API KH test

All: 

The results that some of you have gotten with the API KH test has had be baffled, but upon further inspection of the API website I found this:

What is Carbonate Hardness (KH)?  Carbonate hardness, also known as alkalinity, is the measure of carbonate and bicarbonate concentrations dissolved in water. These minerals are present in municipal, well, and bottled water. The level of carbonate hardness in tap and bottled water depends on the source of the water and the treatment processes it has undergone. Carbonate hardness helps stabilize pH in the aquarium. An aquarium with a low KH level will tend to be acidic. Aquariums with very low KH are subject to rapid pH shifts if not monitored carefully. Water with a high KH level usually has a high pH [1].

This actually explains the results as the KH test does not measure carbonate hardness as the test name indicates, but measures alkalinity. Carbonate hardness is NOT also known as alkalinity, it is that portion of the hardness that is associated with bicarbonate or carbonate ions. The statement that carbonate hardness helps stabilize the pH in the aquarium is wrong: alkalinity helps stabilize the pH (remember alkalinity is the buffering capacity - the ability of a water to resist a change in pH upon the addition of an acid) The statement that waters with very low KH are subject to rapid pH shifts is true - because the water has a low alkalinity. 

I'm sorry that I did not realize this before setting the lab experiment.  However, you should proceed with the lab experiment as specified - it would be interesting to compare the alkalinity determined with the KH and Taylor 1004 kits. You may do that for extra credit.

Dr. Masten

References:
[1] API Brand. GH and KH Test Kits - FAQs: https://apifishcare.com/product/gh-kh-test-kit

Correspondence with API:

Sent via contact form on 9/22/2020:

I am using your test kits to teach our environmental engineering laboratory, which I am teaching entirely remotely. In using the the GH and KH hardness kit, we have observed that with softened water the color of the GH test kit turns from orange to green almost immediately upon the first drop of reagent. On the other hand the KH test does not, and usually requires 15-25 drops, depending on the water source. Is the KH test actually measuring alkalinity? That is the only thing that makes sense to me. The SDS do not provide the reagents for the KH and GH test; that information would be helpful. Your assistance is greatly appreciated.

Response from API Team on 9/23/2020 7:13 AM

Hello Professor Susan Masten,

 

Thank you for contacting API Technical Services. That is correct.  KH is referred to as carbonate hardness or the alkanlinity buffer of the water.  The API KH test kit is testing the carbonate of the water.

 

We hope that we have helped! Please feel free to ask any follow up questions by replying or complete our Consumer Feedback Survey by clicking HERE to let us know how we did.

 

Thank you,

API Consumer Services Team

Technical Services

T 1.800.847.0659

E APITechServices@effem.com

 

 

 

My response back to API Team 9/23/2020:

Dear API Team,

Carbonate hardness is NOT the same as alkalinity. Carbonate hardness is that portion of the hardness that is associated with bicarbonate and carbonate ions. There are situations where the carbonate hardness and alkalinity are numerically equal, but even in that situation they are not the same and alkalinity should never be reported as carbonate hardness. It would be far more appropriate if you referred to the KH as alkalinity not carbonate hardness. 

Thank you,

Susan J. Masten, Ph.D., P.E.

On 9/23/2020 10:04 AM, APITechServices wrote:

Hello Professor Susan Masten,

 There is definitely a difference in reporting conventions for hobbyist type test kits. Our test kits focus on measuring the carbonate ions (CO^2-₃, HCO^-₃) because this is most important for support of nitrifying bacteria, within the aquarium context. The carbonate ions will not be removed by conventional softening.

So it may more appropriate to file this away as “aquarium carbonate hardness,” as most aquarium kits will also use this term and they are equivalent in practice within the context of fishkeeping. I can understand where the lack of nuance of the kits could be frustrating, when trying to source test kits that will not bankrupt the students. This is very similar to the way that most general hardness test kits will not distingush between the different 2+ ions.

 

You would have to move out of this context in order to get kits that express readings in ppm of Calcium Carbonate, such as is common for reporting municipality and ecological data.

 

If you are using the other API kits, I also want to call attention to another difference at this time. The Nitrate is reporting ppm amounts of the ion, rather than “nitrate as nitrogen,” so you will have to divide the result by 4.427 in order to compare back to data using “Nitrate as nitrogen” conventions. Same goes for the nitrite, but the conversion is 3.284.

Ammonia test will read both NH₃ and NH₄⁺, as listed, but the fractions are able to be calculated using an equilibrium equation (accounting for temperature, pH, and salinity). If you were only interested in one ion or the other.

 

Thank you,

API Consumer Services Team

Technical Services

T 1.800.847.0659

E APITechServices@effem.com