An Improved Limestone Analysis for Quantitative Analytical Courses' C . R. JOHNSON University of Texas, Austin, Texas
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RACTICALLY all modem textbooks of elemen- method for determining calcium oxalate by means of tary quantitative analysis include directions for a standard permanganate would ever go back to the proximate analysis of limestone by gravimetric gravimetric way." methods. While this is a good practice exercise, it may In a study extending over four years with six large be doubted whether i t should be considered an inevit- classes, i t was found that out of 350 students making able one, a t least in its usual form. In an effort to triplicate limestone analyses by the modified procedure increase the teaching functions of this standard labo- 94 per cent completed all of the magnesium and calcium ratory exercise, and to adapt i t for use in warm cli- determinations, while out of 340 students analyzing the mates, a modified form of limestone analysis was intro- same samples in triplicate by the standard gravimetric duced and tested in the sophomore quantitative analy- methods only 72 per cent completed all of these detersis courses here several years ago. This analysis was minations. Somewhat unexpectedly, results of the satisfactory enough in use so that it has been con- completed analyses made by the modified method were both more accurate and more precise than those obtinued, with some few minor modifications. The exercise consists in determining the loss on igni- tained by the gravimetric methods. However, under tion, silica, and combined oxides by the usual gravi- the conditions of the study, this may be an indirect metric methods, calcium Ey the permanganate method result of the fact that the gravimetric methods are essenof Kraut ( I ) , and magnesium by the method of Stolba tially more time-consuming. . .' (Z), as modified by Handy (3). This series analysis Both volumetric methods work well in the hands of follows a number of the usual partial gravimetric analy- students using them for the first time. Accidents ses and serves as a transition from the gravimetric to in the volumetric procedures occur no mote frethe volumetric part of the analytical work. Since the quently than in the gravimetric ones. Provision is analysis precedes the presentation of the theory of volu- made for accidents occurring late in the series analysis metric analysis, calculations are made on a titer basis by allowing rerun analyses of calcium carbonate samrather than by the use of the "normal" system. The ples and magnesium sulfate solutions. required titers are derived from the balanced chemical The limestones for analysis-'contain between 23 per equations for the simple analytical reactions concerned, cent and 36 ljer cent of calcium and 2 per cent to 10 by the methods of calculation introduced in freshman per cent of magnesium. For one-gram samples it has chemistry courses. The two volumetric methods may been found best to use 0.4000 N KMnOl having a calthus be developed without logical difficulties. The cium titer of 0.008016 grams per ml. Porous porcelain standard gravimetric methods for calcium and magne- crucibles proved unsuitable for use in the calcium desium are discussed fully in clas's, using the laboratory termination, due to clogging of-the pores by finely work with calcium oxalate and magnesium ammonium divided calcium oxalate. The filter paper technic recommended by Hillebrand and Lundell (4) is more phosphate as a background. The introduction of two standard volumetric methods economical and satisfactory. in the limestone analysis hardly needs any justificaIn the magnesium determination 30.00 ml. of 0.400 tion, but a few points in favor of the modified exercise NHzSO, havinga magnesium titer of 0.00486 grams per may be mentioned. By the time a student has dried ml. are used, and the excess acid is determined by titraand ignited duplicate or triplicate samples of barium tion with 0.400 N NalCOs, with methyl orange or bromsulfate, silver chloride, and hydrous silicon dioxide and blue as indicator. There reagents give a conferric oxide, he does not learn much more by drying and siderably sharper endpoint than the more dilute igniting a few other precipitates with more or less simi- acid and sodium hydroxide solutions used by Handy. lar properties. In the modified analysis the time saved Sodium carbonate was considered preferable to sodium by avoiding the slow gravimetric procedures and such hydroxide as a stock solution for student use. futile repetitions is used to obtain an introduction to LITERATURE CITED the titer system, residual titration, and some of the (1) See MELLOR, "A treatise on quantitative inorganic analysis," technics of volumetric analysis. It is interesting to note Charles Griffin and Co., Ltd., London, 1913,p. 215,or ref. (4). Chem. Zene.. 728 (1866). here the comment made by Handy (3) regarding Kraut's (2) STOLBA. HANDY, J. Am. Chem. Soc., 22, 31 (1900). See also Low, method: "No one who has adopted the volumetric (3)"Technical methods of ore analysis." 9th ed., John Wiley and
' Presented before the Division of Chemical Education at the
l0lst meeting of the A. C.S., St. Louis, Missouri, April 9, 1941.
Sons, Inc., New York City, 1922, p. 140. (4) HILLEBRAND AND LUNDHLL, "Applied inorganic analysis," John Wiley and Sons. Inc., New York City, 1929,P. 501.
