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Revised labscript :
Determination of Zn by EDTA titration and atomic absorption spectrometry
1. 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 two methods, namely:
(1) Titration with EDTA in alkaline solution (Vogel 1961) and
(2) Flame atomic absorption spectrophotometry (FAAS).
2. Experimental Procedure:
Three (3) samples of acidified zinc solutions will be provided for analysis, labelled A, B and C.
The concentrations of each solution will be provided, to allow the accuracy of the two methods to be evaluated, using statistical analyses of results generated.
(A) Titrimetry:
A standardized EDTA solution will be provided. Dilute as recommended (see Demonstrator) and use as the titrant (250mL will suffice for all titrations).
(a) Pipette three 25mL aliquots of each sample into 250mL Erlenmeyer flasks and add 5 drops
of xylenol orange indicator. Add powdered hexamine, with agitation, to each yellow solution,
until it changes to an intense red color.
(b) 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.
(c) Calculate the mean zinc concentration + std. deviation of each sample in µg/mL units.
(B) Flame atomic absorption spectrometry:
(a) Dilute each sample in triplicate with distilled water in 25mL volumetric flasks, using the
dilution factors recommended (see Demonstrator).
(b) Also prepare 25mL each of a range of zinc standards (0.1, 0.5, 1.0, 1.5, and 2.0ug/mL), by appropriate dilution of the strongly acidified zinc standard provided.
(c) The atomic absorption spectrometer will be optimised in your presence. Note all the
following relevant instrument operational parameters: air/acetylene flow rates, lamp current,
wavelength.
(d) Note the procedures followed in the calibration of the instrument.
(e) Aspirate the Zn standards in order of increasing concentration, followed by your sample
solutions, and note the respective absorbances obtained.
3. Exercises:
(A) Plot a calibration curve of absorbance vs Zn concentration. Also determine an equation
for the curve using linear regression analysis. Quote the correlation coefficient.
(B) Use both of these techniques (graphical and mathematical), to determine the sample solution
concentrations.
(C) Determine whether results of analyses by titrimetry and atomic absorption spectrometry
(graphical and mathematical) are accurate, by statistical comparison with the reference Zn
values of each sample, at the 95% confidence level (p<.05).
NB: There is no need to compare the mean values for each sample with each other
in this expt.
(a) Do the results generated by the two calculation techniques used in FAAS differ significantly
(p<.05) in precision with each other, and with those by titrimetry for each of the 3 samples?
(b) List 3 possible sources of error in this experiment and suggest ways of minimizing such
errors.
4. Reference:
1. Vogel's Textbook of Qualitative Inorganic Analysis by Vogel, A.I., 3rd, Ed., Longman (1961)
444445.
analchem resources Chem. Dept. UWI. St. Augustine Campus
Original labscript :
Determination of Zn by EDTA and Atomic Absorption
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.
Procedure
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 erlenenmeyer
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 optimised 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.
analchem resources Chem. Dept. UWI. St. Augustine Campus
