How to quantify metal ion in water

EDTA Chelant Demand Titrimetric Method
EDTA (ethylenediaminetetraacetic acid) is a weak organic acid.  When the sodium salt is applied  to boiler feed water at a relatively high pH it is hydrolyzed to an organic anion able to form stable soluble organometallic complexes. Calcium, magnesium, iron and copper, metals that normally form troublesome deposits on boiler surfaces, remain in solution when chelated. Although the complexes formed with EDTA, in general, are more stable than those formed with NTA (nitrilotriacetic acid), their effectiveness in a boiler treatment program is limited by the concentration of competing anions. Precipitation will occur whenever the potential of the anion to react exceeds the potential of the chelant to form complexes. The complex forming potential, also named the stability constant of the chelant, has been determined for most chelants. Except for a  few special cases, EDTA is the most suitable chelating substances used for boiler water treatment to date.
Proper treatment level is necessary in attaining a successful boiler chelant program. One of the  best means of preventing “overfeed” and determining a proper feed rate of EDTA chelant is to measure the chelant demand of the feed water. This determination can be accomplished by a chelant demand test.
 
Theory of Test
A standard solution of EDTA is added to a feedwater sample. By carefully controlling the pH to insure chelation of the hardness and heavy metal cations, the excess chelant standard is back titrated with a standard metal solution in the presence of all indicator. The amount of chelant standard needed to complex all the cations of interest is calculated in parts per million as feedwater chelant demand.
 
Apparatus Required
Bottle, 2 ounce, screw cap with dropper
Buret, micro, automatic, 2 ml, Teflon stopcock Buret, micro, automatic, 3 ml, stainless steel tip Casserole, porcelain, 340 ml
Cylinder, graduated, 100 ml Pipet, Mohr, 1 ml
Pipet, Mohr, 5 ml Stirring Rod, Glass
 
Chemicals Required
Chelant blank reagent
Chelant Indicator No.3
Chelant Liquid Buffer
EDTA Standard Solution
EDTA Titrating Solution
Sodium Fluoride Solution
3% Sodium Hydroxide
1.0N Sulfuric Acid, 1.0N
 
Procedure for Test Determination of blank
Measure 200 ml of chelant blank reagent in the graduate and transfer to the casserole. With the 1.0-ml pipets, add 1.0 ml sodium fluoride soultion and, with a clean pipet, add 1.0 ml of 1.0N sulfuric acid. Stir.
Slowly add 2.0 ml EDTA standard solution from the 2.0 ml microburet. Using the 5 ml pipet, add
1.5 ml of 1.0N sodium hydroxide and stir. With separate, clean droppers, add 1.0 ml chelant liquid buffer and 12 drop of chelant indicator number 3. Stir. A blue color appears. Stirring constantly, titrate with EDTA titrating solution from the 3.0 ml microburet to the first reddish purple endpoint that persists for 30 seconds.
Record the number of ml of EDTA titrating solution as the feedwater blank value. Retain the titrated solution for use as a reference color when titrating the feed water sample.
Determination of feed water EDTA demand. Measure 100 ml of feed water, cooled to 70 to 85 F, in the graduate and transfer to the second casserole. Add 100-ml chelant blank reagent and stir. With the 1.0 ml pipet, add 1.0 ml of sodium fluoride solution and, with a clean pipet, add 1.0 ml of 1.0N sulfuric acid in that order. Stir, and allow to stand for 30 seconds.
Slowly add 2.0-ml EDTA standard solution from the 2.0-ml microburet. Stir and allow to stand  one minute. Using the 5 ml pipet, add 1.5 ml of 1.0N sodium hydroxide and stir.  After one  minute, with separate, clean droppers add 1.0-ml chelant liquid buffer and 12 drops of chelant indicator number 3. Stir. A blue color appears. Stirring constantly, titrate with EDTA titrating solution from the 3.0-ml microburet to the first reddish-purple endpoint that persists for 30  seconds. This endpoint should match the color obtained for the blank test.
 
Calculation of Feedwater Demand
FORMULA:
ppm 38% EDTA chelant demand = (ml of titrating solution, blank - ml of titrating solution, sample) x 5000 ml samples
Using a 100 ml feedwater sample, the EDTA feedwater demand in parts per million as 38 per cent EDTA is equal to the ml of titrating solution required for the blank test, minus the ml of titrating solution for the feedwater demand procedure, multiplied by 50.
 
Limitations of Test
The concentration of ions normally present in feedwaters suitable for chelant treatment do not interfere with this test.
 
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