Determination of the Extraction Constant for Zinc Pyrrolidinecarbodithioate An atomic absorption experiment for instrumental analysis L. Claron Hosklns, Paul 6. Reichardt, and Richard J. Stolzberg University of Alaska. Fairbanks, AK 99701 Atomic absorption spectroscopy (AAS) is a sensitive analytical technique which is generally free from interferences. Howrver, matrix effects are knonn and are particularly r trwbleswne when analping saline ~ 1 u t i o n s . T h e sd~fficulties may be circumvented by complexing the metal ion and extracting the complex into an organic solvent. The concentration of the metal ion is then determined in the organic solvent. Usually conditions can he selected so that interfering solutes ere not extracted..and concentration hv two orders of magnitude may be realized under favorable conditions ( I ) . In addition. the sensitivitv of AAS mav he increased hv afactor of three when the metal is introduced into the flame as a methvlisobutvl ketone solution rather than as an aqueous solution ( 2 ) . ' Atomic absorption ( A N experiments suitable for an undergraduate course in ins&u&ental methods of analysis are not readily available, although one recent article in this Journal describes an interference experiment (3).I t is the purpose to describe an AAS experiment which . . of this paper . . intrnduces an extraction preconcentration terhniqur commonly used with A M . The student determines the nmditinnal extractlm constant for thr zinc pyrrolidinecarhodithioatr cumnlex . .partitioned betwren methylisobutyl ketone I M I H K ) and an aqueous solution.
Table 1. tl
Solutions Prepared for ZnPCD Extraction Experiment
3 X 1 0 P MZn Irnll
pH 5.2 Buffer Irnl)
H20
lrnl)
2.7 X
lo-' MAPCD frnl)
~~
Theory The aqueous complexation reaction and the extraction process can be combined in an overall reaction
M + nR
580
MR,r,,.,
Journal of Chemical Education
11)
However, since the degree of extraction depends on pH and the concentration of ligands other than R, we define a conditional extraction constant ( 4 ) ,E', as E ' = - [Mlom IM'IIR'I"
(21
where: [MI,,, is the concentration of the metal in the organic phase. IM'I is the concentration of metal in the aaueous phase. [R'] is the concentration of the cornplexing reagent in excess of the stoichiornetric amount calculated as being dissolved in the aqueous phase. T h e distribution coefficient. D, is defined as
. .
Table 2. Rearrangement of eqn. (2) and substitution into eqn. (3) results in a n expression for D in terms of t h e conditional constant and excess ligand concentration:
D
= E'[R']"
I41
T h e fraction, f , of metal extracted into the organic phase is calculated from
where V,, and V,,, are the volumes of the aqueous and organic phases, respectively. T h e value o f f will determine if the extraction is sufficiently complete t o satisfy t h e requirements of t h e analysis. T h e value of E' is valid for a single system of unique composition. However, values of E' valid for other aqueous solutions can he determined by using the methods given in Ringhum's hook (4).
Calculated Values 01 Conditional Extraction Constant
Experimenter
log E'
RS
6.59 6.51 6.34 6.59 6.87 6.42
ML DM
DG RA EC
Standard Deviation 0.03 011 0.09
0.08 0.07 0.11
TION: Methyl isobutyl ketone is toxic and highly flammable. Then the mixture is shaken vigorously for 30 sec, and the flasks are then placed on the rotary shaker tahle for 30 mi". The phasesare separated and the aqueous phase is analyzed for zinc by AA spectroscopy.We found it necessary for the student to aspirate a blank (distilled water saturated with MIBK) after every few samples hecause of uncompensated lamp drift in our single heam instrument. It is suggested that two independent sets of duplicate measurements he made un each sample. This enahles the student to catch a "wild" datum and tn calculate the inherent precision of the AA technique.
Calculations
Experimental Reagents and Apparatus All reagents except t h e ammonium pyrrolidinecarhodithioate (APCD) should he reagent grade. APCD is recrystallized twice from ethanolldiethyl ether. T h e APCD working solution is somewhat unstahle ( 5 ) and, therefore, is prepared immediately before the experiment begins. The zinc working solution(-3 X 10-SM) can he prepared by serial dilution of M ) standard made by dissolving zinc a 1000 ppm (1.53 X metal in a minimum of HC1 and diluting with deionized water. T h e p H 5.2 buffer contains 0.1 M total acetate. Distilled, deionized water is used throughout. It is not necessary t o clean the glassware in any special way, b u t rubber stoppers are known t o contain large amounts of zinc ( 6 )and should he avoided. Glass stoppered, 100-ml volumetric flasks are used for the 30-min extraction on a rotary shaker table. Equilibrium is rapid and could he done in sep~aratorv funnels with D e r h a ~ s1 or 2 min of shaking, h u t t h e 30-mi; break can be'used i o become familiar w i d the AA unit. T h e AA analysis of the extracted a q u w u s phase can he done using virtually any AA instrument operated under standard conditions. We use a 3-sec integration time on a Perkin Elmer model 103 AA spectrophotometer, hut real time measurements with a strip chart recorder would probably work equally well. Procedure A series of 6 test solutions (1-61 and 3 AA standards (1-91 are prepared in 1W-ml volumetric flasks as shown in Table I.' The APCD solution is added last. An aliquot of 20 ml MIBK is added. CAU-
where Cz, is the total concentration of zinc in the aqueous phase before extraction (7.65 X M).The concentration of excess pyrrolidinecarbodithioate, [PCD'], is calculated also on the basis of mass
where Cpco is the total concentration of PCD in the aqueous phase before extraction. The sir independentvalues of E' (or log E'l can he cumbined togive a best value for E' and a standard deviation. Students may then be required to use their calculated value to predict the success of an extractionlpreconeentratinn of zinc in a variety of situations, particularly if they are familiar with the use of conditional constants. Results and Dlscusslon Five students and one of t h e authors performed this experiment in t h e past semester. Results are presented in Table 2. T h e precision of the data is very good, and there is no reason to doubt the accuracy. W e feel t h a t this experiment is a g w d introduction t o AAS. I t works exceptionally well. A number of statistical points can be made, and t h e role of the matrix in affecting atomic ahsorbance can he demonstrated if desired. Acknowledgment T h e AA spectrophotometer was purchased, in part, with funds from National Science Foundation Grant #GZ-3006. Literature Cited
I The matrix of the AA standards must he matched to that of the experimental solutions. We have students prepare a second set of standards of zinc in distilled water to pmve this point. if sufficient time is available, a better approach is to analyze both phases far zinc. This is impractical in a 3-hr lab period.
IS1 Hu1anicki.A.. Tnlonlo. 14, 1:171119671. 161 Hoheroon. D.E.,Anol Them.. 40. LOfi7 119681.
Volume 58
Number 7
July 1981
581
