Using KI and Thiosulfate Titration for FC. What about CC?

[SBax]

I'm using the old standard method.
1 liter water sample
Add 10 mL of 20% sulfuric acid to lower pH.
Add 10 mL of 10% potassium iodide
Titrate with 0.1 N thiosulfate to faint yellow
Add starch
Continue titrating to colorless.
mL of 0.1 N thiosulfate x 3.55 = total chlorine
Should I be worried about only testing for total chlorine?
What about combined chlorine? Is CC a rare issue?

I found this about using spectrum analyzer.
http://www.asaanalytics.com/total-chlorine.php
2. Idiometric Analysis

This is the method of choice where the application requires control of the specific forms of combined chlorine, such as water chloramination monitoring and control (See ChemScan Application Summary # 86, Water Chloramination Process Control). This method is based on the principal that free and combined chlorine will proportionally liberate free iodine from potassium iodide. Free iodine has a strong UV absorbance signature, such that a full spectrum analyzer like the ChemScan UV-2150 can be used to detect the liberated iodine, plus compensate for other oxidizing or reducing agents or interferences in the background (including monochloramine - which can be independently detected), and calculate the resulting concentration of total chlorine. This is also the method of choice if multiple sample lines are being monitored or if other parameters are analyzed in addition to total chlorine. Typical range is 0.05 mg/l to 10.0 mg/l, but can be altered based on path length selection.

Any info or opinion on this method?
Thanks for any comments.

 

[Aquaclear-NZ]

surely a test kit would be a lot easier ? :?: :?: :?:

 

[CaOCl2]

Any info or opinion on this method?

That's (almost) the way the Taylor Bleach test is done - but using smaller sample size and reagent quantities.

You're better off using the drop-count titration of FAS-DPD, smaller sample size, smaller volume of reagents are used, basically simpler to perform, all the reagents are already prepared, the precision is there too, +- 0.2 ppm, and you easily differencitate between the free and combined forms, and it's also listed in Standards Methods. The iodometric method is fine if your sample contains high levels of chlorine, but with the small concentrations used in pool water you'd be better off with FAS-DPD.

 

[SBax]

Yes, a test kit is easier but I work in a lab with excellent equipment. I can easily check Cl2 and also measure pH on a research grade pH meter. I know it's overkill but obsessive minds need to know. I'll check TA weekly and CYA as needed.

 

[PaulR]

But how long is your commute? As I recall, one of the contributing factors to pool-store testing problems is that a long interval between taking the sample and doing the test will make the FC and pH readings somewhat suspect. You can lose FC, and pH will drift if the temperature changes.
--paulr

 

[SBax]

I used a pre-treated glass sample bottle and the pH meter has temperture correction anyway. 15 - 20 min commute.

 

[CaOCl2]

Have fun! It must be cool to play with all that equipment! But keep in mind in all of this that you're dealing with a swimming pool, where the actual volume is very approximate (and varies). It's precision work in a world of uncertainty.