Standard Methods: 4500-Cl G: Chlorine by DPD
|
Official Method Name
|
4500-Cl G. DPD Colorimetric Method |
|---|---|
|
Current Revision
| Standard Methods Online |
|
Media
|
WATER |
|
Instrumentation
|
Spectroscopy (Colorimetry; Photometry) |
|
Method Subcategory
|
Inorganic |
|
Method Source
|
|
|
Citation
|
|
|
Brief Method Summary
|
This is a colorimetric version of the DPD method (4500-Cl F) and is based on the same principles. Instead of titration with standard ferrous ammonium sulfate (FAS) solution as in the titrimetric method, a colorimetric procedure is used. |
|
Scope and Application
|
This DPD method is operationally simpler for determining free chlorine in water than the amperometric titration. Procedures are given for estimating the separate mono- and dichloramine and combined fractions. Used for total chlorine in samples containing significant amounts of organic matter. |
|
Applicable Concentration Range
|
|
|
Interferences
|
(A) Manganese: The most significant interfering substance likely to be encountered in water is oxidized manganese. To correct for this, place 5 mL buffer solution and 0.5 mL sodium arsenite solution in the titration flask. Add 100 mL sample and mix. Add 5 mL DPD indicator solution, mix, and titrate with standard FAS titrant until red color is discharged. Subtract reading from Reading A obtained by the normal procedure as described in this method or from the total chlorine reading obtained in the simplified procedure given here. If the combined reagent in powder form is used, first add KI and arsenite to the sample and mix, then add combined buffer-indicator reagent. As an alternative to sodium arsenite use a 0.25% solution of thioacetamide, adding 0.5 mL to 100 mL sample. Copper: Interference by copper up to approximately 10 mg Cu/L is overcome by the EDTA incorporated in the reagents. Chromate in excess of 2 mg/L interferes with end-point determination. Add barium chloride to mask this interference by precipitation. Color and Turbidity: Compensate for color and turbidity by using sample to zero photometer. Chromate: Minimize chromate interference by using thioacetamide blank correction. |
|
Quality Control Requirements
|
See Section 4020 Quality Assurance/Quality Control. |
|
Sample Handling
|
Chlorine in aqueous solution is not stable, and the chlorine content of samples or solutions, particularly weak solutions, will decrease rapidly. Exposure to sunlight or other strong light or agitation will accelerate the reduction of chlorine. Therefore, start chlorine determinations immediately after sampling, avoiding excessive light and agitation. Do not store samples to be analyzed for chlorine. |
|
Maximum Holding Time
|
0.25 hour (See Section 1060) |
|
Relative Cost
|
Less than $50 |
|
Sample Preparation Methods
|
