B. Krotochvil, ond W. E. Harris University of Alberta Edmonton, Alberta, Conodo T6G 2G2
I
A Nonaclueour Gravimetric Titration Determination of 8-hydroxyquinoline with p-toluenesulfonic acid in acetonitrile
With increasing restrictions on time allowed for learning proper laboratory technique, attention should be given to the development of experiments that provide both practice in modem methods of measurement, and exposure to important chemical concepts. The experiment described here, the titration of 8-hydroxyquinoline with p-toluenesulfonic acid in acetonitrile, demonstrates the techniques of gravimetric titrimetry and photometric end-point detection. I n addition, the advantages of a nonaqueous solvent as a medium for certain acid-base reactions are illustrated. Gravimetric titrimetry' is admirably suited to measurements in nonaqueous solvents because i t provides imnroved nrecision and accuracv in titrant measurement and does not require volumetric equipment, thereby removine" the nroblems associated with calibration, drainage, meniscus reading, and temperature corrections (especially important with organic solvents). Volumetric flasks may be employed a s containers in the procedure described, but glass- or polyethylene-stoppered flasks of any kind will serve a s well. An additional advantage is that small volumes can be used without sacrifice of accuracy. A complete titration is feasible in a spectrophotometer cell, as only 1-2 g of titrant are required. Corrections for dilution have not been found necessary in the procedure described. A syringe rather than a polyethylene squeeze bottle is used for titrant delivery to reduce evaporation of acetonitrile to a negligible level and to improve control of delivery of individual drops. The syringe technique is highly precise; in titrations of ferrocene with copper(lI) in acetonitrile, standard deviations of 0.2-0.3 ppt were obtained ro~tinely.~ 8-Hydroxyquinoline is too weak a base in water ( p K , = 9.8) to be titrated with acid, even if it were sufficiently soluble. In acetonitrile a number of acids strong enough to permit sharp titrations are available, and the base 8-hydroxyquinoline and many of its salts are soluble. Other advantages of acetonitrile as solvent for this experiment are that it is both weakly acidic and weakly basic, is not toxic, and commercial material (such as Matheson, Coleman and Bell, AX149) can be used without further purification. Though a number of other acids are stronger, p-toluenesulfonic acid was selected as the titrant because reagent grade material is sufficiently pure to be weighed directly as a standard and because solutions of i t in acetonitrile are stable for extended periods. The nitro or chloro benzene sulfonic acids3 are more expensive and require standardization because they are not available in high purity. Perchloric acid in glacial acetic acid, another possibility, requires more work to prepare, must be standardized, and is less convenient to handle in student laboratories. The concentration of titrant also is important. Several values of absorbance in the range of 0.1-0.3 are needed if the end point is to be determined to the necessary precision; too concentrated titrant solutions give insufficient points in this region. Deviations from Beer's Law make
points obtained a t ahsorbances above 0.4 of little value. As unknown samples for the experiment we use mixtures of commercial 8-hydroxyquinoline (oxine) and 5-methyl-8-hydroxyquinoline,and have the students report the results as percentage of oxine. Both compounds are titrated, so problems of sample inhomogeniety are less serious than if an inert material were mixed with oxine. (A pure sample of the 5-methyl compound would be reported as 91.24% oxine). The bmethyl derivative is relatively expensive, but 0.3 g of sample per student is sufficient for several titrations, so the cost per student is low. A typical student requires about 3 hr to complete the experimental work, including preparation of solutions and several titrations. Procedure
preoaration of Samole and Titranl ~
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Prepare a standard solution of p-toluenesulfonic acid monohydrate (p-TSA, formula weight 190.22) in acetonitrile by weighing to the nearest 0.1 mg about 0.5 g of pure p-TSA4 into a small beaker. Weigh a clean, dry, glass-stoppered 50-ml flask to the nearest milligram on a top-loading halance.5 Dissolve the p-TSA in a small amount of acetonitrile and transfer the solution quantitatively ta the flask. Rinse the contents of the beaker into the flask with several small portions of acetonitrile. Dilute to about 50 ml with acetanitrile, stopper well, mix, and weigh the flask plus contents to the nearest milligram.e Prepare a solution of the unknown sample by weighing about 0.3 g to the nearest 0.1 mg, transferring it to a clean, dry, glassstoppered 1(M-mlflask previously weighed on a top-loading balance to the nearest milligram, diluting to about 100 ml with acetonitrile, and weighing again. Keep well stoppered to prevent evaporation. Titration Procedure Obtain a spectrophotometer with a cell compartment that can accomodate a large cell (10-15-ml capacity), a cell, a stirrer, and 'Butler. E. A,. and Swift. E. H.. J. Chem. Educ., 49, 425 (1972). Kratochvil, B., and Quirk, P. F., Anal. Chem., 42,492 (1970). Pietrzyk, D. J., and Belisle, J., Anal. Chem., 38,969 (1966). 4 Reagent grade p-TSA, obtained as the monohydrate, typically assays 99.8% or higher. It is stable and nonhygroscopic. If the purity of a particular lot of the acid is unknown or in doubt, test by titration of a pure sample of bhydroxyquinolme (best prepared by sublimation of a few tenths of a gram under vacuum), using the procedure of this experiment. "The concentration of titrant is defined as grams of p-TSA per gram of solution. The flask must he dry, because excess water will reduce the acid strength, and because acetonitrile or water in the flask initially will not he included as part of the solution weight and will cause the calculated concentration to be in error. Stock solutions of p-TSA and mine must be kept tightly stoppered to prevent evaporation of the solvent. Once a portion of oxine solution has been weighed into the spectraphotometer cell, however, evaporation of solvent does not affect the amount of oxine present. The situation is analogous to pipetting an aliquot of sample into a flask for titration. Changes in total volume of solution after pipetting, either from evaporation or from addition of solvent when washing down the sides of the flask, do not affect the amount of sample being titrated. Volume 50, Number 9, September 1973 / 629
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Figure 1. Support for syringe and spectrophotometer cuvette during weighing. Stirrer is left in cuvatte throughout titration.
