Hach AutoCAT 9000 Instruction Manual page 119

Summary of Method
Accounting for Sample Iodine Demand
Required Reagents
Description
Iodine Standard Solution, 0.0282 N ............................................................ varies................ 1000 mL .............23333-53
Phenylarsine Oxide (PAO) Titrant, 0.00564 N............................................ varies................ 1 L ........................1999-53
Phosphate Buffer, pH 7, w/dropper ............................................................. 1 mL................. 100 mL ...............21553-32
Potassium Iodide, SwifTest refill ................................................................ 0.10 g ............... 25 g.......................1077-60
Required Apparatus
Beaker, Glass, 250-mL ................................................................................ 1 ....................... each......................500-46H
Cylinder, Graduated, 250 mL ...................................................................... 1 ....................... each.........................508-46
®*
Magnetic Stir Bar, Teflon
SwifTest dispenser....................................................................................... 1 ....................... each.....................28341-00
Optional Reagents
Chlorine Standard Solution, Voluette
Dilution Water, organic-free ........................................................................ varies................ 500 mL ...............26415-49
Phenylarsine Oxide (PAO) Titrant, 0.00564 N, cartridge ........................... varies................ each.......................1949-01
Sodium Thiosulfate Standard Solution, stabilized, 0.00564 N ................... varies................ 1 L ......................24088-53
*Teflon is a Registered Trademark of EI DuPont Company.
50081_Sulfite_Back.fm
This procedure follows method number 4500-SO
Examination of Water and Wastewater. In the amperometric back titration, a measured
volume of oxidant (iodine) is added directly to a measured volume of the sample at
collection time. This "fixes" the sulfite concentration and decreases the amount of
sample loss throughout the titration. The method is performed at neutral pH to minimize
interference from dissolved oxygen and organic chloramines.
At the time of analysis, the sample is adjusted to pH 7 with phosphate buffer, and excess
potassium iodide is added. The unreacted oxidant is then titrated with standard PAO
solution. The sulfite concentration is derived from the difference in the amount of
oxidant originally added to the sample and the amount remaining prior to titration.
The chemical reactions are:
– 2– –
→
I SO H O 3I HSO
+ + +
3 3 2
–
( )
PhAsO PAO I 2H O
+ +
3 2
(Ph=Phenyl)
Iodine demand in the sample can occasionally have a significant impact on the accuracy
of a titration. This can be accounted for by titrating the sample matrix prior to sulfite (or
sulfur dioxide) addition. For best results, chlorine should be eliminated from the sample
by UV irradiation. The resulting value from the titration is the blank value, and should be
performed at least once daily when sulfite titrations are run. The blank value is
subtracted from the result of each sulfite titration for maximum accuracy.
coated............................................................. 1 ....................... each.....................50085-00
®
Ampules ........................................ varies................ 16/pkg.................14268-10
2–
3
– +
H
+
4
– +
→ ( )
3I PhAsO OH 2H
+ +
2
Qty. required per test
Sulfite
B in Standard Methods for the
Unit Cat. No.
Sulfite
Page 117