Iodine in Drinking Water, Wastewater and Disinfection Solutions

Introduction

Iodine has antiseptic properties and is used for a variety of antimicrobial uses, like emergency drinking water purification, fresh food sanitization, hospital surface disinfection, and disinfection of cooling tower water.¹ Iodine determination in aqueous samples is performed mainly to ensure effectiveness of the disinfection and to prevent negative health impact on humans.

Typical concentrations of iodine for potable water treatment are 2.5 - 7 mg/L.² Iodine has also become attractive for applications, such as drinking water disinfection aboard space vessels at a residual concentration of approximately 2 ppm.³

According to 40 CFR Section 180.940 the iodine tolerance exemption in antimicrobial formulations for food-contact surface sanitizing solutions is defined as “the total end-use concentration of all iodide-producing chemicals in the solutions is not to exceed 25 ppm of titratable iodine”.⁴

This Application Note describes the method that was former available as Spectroquant^® Iodine Test (1.00606) which is now discontinued. The method is furthermore available as preprogrammed method in the Spectroquant^® photometers and colorimeters Prove 100/300/600, Nova 60 and Move 100 and can be performed with the reagent Spectroquant^® Chlorine Test (1.00598).

The reagent Cl₂-1 of Cat. No. 1.00598 Spectroquant^® Chlorine Test has the identical reagent composition as the reagent I₂-1 of the discontinued Cat. No. 1.00606 Spectroquant^® Iodine Test.

The method with a measuring range of 0.050 – 10.00 mg/L I₂ is suitable to determine the iodine concentration in drinking water, wastewater, and disinfection solutions.

Experimental Method

In weakly acidic solution free iodine reacts with dipropyl-p-phenylenediamine (DPD) to form a red-violet dye that is determined photometrically.

Sample Material

Drinking water, wastewater, disinfection solutions

Effect of foreign substances

Other components in the sample - than the target analyte I₂ – may interfere with the detection chemistry described here. Some of them were checked in solutions containing 3.5 and 0 mg/l I₂. The determination is not yet interfered with up to the concentrations of foreign substances given in the table. Cumulative effects were not checked; such effects can, however, not be excluded.

Reagents, Instruments, and Materials

Reagents

• Spectroquant^® Chlorine Test (1.00598)

Instruments

For bromine measurement one of the following Spectroquant^® photometers is necessary:

• Spectroquant^® VIS Spectrophotometer Prove 100 (173016)
• Spectroquant^® UV/VIS Spectrophotometer Prove 300 (173017)
• Spectroquant^® UV/VIS Spectrophotometer Prove 600 (173018)
• Spectroquant^® Photometer Nova 60A (1.09752)
• Spectroquant^® Colorimeter Move 100 (1.73632)

NOTE: Software for data maintenance (Prove instruments only)

The Spectroquant^® Prove Connect to LIMS software package provides an easy way to transfer your data into an existing LIMS system. This software can be purchased under:

Prove Connect to LIMS (Y.11086)

Materials

• Rectangular cell 10 mm (1.14946)
• Rectangular cell 20 mm (1.14947)
• Rectangular cell 50 mm (1.14944)
• Empty cells 24 mm with screw caps (12 pcs) (1.73650)
• Pipettes for a pipetting volume of 10.0 mL

Other reagents and accessories

• MQuant^® pH-indicator strips pH 5.0 - 10.0 (1.09533)
• MQuant^® pH-indicator strips pH 0 - 6.0 (1.09531)
• Sodium hydroxide solution 1 mol/L Titripur^® (1.09137)
• Sulfuric acid 0.5 mol/L Titripur^® (1.09072)

Sample Preparation

• Analyze immediately after sampling!
  
• The pH must be within the range 4 - 8. Adjust, if necessary, with sodium hydroxide solution or sulfuric acid.
• Filter turbid samples.

Preparation of measurement solution

• Add 1 level blue microspoon (in the cap of the reagent bottle) of Reagent Cl₂-1 from Cat. No. 1.00598 Spectroquant^® Chlorine Test into a test tube. Reclose the reagent bottle immediately after use.
• Add 10 ml of pretreated sample (5 - 40 °C) and shake vigorously until the reagent is completely dissolved.
  
• Leave to stand for 1 min (reaction time), then fill the sample into the cell, and measure in the photometer.

Measurement

It is recommended to perform a zero adjustment for this method each new working day. Details regarding the zero adjustment can be found in the user manual of your instrument.

It is recommended to use the same cell for zero adjustment and for sample measurement. For zero adjustment fill the cell with distilled water (or water for analysis) and follow the instructions in the user manual of your instrument.

1. Prove and Nova instruments

• Select method no. 146 from the method list.
• Fill the pretreated sample into a 10-mm, 20-mm or 50-mm rectangular cell and place the cell into the cell compartment, the measurement is performed automatically.
• The Iodine content in mg/L I₂ appears in the display.

2. Move 100

• Select method no. 90 from the method list.
• Fill approx. 10 mL of distilled water into a 24-mm cell, close with the screw cap (Blank cell).
• Insert the blank cell into the cell compartment.
• Align the mark on the cell with that on the photometer.
• Press “Zero”.
• Fill the prepared measurement solution into a 24-mm cell, close with the screw cap (Sample cell).
• Insert the sample cell into the cell compartment.
• Align the mark on the cell with that on the photometer.
• Press “Test”.
• The Iodine content in mg/L I₂ appears in the display.

Tips for the measurement

• When using the 50-mm cell, it is recommended to measure against an own prepared blank sample (preparation as per measurement sample, but with distilled water instead of sample) to increase the accuracy. Configure the photometer for blank measurement (Prove and Nova instruments only).
• For photometric measurement the cells must be clean. Wipe, if necessary, with a clean dry cloth.
• Measurement of turbid solutions yields false-high readings.
• The pH of the measurement solution must be within the range 4.0-5.0.
• The color of the measurement solution remains stable for 30 min after the end of the reaction time stated above.
• In the event of iodine concentrations exceeding 25 mg/L, other reaction products are formed, and false-low readings are yielded. In such cases it is advisable to conduct a plausibility check of the measurement results by diluting the sample (1:10, 1:100).

Analytical quality assurance

Note: Recommended before each measurement series.

To check the photometric measurement system (test reagent, measurement device, handling) and the mode of working, a freshly prepared iodine standard solution containing 5.00 mg/L I₂ (application see the website) can be used. Sample-dependent interferences (matrix effects) can be determined by means of standard addition.