California
Brigham Young University Provo, Utah 84601 and Ernest H. Swift Institute o f Technology Pasadena, 91 109
I
chemists have made use of gravimetric burets for over 70 years (I-$), and since the development of rapid means of weighing there have been favorable reports concerning their use (4-6). It was, therefore, surprising to find that even very recent texts on analytical methods either mnke no or only passing mention of gravimetric titrations. A technique which has much to recommend it is being largely ignored, it appears, while the inherently inconvenient,, if t,imehonored, volumetric technique is being retained. It appears that this neglect is caused mainly by the time and effort expended in the conventional practice of making repeated weighings of the buret as the end point is approached. Experiments with burets and titration procedures have led to the development of a simple plastic buret and a titration method which have so minimized these objections that there seems to be justification for presenting a discussion of the merits of the technique and data resulting from its use with representative groups of students. Tbohurn (4) has given a detailed discussion of the advantages of gravimetric titrations. They can be summarized as follo\vs: (a) Elimination of drainage problems, therrfore, clcaning of interior walls of vessels is easier. (b) Elimination of troubles with stopcock and stopcock grease. (c) No reading of menisci. (d) Elimination of volumetric calibrations. ( e ) Elimination of tcmpcrature corrections. (f) Equipment requirements and hreakagc reduced. No volumetric burets and fewer fla7lcs and pipets are required. (g) The gravimetric measurements can be made more accurately. Thohurn used glass syringes as burcts. Gaddis ( 6 ) used a small plastic dropper bottlc, and others (7) have used a plastic wash bottle with capillary-tippcd delivery tuhe. This has been found to be a very satisfactory gravimetric hurct for student use; it is inexpensive, durable, and easily modified to fit the requirements of gravimetric titrations. The gravimetric buret and the titration procedure used arc described belolr., followed by data obtained in two classes at Brigham Young University having students of significantly diffcrrnt laboratory expericnces. The data that are reported wcre taken during thc development of procedurcs and instructions for a forthcoming text. The Buret
A 2-02 (60 ml) polyethylene vash bottle is used as the gravimetric burpt (Fig. 1). Thin, easily-flexed 'The Nalgene 2402 wash bottle has been satisfactory in this resvect.
The gravimetric buret.
walls are essential in order to control the delivery of a single drop.' A capillary tip on thc delivery tuhe is essential; this is casily prepared from thc commercial tip as follo~vs Adjust a micro burner t o give the smallest flame which will burn continuously. Then heat the delivery ttuhe shout 3 / ~in. from the tip; hold i t approximately 1 in. sbove the top of the flame and rotate it slowly. The polyethylene hecomes transparent when i t is heated; when this transparent section has thickened about 5015,remove the tuhe from the heat, hold i t vertically, and draw it to the desired diameter. Hold i t a few seconds until i t becomes translneent,, after which trim i t with a sharp knife.
Onc accustomed to \\.orking with glass tubing has a tendency to overheat or cvcn ignite the polyethylene, but after only a few practice attempts can dran- a very satisfactory tip. Students with manual dexterity soon learn to draw acceptable tips. Occasionally, an air leak occurs xherc the delivery tube passes through the cap. This can be sealed ~ i t h a silicone cement, or a new cap can be bored to fit the delivery tube tightly. Any leakagr of air makes the delivery of a singlc drop of polution almost impo.isibk. Students are rcquired to practicn until thry can consistently deliver from the buret a port,ion of water which vcighs less than 5 mg. Usually, this is a split drop, for while a tip is casily prepared xhich dclivers drops weighing less than 5 mg, surh a tip ordinarily so reiitricts the flow of solution that a titrittion is frxstrat,ingly slow. For this reason, studcnts arc directed to chcclc that thcir burets will deliver about 4 ml in 10 sec. Volume 49, Number 6, June 1972
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When making titrations very few students find it necessary near the end point to go through a sequence of weighing, adding a drop, and weighing again. Such repetitive weighings would makc gravimetric titrations slow compared to volumetric titrations, even with top-loading balances available in the laboratory. The students soon learn that they can add portions of such size that at least three such portions would be required after the equivalence point to cause a 0.1% error in a titration that requires about 15 g of titrant. Therefore, they approach the end point with more confidence than when making volumetric titrations in which the portions added are ordinarily much larger. For convenience, approximate volume graduations are put on the buret with a marking pen. A buret, measuring pipet, or 10 ml graduated cylinder is adequate for measuring 5 ml portions of water into the gravimetric buret. The 5 ml marks serve as a rough guide in repetitive titrations. Titrations that use about 15 g of titrant are recommended for student work. For *@.I% accuracy the titraut weight must be known to 15 mg and thc individual w~ighings(before and after) of the buret and solution must be made to 7.5 mg. With top-loading balances used in the laboratory, a 5 mg reproducibility is easily obtained. With analytical balances, the weighings can bc reproduced to *@.I mg and considerably smaller quantities of titraut can be used with +O.l% accuracy (7). However, the use of about 15 g of titrant has the advantage that if fingers are wiped carefully before the buret is touched, the weight of fingerprints is insignificant, and handling techniques are very simple. (Seils, Meyer, and Larsen (7) suggest a convenient means of avoiding touching the buret when small titrant weights are used.) Sfudent Resullr
