Erwin Boschmonn
Indiana-Purdue University Indianapolis, Indiana 46202
I I 1
Efficient Use of Washing- Solvents A quantitative treatment
There are a number of techniques employed routinely in the chemistry laboratory which, unless properly performed, may easily deteriorate into inefficient, expensive, and time-consuming operations. One such frequently abused technique is the manipulation of laboratory solvents used for washing glassware, precipitates, and liquids. Thus it is not uncommon to see a student filling a flask with distilled water "in order to rinse it well." As in many other cases, a correct washing technique is seldom acquired through int,uition, but can readily he developed through an understanding of the underlying basic principles. While it is common knowledge among practicing chemists that washings are best performed by several successive operations using small portions of solvent, rather than by a single operation using the same total volume, the reasons, while treated in some advanced texts,' are usually not adequately emphasized in the teaching of introductory quantitative chemistry. The considerable waste in solvents and time alone would seem to justify a feu. minutes of class time to prove that for a fixed volume of available solvent theewashingefficieny is a very sensitive function of two simple parameters. Experience has shown that such a proof, when accompanied by several examples ill impress, convince, and usually convert most students. Presented s t the Third Central Regional ACS Meeting, June 6 1 1 , 1971, Cincinnati, Ohio. See for instance KOLTHOPP. I. M.. SINDELL.E. B..MEEHAN. E. $., A N D BRUCKENGTEIN, STANLEL, "~uantitativ; chemical Analysis" (4th ed.), The Macmillan Company, Collier-Macmillan LOUIS,AND Ltd., London, 1969, p. 533; also WALDB.AUER, CORTEYLOU, W. P., J. CHEM.EDUC.,18,341 (1941); and MARSH, G . EVERETT, Ind. Eng. Chem., 20, 1241 (1928).
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650
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Journal o f Chemical Education
The two factors affecting the overall efficiency are ( 1 ) the number of washings; and (2) the extent of drainage of each washing. Thc following generalized discussion accounts for these factors, and may easily be adapted to specific situations such as the washing of laboratory glassware or precipitates, and to separation equilibrium equations. Let V, = total volume of available solvent Vr = amount of solvent remaining in the washed item after drainage n = number of washings carried out with equal fractions of Vt mi = initial amount of solvent-soluble impurity contained in the item to be washed mi = amount of impurity dissolved in Vl
For a single washing, using V ,of solvent, there would remain mi
=
(VilVdrni
(1)
Two successive vashings using V,/2 portions of solvent each time would leave mA1) = (V,l(V1/2))rni
(2)
for the first of the two washings, and md2) = ( V J I ( V ~ I ~ ) ) ~ , ( ~ )
(3)
for the second of the two washings. Rote that in equation (3) the term m,(l) is used in place of m,, since it now represents the amount of impurity present in the washed item at the start of the second washing. Suhstituting the value of m,(l) from eqn. (2) into eqn. (3)
Generally, for n washings using V , / nportions of solvent for each washing
Table of Washing Improvement Factors Poor Drainage: V,/V, =; 1/100 Number of washings,
n
18
Good Drainage: V,/V, = 1/1000
Number of wsshings,
I.F.
The fraction of remaining impurity, washings then becomes
=b EI 614
I.F.
n
ml/mt,
and the efficiency
- mj/mi)/mi
-
=
2 3 4 5 6 7 8 9 1 0 No. of Woshings with V,/n s~zedportions
v~/vt
(s)
Pbtr of the logarithm I.F. of impurity removal,log (V,/V,)-n log lov,/vd. varrur n, the number of washings with Vr/n sized The VI/Vcvaluss ore, for curve a: 1/1000; curve b: lj500; c v n e c: 1/20O; and for curve d: 111 00.
Taking an example from another facet of quantitative chemistry, suppose that 8.0 g (or any other amount!) of a given solute arr to be extracted from one solvent into a second, slightly miscible solvcnt, the distribution coefficient being about 1. If 0.30 ml of extracting solvent remain dissolved in 100 ml of the first phase, the efficiency of four 15-ml extractions compared to that of one 60-1111 extraction is
(nV//Vt)"
The table eives some tvoical exam~lesof Imnrovement Factor'obtained from eqn. (8): Note for incomplete drainage (1% of original volume remains) two successive washings remove 25 times more impurity than a single washing using the same total volume. Three successivr washings remove 371 times more impurity than a single washing, while four washings increase the efficiency to 3910 times. For better drainage (only 0.1% of original volume remains) the improvements of n washings over one washing are 250 times for n = 2; about 37,000 times for n = 3; and close to 4,000,000 for n = 4! Information such as this may be summarized by plotting the Improvcment Factor (or logarithm of this factor to keep the values on scale) versus n , the number of washings for fixed values of VI/V,. A glance at the plot clearly shows the improvement attained by using several washings with small amounts of solvent over that of the same total volume used once. Thus for curve b (relatively good drainage at Vl/V, = 500) four successive washings increase the efficiency by a factor of approximately lo6as compared to the efficiency of a single washing using the same volume of ~ o l v e n t . ~ By way of illustrations, sumose that a braker is to be rinsed with 30 ml of water.. 'Assume that V, = 0.30 ml, and that mi = 1.0 g. The weight of impurity remaining in the beaker aft,er a single washing using 30 ml of ~vateris 0.01 g; while three successive rinsings, using 10 ml of water each time, leave only 2.7 X lo-' $4 an Improvement Factor of 371. " A
I;(t
(7)
Equation (7) relates the amount of impurity removed to the amount of impurity originally present. While the values obtained from this equation are quite convincine in themsleves. the obiect here is to comnare the rfficiency of one washing against that of several washings. Therefore, a more useful term giving relative efficiency, hereafter called the Improvement Factor, I.F., is defined as the fraction of remaining impurity after one washing with V, volume of solvent, divided by the fraction of remaining impurity after n washings with V,/n sized portions of solvcnt I.F.
B e sDf r a i n a g e
after n
-I
efficiency = (ini
1
teat
I.F.
=
0.30/60 = 31,000 [4(0.30/60)14
The plot also points out the importance of good drainage. Thus, while in curve d the drainage is poor, and its Improvement Factor a t , say, n = 5, is about the efficiency is improved to about lo8 when the better drainage curve a is used. Illustrating the point again, assume a precipitate is to he washed with three successive 10-ml washings. Again let m, = 1.0 g. Now if in one case 0.30-ml wash solution were allowed to remain in the precipitate, the amount of impurity retained would bc 2.7 X g. If, however, each of the three washings is followed by good drainage such that only 0.030 ml of solvent remain in the precipitate, then the amount of impurity retained g, an Improvement Factor of 3000. is only 2.7 X Clearly then, solute removal via solvent washings is a very sensitive function of the number of washings and the extent of drainage--two facts which when properly understood, are likely to be made use of in daily laboratory work. Generally, loa or lo4 efficiency factors, sufficiently high for most purposes, may bc attained after three of four successive washings and a severalsecond drainage time.
* Among the assumptions made are that all of the solute will dissolve in the solvent used; that the solvent-solute dissolution is the main process, and that other processes such as evaporation, that V, is onto the glass, etc., are negligible; small enough to be negligible in the volume of the succeeding washing. Volume 49, Number 9, September 1972
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