CH test reads 0?

[waterbear]

I re-tested with the HTH kit and got the same weird result - the sample had a faint yellow tint after the 5 drops of hardness indicator (which is listed as 23% isopropanol and 77% Triethanolamine).

warning...chemisty nerd info follows but it might help explain what is happening
The Isopropal alcohol is to dissolve the indicator dye (there are a few different ones used to test for hardness, some of the more commen ones are eriochrome black T and calcon (eriochrome black B), there are others also) and the triethanolamine is used to raise and buffer pH, it is basically just ammonia with alcohol attached to it. The reason that they are listed as ingredients is that they are the actual "toxic" substances in the indicator solution.

The Taylor test (and Ben's) uses sodium hydroxide to raise and buffer the pH to about 12 and cause any magnesium in the sample to preciptate out as magnesium hydroxide so the titration will just show the calcium in the sample. This is added before the indicator solution and is a much stronger base than triethanolame.

A test for total hardness (both calcium and magnesium) is usually perfomed at a pH of 10 so the magnesium does not precitipate out and both are measured. Below ph 10 the test does not work properly.

The same indicator dyes are basically used for either test with some better at one than the other. Like I said, there are several dyes that can be used as indicators. Eriochrome black T and calcon both change from wine red to blue endpoints. Eriochrome black T is more of a blue grey at endpoint such as the color seen in the 2 reagent tests. This might possibly be the indicator used in the 2 reagent tests.

The assumption I am making from this about the 2 reagent kits (and I might be wrong) is that the indicator/buffer might or might not get the pH high enough to remove the magnesium interference, or only partially remove it. The pH and alkalinity of the water being tested will have an effect on just how high the pH can be raised (and how much interferance can be removed or whether the test works at all). If the pH is not in the proper range (10 for total hardness, 12 for calcium hardness) then the indicator does not develop the proper color (which might explain the yellow color that has been observed by myself and others since these dyes are also pH indicators).

Therefore it seems that the results, when obtainable, might be giving total hardness, calcium hardness, or somewhere in between. Therefore, it seems that the 2 reagent test is NOT a reliable test for calcium hardness.

This is just my attempt to explain the results of what has been happening with the 2 reagent test kits and the preceding two paragraphs (in blue) are supposition on my part.