The data presented here came from two types of courses. Course I is a one-semester course in quantitative analysis taken by ~tudentsin the biological or agricultural sciences. These students had had a year's course in general chemistry which included 30 3-hr laboratory sessions. The data were obtained near the end of the quantitative analysis course and, cousequently, represent the students' performance after nearly three semesters of chemistry at the university level. Chemistry was not the prime academic interest of any of the students and, indeed, it appeared to receive only minor interest at times. Course I1 is a freshman chemistry course taken by chemistry majors together with selectcd students from the other physical sciences. These students were in their second semester of university chemistry and in their first semester of laboratory work. Prior to the experiment reported below, thcy had calibrated their volumetric glassware and had performed just two quantitative determinations; a volumetric titration of chloride by the Mohr method and a gravimetric titration of iron by the Zimmermaun-Reinhardt method. Their total previous laboratory experience other than that gained in high school covered a period of eight weeks. In both courses the data reported bclow were obtained in a single 3-hr laboratory period. That is, the experiment w i ~ sin the form of a practical laboratory examination in which the measurements were made, 426
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calculations done, and results handed in by the end of the period. In each case, the experiment was new to the students, but they were told a week in advance what the determination would be in order that they could study for it. In Course I, the students received a standard arsenious acid solution, a ceric sulfate solution, and an unknown arsenious acid solution. To decrease the work of preparing solutions, all students were given portions of the same three solutions. This also facilitated determining sources of student errors. The students first standardized the ceric solution and then used that to titrate the unknown arsenious acid. No top-loading balances were available in the laboratory used by these students and all weighings were made in the adjacent balance room on single-pan analytical balances. Despite the time spent walking between laboratory desk and balance room, all students in the class finished the experimental work and the setting-up of the calculations, and more than 90% completed their calculations. In Coune 11, the students received a standard silver nitrate solution and a potassium thiocyanate solution. They standardized the thiocyanate solution for subsequent use and reported its concentration at the end of the laboratory period. Four top-loading balances were available to the 22 students and most weighings were made on these, which were in the laboratory and were shielded on three sides from air currents. However, some students used their analytical balances (single-pan) when the waiting period for a toploading balance appeared to be too long. Every student's results are given in Tables 1 and 2 and are their averages of three titrations. While these numbers lend themselves to statistical treatment, we found more impressive the fact that in each group, despite the time-pressure of the experiment, a majority of the students were within +O.lOjo of the accepted value. Overall, it is seen that in the more experienced Group I, 28 of 47 values were within *@.I%, 42 of 47 were within +0.2y0, and the remaining five had deviations of -0.4, -0.3, +0.5, and +1.2%. In the freshman Group 11, 14 of 22 values were within *O.lOjo, Table 1.
Data from Course I
Reported normality
%
Error
Reported normality
Error
%
Reported normality
Error
0 1030 1029 ,1028 1030 ,1081 1080 ,1029 ,1029 .I029 ,1032 1029 ,1081 ,1030 .lo29 ,1029 ,1028
0 -0.1 -02 0 f0.l 0 -0.1 -0.1 -0.1 f0.2 -0.1 fO.1 0 -0.1 -01 -0.2
0.1029 .I032 1030 .lo12 .I028 ,1029 ,1028 .I029 .I026 .10.30 .I028 ,1028 1080 1027 ,1035 ,1081
-0.1 +02 0 +1.2 -0.2 -0.1 -02 -0.1 -0.4 0 -0.2 -0.2 0 -0.3 +0.5 +O.l
0.1030 ,1029 1031 ,1032 ,1028 1082 ,1032 ,1028 1086 ,1028 ,1031 ,1029 ,1029 ,1030 ,1030
0 -01 +Ol +0.2 -02 +02 +02 -0.2 COB -0.2 fO.l -0.1 -0.1 0 0
Table 2. Reported normalitv
Error
%
Data from Course II
Reported noirnahtr
Error
Reported normalitv
Error
18 of 22 were within *0.2y0, and the remaining four had deviations of -0.4, -0.5, -0.6, and -0.4%. An additional advantage of gravimetric titrations is that standard solutions and solutions for unknowns can he easily prepared with excellent accuracy. For example, to prepare the standard silver nitrate solution, a weighed portion of dry AgNOa was transferred to a large dry polyethylene container which had been tared on a 10 kg top-loading balance. Then water was added to the desired total weight. By this method standard and unknown solutions can be prepared with uncertainties in weights of considerably lees than +0.10/0. Buoyancy cffccts on the solutions are eliminated because the weight-in-air is used both in preparation and titration.
In Course I, the students were allowed to choose which titration technique to use in experiments suhsequent to that reported in this paper. Every student chme to use the gravimetric titration method. Faculty members who have instructed laboratory sections in which gravimetric titrations were made have been unanimous in preferring these to volumetric titrations. Literature Cited (1) (2) (3) (4) (5) (6) (7)
RIPPER, M . . Chem.-Zlg., 16, 793 (1892). W ~ s n s u n r E. . W., J . Amer. Chent. Soc., 30, 31 (1908). B ~ r ~ e H. u . S.. Ind. En.. Chem., 6. 941 (1914). T ~ O B U RJ.NM.. , J. CHBI. EDUE.,36,616 (1959).
C a n r s ~ ~ R., n ~ J. . Cmr.Enuc.. 44, 144 (1967). G ~ o o r s S. . W.. J. Cnear. Eouc., 40, 324 (1963). SBILB. C. A,. METER,R.J., LABBIN,R. P., And. Chern., 35, 1673 (1983).
Volume 49, Number 6, June 1972
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