R. Adamson
and P. C. Parks Florida Atlantic University Boca Raton, Florida 33432
An Experiment in Coupled Equilibria
This experiment was designed to introduce students to the quantitative treatment of equilibria which involve a common component. The equilibria are the ionization of 2,4-dinitrophenol (DNPL)in water and its distribution between carhon tetrachloride and water as shown in Figure 1. The
Experimental determination of a, pK, value. Quantitative treatment of simultaneous equilibria.
Although this experiment was designed for a one-quarter survey course in physical chemistry, it. should also be suitable for a quantitative analysis course or an advanced general chemistry course. Four to six hours are required to complete the experiment. The Experiment Deferminafion of the pK. o f DNP
CCI, PHASE
Figure 1. Diogramofis illustrotion of the experiment in coupled equilibria. llonizotion in the non-aqueous phase ir negligible.)
equilibrium constants for these two processes are determined in separate parts of the experiment. I n a third part of the experiment, D N P is equilibrated between aqueous buffers and carbon tetrachloride. The ohserved values of the D N P anion in the aqueous phase are compared with those calculated using the experimentally determined equilibrium constants for the distrihution and the ionization. Analyses can be performed on a simple spectrophotometer using the absorption of the D N P anion at 400 mp. This experiment serves to introduce the following ideas Quantitative applications of the spectrophotometer. The effect of phase volumes on the equilibrium distribution of a third component between two phases.
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Journal of Chemical Education
The DNP stock solution is mixed with buffers of various pH and the concentrations of DNP anion are determined from spectrophotometrio measurements. The concentrations of the protonated form are computed using the corjservation equation. The pH values of the buffers aye determined by direct measurement. 1 ml aliquots of approximately 1 X 1 0 - W DNP stock solution' are mixed wit,h 10 ml aliquots of 0.1 M ionic strength citrate buffers' whose pH values range from approximately 3 . 3 to 4.7. The a d . values are measured a t 400 mp and the concentratians of DNP anion are computed using an extincbian coefficient of 1.085 X 104 I M - I cmP. The extinebion coefficient of the protonsted species is negligible a t this wavelength. Table 1 shows s set of typical data for this experiment. The ~ K isAdetermined from the intercept of a plot of pH versus log ( C A - / C H I ) as shown in Figure 2. The ~ K value A determined from this plot is 4.00. Deferminotion of the Distribution Coefficient of DNP Between Carbon Tetrachloride and Water
Aliquots of 10 ml of approximately 2 X lo-' M DNP stock solution4 are equilibrated with 2-5 ml aliquots of carbon tetra1 Abbreviations used are DNP for 2,4-dinitrophenol and 0.d. for optical density. 'The stock solution is made in 0.1 M KC1 to maintain constant ionic strength. The DNP may be dissolved in 8. slightly alkaline solution. The stock solution should then be adjusted to about pH 4 using eonc. HCI solution. a LONGC., (Editor), "Biochemist's Handbook," D. Van Nostrand Co., Inc., New York, 1961, p. 38. 4 This solution is made by 5-fold dilution of the DNP stock solution with 0.1 M KCI. This solution should be adjusted to pH 2 to repress the ionization of DNP.
Table 1. Determination of the pK. of 2,4-Dinitrophenol in 0.1 M Ionic Strendh Aoueous Solution at 25'C
Table 2. Determination of the Distribution Equilibrium Constant of 2.4-Dinitrophenol between CCll and Aaueous
Vccl, (ml)
Stock 2 4dinitrophenol (cone. 9.91 X citrate buker.
lo-$
diluted 1:11 with
0.d. aqueous (em-')
0.d. CCL [HAIH,~ extract X 106M (cm-')
[HAIccj, X 106M
KD
10 ml alicluots of 1.98 X 1 0 P M DNP solution s t DH 2 were equilibrated with the indicated volumes of CCL.
Table 3. Comparison of Observed and Computed Concentrations of 2.4-Dinitrophenylate Anion in Experiments Involving Simultaneous Equilibria
vH
0.d. (em-')
Observed IA-1 . . X 106 M
.[A-1. X 10aM
3.26 3.53 3 71
0.037 0.057
0.34 0.53
0.30 0.54
n nx7
nxn
Calculated
n nz
2 ml of 9.91 X 10-6 M DNP stock, 5 ml of CC4 and 10 ml of the indicated buffer were equilibrated at 2 5 T . Optical densities and concentrations refer to the aqueous phase. Grophic determination of the pKa of 2,Cdinitmphenol. doto werecomputed from thevaluer given inTable 1.
Figure 2.
The
chloride. (The carbon tetrachloride is most easily dispensed from a buret.) Equilibration can be accomplished by persistent mixing in a, test tube or by gentle shaking in s. separatory funnel. Three milliliters of the aqueous phase is added to 1drop (approximately 50 pl) of 10yo NaOH and the 0.d. is memured. The concentration of DNP in the CClr phase can be computed from the conservation equation or may be measured by extracting 1 ml of the CC1. ~ h a s ewith 10 ml of 0.1 M NeOH. The a d . of
Equation (3) is obtained by dividing hoth sides of eqn. (2) by the product of thevolumes of the two phases, V c c l ,VX,O.
[HAlcc~,is eliminated using the K o expression. is eliminated by means of the K A expression. Equation (4) gives the final expression.
Table 3 gives typical results for this part of the experiment
Discussion Typical results are shown in Table 2. The average value of K Dbased on these data is 21.1.
Coupled Equilibria Two milliliters of. 1 .X M DNP solution and 10 ml of aqueous buffer (both from the P K . determination) are equilihrilted with 5 ml of CC4 a t 25°C. The 0.d. of the aqueous layer is measured and the concentration of DNP anion is outed. The observed values of DNP anion concentrstion are compared with those calculated by means of the K A and K D values determined previously. The expression for this computs, tion is derived as shown below.
+
MT = M H ~ O MOCL,
(2)
Where = total moles of DNP in hoth phases Mr Mcol, = moles of protonated DNP in the CCL phase (a negligible amount of the aniondissolves in CCL) ME,, = total moles of DNP in the aqueous phase
Although this is a simple experiment, good quantit,ative techniques are required to obtain adequate results. Careful volumetric and spectrophotome~icmeasurements are required. The choice of one of the equilibria as a ~ h a s edistribution eauilibrium is narticularlr instruc&e it demonstrat& the effectsdf voiume 0" the amount's dissolved in each ~ h a s e and shows that such equilibria can be treated as a chemical process. The spectrophotomet,ric determination of ionization const,ants is a widely used technique,, and that portion of the experiment has considerable instructional value for that, reason. The investigation of the simult,aneous equilibria emphasizes the usc of conservation equations. This is an area where many shdents lack practical experience. 84ore advanced students can be asked to derive eqn. (4). The computations can be performed using a very simple FORTltAN program if desired.
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Volume 48, Number 2, February 1971
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