API's KH test kit - anyone know how to quantify "bright yellow?"

[Emika_B]

Aloha!

If you've been following my 40 gallon journal, you'll know I'm having issues with KH.  I started off using Tetra's Easy Strips and have always gotten a result of 0 KH, 300 GH and pH of 7.2.  That's been steady for a week now.  I picked up some API test strips (bad reviews, I know) just to compare, and got the same results.  Huh.  Today I picked up API's liquid test for KH.  Followed the instructions to add 1 drop to 5 ml tube of tank water, cap and invert several times.  That gave me blue water -  yay!  Added 1 drop, capped and inverted and it turned yellow.  Step 4 says, "The test is completed when the water in the test tube, after having been shaken, turns from blue to yellow."  Okay, so 2 drops for my test.  Then, step 5 says, "The KH value is determined by the number of drops of test solution that mus be added to turn the water in the test tube bright yellow."  Okay, keep adding drops.  I stopped at 8 - it was getting darker, not brighter.  What does "bright yellow" mean?  Banana yellow?  Box label yellow?  Wish they'd included a color chart.  2 drops is 35.8 ppm, 8 drops is 143.2 ppm, so very different.

Anyway, I'm going to go with 35.8 ppm since that's close enough to what 2 other tests have told me.  Time to modify one of my fluidized filters and add a media bag of aragonite to it.  Let's see what that does.

[Brandy]

The chemistry here is for the color flip, not the intensity. 

[Brandy]

Longer nerm version:

The drops are added until they exhaust the buffering capacity of your water, which triggers a ph shift and a subsequent indicator color shift. The color is probably brighter in more buffered water, as there would have been more "blue" drops added before the color flipped.

[Emika_B]

Brandy - Well, the color 'flipped' on drop 2, so it would seem that I have practically no KH.  Thanks for the info!