How to determine chelate demand by titrimetric method
NTA Chelant Demand Titrimetric Method
NTA (nitrilotriacetic acid) is a chelant commonly applied in boiler water treatment to prevent the deposition of hardness and other heavy metal cations under boiler water conditions. NTA reacts with the metal cations, forming a soluble ring structure. The stability of the complex depends on the strength of the cation chelant interaction and is measured by how completely the metal is: “Locked - in” by the chelant. NTA complexes are susceptible to reactions of the metal is locked in by the chelant. NTA complex are susceptible to reaction by competing anions and that fraction of the metal cations which are not chelated are free to deposit.
Too great an excess of NTA can cause corrosion of the boiler metal surfaces. A desirable treatment level is one where there is a slight excess of the chelant needed to complex the metals contained in the boiler feedwater. The balance, between too much NTA in the boiler on the one hand and too little to achieve complete chelation on the other, is best achieved by measuring chelation requirement of the incoming feedwater.
Theory of Test
A standard solution of chelant is added to a feedwater sample. The pH is controlled to insure chelation of the heavy metallic cations. Then the excess chelant is back titrated at a buffered pH with a standard metal solution in the presence of an indicator. The amount of chelant standard needed to complex all of the cations of interest is then calculated in parts per million of feedwater chalant demand.
Apparatus Required
Bottle 2 ounce, screw cap with dropper
Burett micro, automatic 2 ml Teflon stopcock
Buret, micro automatic 3 ml stainless steel tip
Casserole, porcelain 340 ml
Cylinder, graduated 50 ml Measuring dipper, brass
Pipet Mohr 1 ml
Pipet Mohr 5 ml
Stirring Rod Glass
Chemicals Required
Chelant Blank Reagent
Chelant Buffer Reagent (dry)
Chelant Indicator No.1
Chelant Indicator No.2
Isopropyl Alcohol, 99%
NTA Standard Solution
NTA Titrating Solution
Sodium Hydroxide 1.0N
Sulfuric Acid, 1.0N
Procedure for Test Determination of Blank
Measure 100 ml of chelant blank reagent in the graduate and transfer to the casserole. Add 2.0 ml NTA standard solution, using the 2.0 ml microburet, and stir. With separate, clean 1.0 ml pipets, and 0.5 ml of 1.0N sulfuric acid and 1.0 ml of 1.0N sodium hydroxide. Stir after each addition.
Add three level brass dippers of chelant buffer reagent. Stir until dissolved. By means of the graduate, add 20 ml of isopropyl alcohol. With separate, clean droppers, add two drops chelant indicator number 1 and 10 drop chelant indicator number 2. Stir. A green color appears. Stirring constantly, titrate with NTA titrating solution from the 3.0 ml microburet until the green color disappears. A bluish purple color remains.
Record the number of ml of NTA titrating solution. This value, which approximates 2.00 ml usually, is the blank to be used when calculating the NTA demand for feedwater. Determine this at least weekly, or whenever using a new supply of the chelant blank reagent.
Determination of Feedwater NTA Demand
Measure 100 ml of feedwater, cooled to 70 to 85 F, into the graduate and transfer to the second casserole. Slowly add 2.0 ml of NTA standard solution, using the 2.0-ml microburet and 0.5 ml of 1.0N sulfuric acid, using the 1.0 ml pipet. Stir and allow to stand for five minutes. Add three level brass dippers of chelant buffer reagent, and stir until dissolved. By means of the 5.0 ml pipet, add 1.5 ml of 1.0N sodium hydroxide and using the graduate, add 20 ml of isopropyl alcohol. Stir, Stir and then with separate clean droppers, add two drops of chelant indicator number 1 and 10 drops of chelant indicator number 2. Stir the solution and a definite green color will appear. Stirring constantly, titrate with NTA titrating solution from the 3.0 ml microburet until the green color just disappears. The blush purple color remaining is the endpoint color.
Calculation of Feedwater Demand
FORMULA:
ppm NTA chelant demand = (ml of titrating solution, blank - ml of titrating solution, sample) x 2000 ml sample
For example, using a 100 ml feedwater sample, the NTA feedwater demand in parts per million 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 20.
Precaution
Lighting and individual color perception determines the exact blank to be used for local conditions. Satisfactory lighting is very important.
Limitations of Test
Ions normally present in boiler feedwaters suitable for chelant treatment do not interfere with this test.
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