Supertitrations: High-Precision Methods W. B. Guenther University of the South. Sewanee, TN 37375 Although wet chemical analysis is still part of analytical courses, too often modern rapid balances are not used to best advantage. This paper offers challenging work a t a higher level of technique than students meet in elementary laboratory work. It adapts work on redox standards done a t The National Bureau of Standards ( I ) to produce measurements having average deviation well under 1 ppt. It uses a combined weight and volumetric sequence, faster than historic weight titrations, which has appeared in other literature (2) but not in textbooks. Benefits of wmk of this kind include t1) practice in highprecision measurements in a context requiring them: testtng of standards, (2) useof more than a single standard toassure reliability, and (3) testing one's skill in purification hy rerrvstnllination. Fern,usammonium sulfate ~. F A .Sand I wtai.-"..-~ ---sium dichromate are recrystallized and compared. Arsenic(II1) ~~., oxide can be used as a second standard as in the NBS work ( I ) . Commercially available standards can he used as well as. or instead of. the student's own preparations. This work tdsts the reliability of repurified FAS i s a standard as recently reported (3). This exercise cold precede iron or chromium determinations. Historically, weight titrations were tedious because of the slow weighings. That factor also lowers endpoint precision because of evaporation and drop size of the concentrated solutions weighed. The NBS paper presents the way to simplify the technique and avoid the serious fault in endpoint nrecision. In the revised method oresented here, each reactant is weighed to a t least five figures, one in slight excess, which is then titrated with a diluted solution of the other to throw the endpoint uncertainty into the fifth figure. An allweights method, and an alternative with large pipets are described. This work began after use of a weight method published by Ramette (4). First, let us review the NBS method, which makes an impressive case history of modern highprecision work with some venerable and reliable redox chemistry. ~~~~~
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ing 99.9822%, and hy comparison with NBS primary standard ASZOB,giving 99.985%. Giving greater weight to the coulometric, they chose 99.983% as their best value for the new standard (I, pp 45-48). The chemical values show what accuracv can be exnected under the most favorable and conditicks. The NBS Inorganic Analytical Research Division is nrenarine . " a new standard. 136d. to replace 136c, now depleteb. The nuritv of their FAS cancels in the calculation, but we can caiculatk it from dichromate purity and weight ratio as 99.84%. This was a commercial reagent grade, not further purified. Student Procedure: Comparlson of FAS and Dichromate In this work, "apparent weight" refers to the raw data fromweighings in air of density 0.0012 g1mL against weights of density 8.0 gl mL. (See discussion at the end of this paper.) Dry 2.8 g potassium dichromate (primary standard, or recrystallized reagent grade, procedure below) for 1 h a t 160 ".Weigh a clean, dry, stoppered 250 mL volumetric flask to 0.05 g, and weigh the dichromate to 0.1 mg into this flask. Dissolve and make to volume with water and weigh the total. Calculate the apparent normality (or molarity if desired) of the solution by applying to the apparent weight of dichromate the buoyancy factor' B = 1.00030 and dividing by the apparent weight of the salution. The apparent normality is then, milliequivalents of Cr(V1)per apparent gramof solution. This will cancel the buoyancy on the future weights of solution used and avoid repeated corrections. Thevolume N(or M) isalso needed for use with a 111Odiluted portion of this to he placed in a buret (10 or 25 mL if possible).
