Use of aquarium alkalinity test for spa water

[Mad_caddy]

I have some aquarium drop tests that I decided to try out for balancing my hot tub. They mostly seem to work ok but I’m running into an inconsistency when it comes to alkalinity. When I test my alkalinity with a test strip I’m getting around 160 ppm. When I use the drop test I’m getting about 70 ppm. The drop test is called carbonate hardness (KH). I read that this was the same as alkalinity. Is that wrong? Which number should I rely on?

 

[PoolGate]

You shouldn't rely on any test strip test. We recommend Taylor tf-100 test kit or other Taylor test variants.

 

[ajw22]

You shouldn't rely on any test strip test.

Agreed.

We recommend Taylor tf-100 test kit or other Taylor test variants.

Taylor test kits are difficult to get in Canada so I appreciate the OPs problem.

 

[ajw22]

I have some aquarium drop tests that I decided to try out for balancing my hot tub. They mostly seem to work ok but I’m running into an inconsistency when it comes to alkalinity. When I test my alkalinity with a test strip I’m getting around 160 ppm. When I use the drop test I’m getting about 70 ppm. The drop test is called carbonate hardness (KH). I read that this was the same as alkalinity. Is that wrong? Which number should I rely on?

Welcome to TFP.

I would trust putting my finger up to the wind rather then test strips.

Post a pic showing your KH test kit and we will see if anyone is familiar with it.

Canadians have used creative ways in obtaining Taylor test kits. There are shipping companies who help get it across the border. Or if you are near the border going across to pick it up. Using the search bar at the top should find threads on Canada test kit shipping.

Search results for query: canada test kit

www.troublefreepool.com

 

[JoyfulNoise]

While both tests (KH and TA) use a similar protocol, the test themselves express fundamentally different things.

Typically KH is used in freshwater and saltwater aquariums where things like chlorine, cyanuric acid and borates are not present. The presence of any active chlorine can cause the KH indicator to bleach out.

Also, typical aquarium test reagents are calibrated in degrees Karbonathärte (German for “carbonate hardness”), or dKH. There is an equivalence between dKH and ppm CaCO3 -

1 dKH = 17.85 ppm

So you need to know what the titrating reagents are calibrated in for dKH/drop. Some are 1 dKH/drop while others are 0.5 dKH/drop.

Now the concept to understand is that carbonate hardness and carbonate alkalinity are not the same thing. Carbonate “hardness” simply describes how much dissolved inorganic carbon is in solution -

Carb Hardness = [HCO3^-] + [CO3^2-]

Whereas carbonate alkalinity describes how many hydrogen ions can be “absorbed” or neutralized by the alkaline species in water -

Carb alkalinity = [HCO3^-] + 2 [CO3^2-]

However, when the initial pH in the sample is below 8.3, the amount of carbonate anion (CO3^2-) is so small that the two can be used interchangeably. But the issue to realize is that in pool water, you have cyanuric acid which acts as a buffer and, in some pools, borates which also act as a buffer. So in that instance, you can’t really call the KH test a true carbonate hardness test because there are chemical species in the water that can significantly affect how much titrating reagent is used.

All this is to say that using a KH test is ok for testing TA but you’re going to need to adjust your units to convert from dKH to ppm.

 

[mgtfp]

As Matt has mentioned, chlorine can bleach out the indicator dye. If that's the case, then add a drop or two of Thiosulfate (Taylor R-0007, but any brand should do) before adding the indicator.