Clarification on CH Test (Taylor K2006)

[Bigdog43]

I'm getting the hang of the test kit and I have a question around the calcium hardness test (and the TA is kind of the same way). When I add the R-0011L and swirl, the sample turns red. I begin adding R-0012 and start counting. At some point the sample turns a light purple almost, but it's definitely not red...or blue for that matter. As I keep dropping, the sample will turn a very light blue and as I continue to add it eventually becomes BLUE BLUE. At what point in this process exactly is "until the color changes from red to blue"?

 

[JoyfulNoise]

BLUE-BLUE.

What water sample size are you using?

The color transition of the R-0011L is probably one of the hardest to detect. People often don't wait for it to develop the full blue color but call the test done at the first flash of baby blue. The best way to see the color transition is to either buy the 250ppm CH standard from Taylor (I think TFTestKits sells it) or to simply take some tap water with roughly known hardness, dilute it a little with distilled water (no hardness) and see what the full spectrum of color transitions look like. Using a diluted sample is just so that the number of R-0012 drops needed is small in order to conserve expensive reagents.

 

[Bigdog43]

I am using the 25 mL per the instructions. So do I take your answer to mean that when the sample stops getting "bluer", then that drop before the last one is my number to record? I guess I'll eventually get used to the shade of blue that I expect to see and recognize it when I get there.

 

[JoyfulNoise]

The color change should stop at blue. However, there is always the possibility of a fading endpoint with this test as trace metals (iron and copper) can interfere with the end point causing the blue to backslide to purplish color. So here's a way to modify the test to avoid that -

It depends on the overall number of R-0012 drops you think you'll add, but typically you can add 5-10 drops of R-00012 FIRST. Then, add the R-0010 as you normally would followed by the R-0011L indicator. Then add the remainder of the R-0012 drops until you get to the final blue color. You then count up all of the drops of R-0012 reagent you've added and that's the final number.

I do the above modification to my CH testing because I have VERY high CH (~1000ppm). I will often dilute my CH test water with distilled water to reduce the CH prior to testing. Very high CH levels can also cause an indeterminate endpoint so tying up some of the calcium prior to adding the indicator helps with the sharpness of the endpoint transition.

See this Taylor webpage for a good visual of the CH test colors - https://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=35