Determination of chlorine by DPD methods

Determination of chlorine by DPD methods
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Introduction :

We use DPD as indicator because of it's high sensitivity  for chorine ,DPD reacts with chlorine producing a bright red color ,so we can use it  by two methods colorimetric or titration with ferrous ammonium sulphate (FAS) Fe(NH₄)₂SO₄ .       

This DPD  methods very accurate but  the main  defect that it is can not directly  determine the concentrations exceeds 5 mg/l . this can achieved by using dilution 

DPD oxalate

A)colorimetric methods.

since the degree or the strength of red color  that is yields from reaction between DPD and chlorine proportional to the concentration of chlorine  we can  compare the   color  which developed when DPD added to a sample and standard colors  known it's concentration  

 

The comparison may takes place either by simple device such as above in which a cycle has a graduated red or pink color ,each degree of color equivalent to Known concentration  or by spectrophotometric   methods 

 The simple comparison colorimetric method:

 

Preparations:

1) prepare DPD solution  as the following :

Dissolve 1 g DPD oxalate in little amount of distillated water containing 8 ml 1:3 sulfuric acid and 200 mg EDTA  complete to 1 L freshly boiled and cooled distillated water (must be sure that is free from chlorine ) , store in brown or dark bottle

2)prepare phosphate buffer :

a)Dissolve 24 g Na₂HPO₄  and 46 g  KH₂PO₄ in little amount distillated water

 b)dissolve 800 mg EDTA in 100 ml distillated water 

mix the phosphate solution and EDTA solution ,dissolve well ,and complete to 1 L

see altrnative methods for preparation this buffer solution 

 Buffer Solution preparation 

Procedure:

Add 5 drops Buffer solution +5 drops DPD solution  to sampling tube , then add to it 10 ml sample water and put it in comparison device , read the concentration of the color strength equals to the strength of color developed in sampling tube .

The spectrophotometric comparison colorimetric method

Prepare DPD and buffer solutions as above: 

1)Prepare known concentrations of chlorine (use thiosulphate or DPD tablets or DPD – FAS method as we see)

2)set the spectrophotometer on 515 nm  and blank it with distillated water

3) add 5 drops buffer +5 drops DPD +10 ml known concentration chlorine  solution to spectrophotometer cell and read the corresponding absorbance

Plot a curve between absorbance in horizontal axes and concentration in vertical axes and detect the slope as the following 

Concentration = slope *Abs   Concentration = slope *Abs

 

B) Titration Method (Ferrous ammonium sulphate method)  

Preparations:

1)Prepare DPD and buffer solutions as above 

2)Prepare standard  ferrous ammonium sulphate solution  (FAS)   Fe(NH₄)₂SO₄ as the following :

Dissolve 1.106 g  Fe(NH₄)₂SO₄.6H₂O in distillated water containing 1 ml 1:3 sulfuric acid  ,then complete to 1 liter freshly boiled and cooled distillated water .

3)KI crystals  or 0.5% KI solution (500 mg KI in 100ml distillated water )store in brown bottle

Procedure:

a) Free chlorine and chloramines

add 5 ml buffer + 5 ml DPD solution in conical flask , mix , add 100 ml sample and mix well.

i) Free chlorine

titrate rabidly with standard FAS solution until red color discharged (reading A).

ii) Monochloramine ( NH2Cl).

add very small crystal of KI (about 0.5 mg) or 0.1 ml (about 2 drops) KI solution ,complete titration  with FAS solution  until red color discharged again.(reading B).

iii)Dichloramine. ( NHCl₂).

Add several crystals KI (about 1 g) mix, stand for  2 minutes ,continue titration until red color discharged too (reading C).

iv) Trichloramine. (NCl3)

add very small crystal of KI (about 0.5 mg) or 0.1 ml (about 2 drops) KI solution +100 ml sample in conical flask ,add the content to another conical flask containing   5 ml buffer + 5 ml DPD solution ,titrate rabidly with FAS (reading N)

.

b) Total chlorine (free +combined) in one reading 

add the full amount of KI at the start of titration +100 ml sample in flask ,add the content to another conical flask containing   5 ml buffer + 5 ml DPD solution ,titrate rabidly with FAS

Calculations:

Determine the free chlorine, mono, di and tri chloramines as explained in the following table:

 

NCl3 present	NCl3 absent	Reading
Free Cl NH2Cl NHCl2 +1/2 NCl3 Free Cl  +1/2 NCl3 NCl3 NHCl2	Free Cl NH2Cl NHCl2	A B-A C-A N 2(N-A) C - N