time permits the endpoint to be sharper. For example, bromcresol green has a broad color transition between pH 3.8 (yellow) and 5.4 (blue-green) and methyl red has a broad transition between pH 4.4 (red) and 6.2 (yellow). A mixture of the two (used in the Hach alkalinity kit) has a sharp transition (orange to blue-green) around pH 5.1 in fresh water (which may be slightly different in salt water).

Five point 1 you say? Based on the discussion above, is that low enough? Well, the Hach kit was designed for use in fresh water where the pKa of the bicarbonate is much higher than in seawater, and in that situation, it is appropriate. In seawater, however, it is marginal. My tank water took 3.4 meq/L to get down to pH = 5.03, and then an additional 0.4 meq/L to get down to pH 4.00. Consequently, this kit (and others with a similar dye mix) may be missing out on 10% of the alkalinity simply because it isn't titrating low enough. This difference obviously isn't significant to most reef keepers, but is something to keep in mind when doing such things as comparing test kits to standards (in seawater) or to each other.

Some test kits also provide a different dye for a different measure of alkalinity. Frequently, this other dye is phenolphthalein. This dye has a color change between pH 8.2 and pH 9.8. In fresh water, carbonate is almost completely converted into bicarbonate at pH 8.3, and that is the purpose of phenolphthalein titrations: to determine alkalinity in freshwater due to carbonate only (discussed in detail below). This test serves no purpose in a reef tank or seawater for two reasons: 1) the water is probably already more acidic than the endpoint of this dye, and 2) the carbonate in seawater is not completely converted into bicarbonate at this pH anyway. That is, even if the pH were higher than 8.3 (say, 8.6), titrating down to the phenolphthalein endpoint will not effectively "count" all of the carbonate because in saltwater there will still be substantial carbonate present at the phenolphthalein endpoint.

but, I'm still none of the wiser into finding out my KH, and how to decipher the test?

*Edit 2:

I've just found the water analysis for my area:

pH pH
7.49
Alkalinity mg/l CaCO3
150
mg/l HOC3
183
Calcium mg/l Ca
79
Magnesium mg/l Mg
9.2
Total Hardness mg/l CaCO3
237
Clark° (UK)
17
French°
24
German°
13

I'm looking at the CaC03 figure for the KH, which would be 150ppm, giving me a dKH of 8-9. Correct?

Cheers

 

[Alastair]

Funny you mention that mate, I have the Api kh and gh test kit.
It states in mine that add drops until the tube water goes from blue, to yellow. It isn't blue from the bottle and is yellow immediately in the water.
Your description says it goes from orange to green.
Both Api kits but both different colours??


Sent from my iPhone using Tapatalk

 

[dw1305]

Hi all,
The different colour changes are just to do with different pH reagents. All the tests work the same way, in that you add an acid until the alkalinity is consumed and the pH drop causes a pH indicator to change colour. It is exactly the same technique as the drop checker uses with bromothymol blue changing colour as the CO2 (the fraction that changes to carbonic acid) changes the balance of H+ ion donors (acids) & acceptors (bases). pH is the ratio of acids to bases, so it should work pretty well. The acid added will be hydrochloric (HCl) or sulphuric (H2SO4) and the pH indicator may be a mixture, but will be based on the 1 or 2 of the indicators in this list.
<http://www.ph-meter.info/pH-measurements-indicators>

It is the German measure for dKH, 17.9ppm "1 dH is "One degree German" and defined as 10 milligrams of calcium oxide (CaO) per litre of water. This is equivalent to 17.85 milligrams of calcium carbonate per litre of water, or 17.85 ppm."

So 237/17.9 is approx. 13dKH.

Some water details here:
<http://www.ukaps.org/forum/viewtopic.php?f=51&t=8480&p=93934&hilit=corsham#p93934>

For those who are more chemically minded, and it is based on marines, there is a really good article on alkalinity here, including the description of why CO2 addition changes pH, but not alkalinity:

<http://www.advancedaquarist.com/2002/2/chemistry>

cheers Darrel

 

[Alastair]

I've never personally used test kits ever, I only got these as I'm about to start a soil tank journal and gave discovered that certain soils massively increase gh.

 

[Sentral]

Thanks Darrel, but why doesn't the reagent change colour? I'd like to be able to test kh myself

 

[dw1305]

Hi all,

Thanks Darrel, but why doesn't the reagent change colour? I'd like to be able to test kh myself

I'm not sure, but it definitely isn't working, so you need a new test kit. Anywhere in the Bristol area will have water from either the Chew Valley lakes or the River Severn (via the Sharpness canal) if it is "Bristol water" water? or Wessex Water from an aquifer but it is all pretty hard.

cheers Darrel