CH test reads 0?

[waterbear]

You win the prize!

I was careless, and didn't notice on my quick read through earlier. The '2-reagent' system combines the buffer and the indicator. But, on pools with lower pH, and relatively high alkalinity, there's not enough buffer in the indicator. That's why extra drops of indicator -- which add extra drops of buffer -- works, when the 'normal' number of drops don't.

(The pH of the sample has to be pretty high, for the indicator to work, and the 'buffer' tries to move the pH up high enough to work. But, pools with high alkalinity are resistant to pH change, thus the problem.)

Ben

So this could also explain my last test results. If the pH is not brough up high enough then the interfering magnesium would NOT precipitate out (or not completely) and the titration would show more of a total hardness reading instead of a true calcium hardness reading! (Current ALK at 110 ppm and pH at 7.4) When I did the experiements previously my pH was at 7.8-7.9, ALK 110. My ALK never seems to change much unless I really lower the pH and airate.

Bottom line, IMHO, is that the test in the Aquachem kit is not reliable for CH!

[prh129]

At the time I did the HTH hardness test with the weird results (yellow solution to faint green), my pH was low and so was my alkalinity. (I did the test twice with the same result both times - as I swirled the blue reagent the wisps turned red before disappearing.) The numbers at the time were:
pH - 7.0
TCL - 2.5
ALK - 30 ppm

I have since brought the pH up (7.3) and the alkalinity up (90) also. I will repeat the HTH test tonight and see if it still acts weird.

Peter

[prh129]

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).

I tested with the PS234 and after 20 drops of cal 1 and 5 drops of cal 2, the sample was pink. After two drops of cal 3 the sample turned blue. This makes sense and is about what I would expect.

I then started over with 20 drops of cal 1, then 5 drops of the HTH indicator and the sample turned pink although deeper than with cal 2. I then tried the HTH titrant and after 10 drops it was more of a greyish color but nowhere near the blue from the last test. I repeated this test using cal 3 as the titrant this time and got the same grey result so it is clearly the HTH hardness indicator that is not working with my water for some reason.

I also tested pH (7.3) and alkality (90) on the sample.

[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.