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The NBS Primary Standard Potassium Dlchromate When the analytical chemistry team a t NBS needed to reolace their 136b K d h O v . they . elected to compare the old wLth the new 136c through reaction with solid te&usammoniumsulfate ( 1 J.Known . vurit\f - o i t h e PAS was not required, only that i t be homogeneous and stable. They had to grind and mix it to obtain reproducible assay to within a few parts per 100,000. They weighed both solid reagents to 0.01 mg, taking about 1g of dichromate and 7.8 g FAS. Mixed in ice cold sulfuric acid solution, the excess Cr(V1) was titrated with 0.02 M Fe(I1) using a potentiometric endpoint detection. This example illustrates the precision. It is simulated from averages since raw data were not reported. True masses (weiehines corrected for buovancies) are used in this section. F i k t ::85361 g FAS and 0.68162 gof 1:ltirdirhnmiite were dissolved in the arid. Then 0.842 ml. of FAS solution having 8.116 mg/mL were required to complete the reartion. yirlding a total of 7.86214 g PAS. or. 8.00966 g 1 g of dichromate. The old standard l36h gave 8.00882 g g. With the old taken as 9 9 . 9 3 7 ~the ~ new becomes 99.990";. Other assays were pert'(~medcoulometrirally with elertrogenrrated Fell11giv-
Titration Method Obtain a thin-walled 125-mL conical flask fitted with a plastic stopper (total weight about 60 g). Weigh this dry to 1 or 0.1 mg as convenient. Pipet into the flask approximately 25 mL of the standard 0.2 N dichromate solution and weigh it. Add 5 mL each of 6 M sulfuric and85%phospharicacids, and ahout 40 mL ofwater. Weigh into this, from a weighing bottle, a sample of the FAS being investigated large enough to be 1% in excess of the diehromate taken. Dissolve, add 3 drops 0.2% diphenylaminesulfonate (DPS) indicator, and titrate with the diluted (0.02 N) dichromate to the purple endpoint. Add up the total milliequivalents of dichromate used, and multiply by the equivalent mass of FAS, 392.14, to get the mass of FAS indicated. Compare with the weight taken ( B = 1.00050)to get the purity. Note that the buoyancy effect is 5 parts per 10,OM).The density of FAS is 1.86 g1mL. Since the material quantities are so large, the endpoint indicator correction found was small. Checks with preoaidized DPS, with ferroin, 4,7-dimethylferroin, and with Pt-calomel EMF detection showed no significant differences.With DPS, the first permanent gray-purple color was taken, usingsplit drops of the diluted dichromate. With the ferroins, excess dichrornate had to he used, and the titration finished with 0.02 N Fe(I1) because of the slow reaction with the indicator in the opposite direction. The dark solutions made the color change harder to see with the ferroins.
1 R = 11 ~.- 0.0012/8\/11 . ~- . ,~ , - 0.0012/&L The densitv of the obiect (dichromate) is 2.68 g/mL, of weqhts, i.0, an0 of a;, 0.0012. ihe correct on. 3 pans per 10.000. is needed for tnis method. which aims at 1/10.000prec sion. For detal s of buoyancy corrections, see ref 4. p 75, and ref 6, p 4264. ~~~
Volume 65
Number 12
December 1988
1097
For preoxidation of the DPS, place 3 drops in the acids-plus-water mixture, add 2 drops of 0.02 N Fe(II), and titrate just to purple with thediluted dichromate. Pour this into the flask after weighing in the dichromate aliquot and the FAS sample. This pouring need not be quantitative. Preaxidation might be desired for highest possible accuracy.
Example Determination All weighings are given in apparent weight as defined above. Standard solutions: 2.4577 g of dry dichromate (G.F. Smith Co., 100.00%certified) was made into 250.90 apparent grams of solution, 250.00 mL at 21PC. Calculating as explained above gives 0.19983 meqlapparent gram of solution, and 0.20055 meq/mL. The 1/10 diluted titrating solution was then 0.020055 N. The first dichromate solution (25.0810 g) was mixed with the acids and water and 1.9963 g of the FAS sample was weighed in. DPS indicator was added, and 3.97 mL of diluted dichromate gave the final endpoint. The total milliequivalents of dichromate was, thus, 5.0916, which signifies 1.9957 apparent grams of FAS, or 99.97% purity for the sample. Reasonable estimates of uncertainties. are in Darts Der . . ten thouranu: .A'. 1, weight of solurion < I , weight of PAS,
