
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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