Reading Borderline Color Changes In Titration Tests...

[Ari]

Hi all, my question is centered around doing various titration/dropwise tests with a Taylor kit, for example, a chlorine powder/titration test, where each drop is .5 PPM, and you're waiting for the reddish color to turn totally clear, let's say you just put in drop #9, and it didn't turn totally clear, but it will on the next drop since it did turn a very light, pale pink on the 9th drop, is it correct when you add drop #10, and it does turn totally clear to figure that the accurate measurement is somewhere in between 9 and 10, since 9 made it almost clear, but not quite, so, in this case, instead of calling it 4.5 ppm, or 5 ppm, it would be more like 4.75 PPM? Or, is it correct to not take into account that the second to last drop brought the solution almost to the brink of complete color change, and in this case, call it a full 10 drops, ie. 5 ppm, and not split the diff between drops 9 and 10?

Now I realize with chlorine, a quarter PPM isn't crucial, but how about when doing total alkalinity or calcium, where you're waiting for the green to turn red, and respectively, the red to turn blue, what I find with calcium is, I get very close to the change from red to blue, but, generally the second to final drop changes the red to something in between red and blue, a purple-ish color, the next drop of course makes it true, blue, same with alkalinity, the second to last drop generally makes the green a sort of dull reddish gray, then that final drop puts it over, and it turns completely red. So, for these tests, is it as I said above, I should split the difference, ie. with calcium if drop 26 makes it purple, and drop 27 makes it completely blue, should I consider my calcium to be 265?

I realize that differences like this aren't too important, but with TA, the difference between 60 and 70 is somewhat significant, so, I'd appreciate any answers you all might have, thanks!

Ari

[PaulR]

The Taylor tests are generally stated as +/- 1 drop, so 60 and 70 on the TA test aren't really that different.

As a rule, for TA and CH you add drops until there is no further color change. Any drop that caused a change should be counted.

See these instructions from Taylor.

--paulr