Zinc by EDTA and AA

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Determination of Zinc

Introduction

Zinc in solution may be determined by several means, including titrimetry, polarography, colorimetry and atomic absorption spectroscopy. In this experiment, Zinc will be determined by

(1) Titrations with EDTA in alkaline solution(Vogel 1961) and
(2) Atomic absorption spectrophotometry. Three (3) samples of acidified zinc solutions will be provided for analysis, labelled A, B. and C. The concentrations of each solution will be given to allow the accuracy of the respective methods to be evaluated.

Titrimetry

A standardized EDTA solution will be provided. Dilute (x20) as recommended and use as the titrant (250 mls will suffice).

Pipette three (3) 25 mls aliquots of each sample into 250 ml erlenmeyer flasks and add 5 drops of xylenol orange indicator. Add powdered hexamine, with agitation to the yellow solution until it changes to an intense red color.

Titrate with EDTA until the color changes from red to yellow. Carry out a blank titration to determine the indicator blank for possible correction of your sample titre values.

Exercise

Calculate the mean zinc concentration of each sample as µg/cm³ with suitable expression of precision.

Atomic Absorption Spectrophotometry

Dilute each sample in triplicate to 1 ppm, with distilled water in 25 ml volumetric flasks.

Also prepare a range of zinc standards (25 mls) using the strongly acidified zinc standard provided. Use this range of values: (0.1, 0.5, 1.0, 1.5, and 2.0 ppm)

The atomic absorption instrument will be optimized in your presence. Note all relevant instrument parameters; air/acetylene flow rates, lamp current, wavelength etc.

Aspirate the standards in order of increasing concentration, followed by your sample solutions, and note the absorbances obtained.

Exercises

Plot a calibration curve of absorbance vs concentration as well as determine an equation for the curve using linear regression analysis. Quote the correlation coefficient.

Hence, determine the sample solution concentrations using both graphical and mathematical methods.

Compare the accuracy of each method with the given values at the 95% level.

Do the methods differ significantly (p 0.05) in precision with each other? Comment on your conclusions, with possible explanations for any differences obtained.



Reference

Vogel's Textbook of Qualitative Inorganic Analysis by Vogel, A.I., 3rd, Ed., Longman (1961) 444, 445.


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