Figure 2. Plot of absorbance versus weight of p-TSA solution for the phatometric titration of a sample of oxine.
a 10-ml syringe.' (A Bausch and Lomb Spectronic 20 with a sample compartment accomodating a l-in. test tube is convenient). Set the spectrophotometer wavelength to 540 nm. Weieh the cell (Fie. 1) on a ton-loading balance to the nearest millifram, add 10-13 g of sample culut~un,and wergh again. Srr the ipertruphutometcr tu zeru un the percent-transnlittnnrr scale. Insert the d l and add 10nrnps n l n 0.1%.solution uf 4-phenylamdt. phrnylamine mdirator m scetmitrile. Insert a s t l r r ~ r mix, . ~ rowr
calculate t h e concentration of p - T S A t i t r a n t i n grams of p - T S A per g r a m of solution. From t h e weight of t i t r a n t (obtained from graph), a n d s a m p l e weight calculate t h e concentration of t h e sample solution. T h e percentage of oxine in t h e s a m p l e i s t h e n obtained b y
% oxine = the cell, and set the instrument to zero absorbance (100% transmittanee). Draw from 5-10 ml of p-TSA titrant solution into the syringe. Weigh the syringe to the nearest milligram, record the t eight,^ and add 0.4-0.5 g of titrant solution to the cell. Retract the plunger slightly after each portion of titrant is added to minimize evaporation of the drop that would otherwise remain on the delivery tip. Stir the cell solution, cover, and record the absorbance. Record the syringe weight, add another portion, and continue as before. When the ahsorhance reaches 0.08 to 0.1, take readings after each 1 or 2 drops of titrant up to an absorhanee of about 0.4, when the titration may he terminated. To determine the weight of titrant required, plot weight of titrant added versus absorbance and draw straight lines through the points before and after the break point (Fig. 2). A major source of error in this experiment is insufficient care in drawing the lines used to select the end-point weight. Calculations F r o m t h e weights of p - T S A , a n d solvent p l u s p - T S A ,
630 /Journal of Chemical Education
(g p - T S A l g t i t r a n t ) ( w t t i t r a n t ) ( f o r m u l a w t oxine) ( m o l w t p - T S A ) ( w t s a m p l e soln)(g s a m p l e l g soln)
(100)
T h e formula weight of oxine i s 145.16. 'The i y i n ~ eia fitted with a YL-cauge stainless-steel needle or with a glass or polyerhtlene rip hnvinr: a fine orif:ce at the end. A thin film of silicone xrense on the . plunger . allows control of titrant addition and titrant loss along the ground-glass surface. The syringe and cell are supported during weighing with a cradle constructed from a 16-oz polyethylene bottle, the upper part of the bottle being cut at the shoulder so as to fit snugly into the bottom half (Fig. 1). 8 The stirrer, a polyethylene rod or 16-gauge copper wire sealed in polyethylene tubing (Fig. 1). must be positioned in the cell so that it does not ohstruet the light path during measurements. aIf the top-loading halance has a built-in taring device, set the optical scale to its upper limit, usually 10 g, at this point. (A check of the optical-scale sensitivity at this time is suggested.) In this way the total weight of titrant added at each step, obtained by subtracting the valies recorded from the initial reading, is determined more readily. Some balances have a built-in complementary scale that allows the weight of titrant rernoued from the syringe to he read directly from the scale.
