Alkalinity titration questions

[cypho]

Thanks. There is certainly no practical use for of 4 significant figures of precision on the alk. But if the measurement is as precise as it seems to be, I don't like the idea of introducing unnecessary rounding errors when doing the conversions.

Thanks for pointing out that the and ppm conversion should take density into account. My controller is already doing a density calculation from conductivity and temperature sensors using the GSW toolbox to do the math, so no reason not to feed the density into the conversion equation.

Should the dKH conversion factor also include density?

I have noticed the inflection point tends to slowly rise over the course of the day and then slowly fall back down during the night. The daily swing in the inflection point is about 0.05 pH.

Here is my most recent test (using the extra digits in the conversion, but not yet taking density into account).

Starting prime and drain.
   Done
   It took 2.23 minutes to prime and drain

Starting rinse.
   Done
   It took 3.20 minutes to rinse

Preparing sample.
   Adding 17.00 ml sample and 0.4586 ml 0.1N HCl
   Done
   It took 2.77 minutes to prepare sample

Starting Titration.
  Titrating to pH of 3.60
   meq/l         pH
   2.6977 5.22667820069
   2.8065 5.07989619377
   2.8767 4.91214532872
   2.9188 4.84923875433
   2.9539 4.74439446367
   2.9797 4.63955017301
   2.9984 4.53470588235
   3.0113 4.53470588235
   3.0241 4.47179930796
   3.0347 4.45083044983
   3.0452 4.40889273356
   3.0546 4.34598615917
   3.0651 4.30404844291
   3.0768 4.26211072664
   3.0908 4.17823529412
   3.1084 4.17823529412
   3.1259 4.05242214533
   3.1528 3.98951557093
   3.1844 3.90564013841
   3.2254 3.80079584775
   3.2827 3.71692041522
   3.3564 3.59110726644
   Done
   It took 14.17 minutes to titrate.

The alkalinity is 3.046 meq/L
                 8.539 dKH
                 152.4 ppm

 

[Want2BS8ed]

I'd be careful to not carry this concern too far. Besides being a possible sign of OCD ( ), it may also not be the full story.

I prefer CDO. It's a lot like OCD, but the letters are in the right doggone order!!!

M

 

[cypho]

Calmagite is only useful as a complexometric indicator at pH values midway between its two pKa of 8.14 and 12.35, and acts as a pH indicator at pH values near its two pKa, so it is not useful for a Ca titration at high pH (but is excellent for the Ca + Mg titration). The Hach method for Ca uses Hydroxy Naphthol Blue as an indicator, which is also what I use when manually titrating Ca after precipitating the Mg as the hydroxide.

Helpful hint for the Ca titration: If you have a pretty good estimate of what your Ca level is, you can greatly improve the accuracy and precision of this titration by delivering about 90-95% of the EDTA necessary to complete the titration *before* you add the hydroxide to precipitate the Mg. Then, complete the titration to the pure blue endpoint. The endpoint will be sharper, and you should find it to be much more precise. In general, this approach will give results that are a little bit (1% to 2%) higher than the standard approach. This is because the with the standard approach, some Ca gets taken down along with the Mg as the hydroxide, and then it wants to come back into solution near the endpoint, causing you to chase the endpoint somewhat, while still giving a slight negative bias. Adding the bulk of the EDTA at the beginning of the method binds that Ca up, preventing it from being precipitated out to begin with.

FWIW, the mixed indicator dye described in this paper produces an incredibly sharp endpoint for the Ca + Mg titration, and you can substitute Calmagite for the EBT in that recipe. It goes from red to grey to green very quickly at the endpoint, and if you consider the very first appearance of any green tint as the endpoint, you can achieve very precise results (relative standard deviation around 0.25%).

Jim,

I ended up using Hydroxynaphthol Blue for the Ca test as you suggested and Calmagite for Ca+Mg. Both tests are working great.

The Ca seems to be +/- 1 ppm and Mg +/- 10 ppm.

Thanks for all of your help.

 

[Randy Holmes-Farley]

Nice!