Gas-Phase Chromatography: A Class Experiment See BRENNAN, D., AND KEMBALL, C., J . C h m . Edue., 33,49&492 (1956). As an analytical tool, gas chromatography can be applied both to gaseous mixtures and to mixtures of volatile materials. With sufficient attention t o detail, the data that are obtained can he interpreted quantitatively. This article describes the construction and operation of a gas chromatograph; two articles immediately preceding' discuss the principles. The chromatographic column can be filled with w one of the several available materials now on the market instead of the specific material recommended. The device immediately on the right of flask BXin Figure 1 is a vacuum pmnp.
1 CASSIDY, H. G., J. Chem. Edue., 33, 482 (1956); LOTZ.J. R. and WIL~IINGHAM, C. B.,J . Chena. Educ., 33,485 (1956).
Questions:
Should peak heights or peak areas of a chromatogram be used ,,fa mi*ure? What factors determine the aptimtlm temperature of operation d an adsorption column? Does the precision or the accuracy of an analysis, or both, deof flow of gas through a gas chromatograph? ~ , d upon the Is distilled cider vinegar merely an aqueous solution of acetic acid? The ethyl alcohol content of most liquid flavoring agents is given on the label of the battle. Can you verify such a statement? Is the efficiency of an adsorption column for each component of a mixture of hydrocarbons a linear function of the molecular weight of each hydrocarbon?
to determine the
A Quantitative Sodium Flame Test See KIPLINGER, C. C., J. Chem.Educ., 28,641 (1951). \ n iia.xpmjiv+spnn rwwpc i s mndiht4 I,? t i l e use of I ao pirces of polnroid ro scrvt. s i :t s~tnplcr p ~ ~ c t n j ~ ~ l . o r o n i t ~One t t . r . piwr
filter paper wick, into a burner flame.. The angle of rotation of the ~olaroidwhich is necessarv to "extineuish" the emitted lieht This anile is an approximat& is:n;ertsured on the h e m function of the sodium ion concentration, up to about 0.1 molar.
Questions:
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Kiolineer mentions certain ~reoautions: the heieht of the burner flame must be kept constant; if crystals appear on the paper wick, the data are useless, etc. Can you modify this procedure so that it will he less subject to error and easier to obtain reproducible results? Kiplinger states that the amount of sodium present as an impurity in certain reagents was found to be greater than that indicated as the maximum amount on the labels of the bottles containing these reagents. If true, this is distressing. Can you conhm this result? Is it po~iildeto us? this rncthod for tlrr qonntitntiw d+r~.rniinatirm of ot lwr ion+,auelr us potnssium, orrslriqlm, or honm?
Thioacetamide as a Sulfide Precipitant in Qualitative and Quantitative Analysis See GUNNING, H. E., J. Chem. Edw., 32,258-259 (1955).
Aqueous solutions of thioacetamide,
s CHI&-NHr hydrdyse rapidly at temperatures above 80' C, forming hydrogen sulfide and other products. Cations which form insoluble sulfides will precipitate as sulfides in a hot solution of thioacetamide. Hence, thioacetamide can be used in quslitative analytical procedures instead of gaseous hydrogen sulfide. No essential change in the analytical procedure is necessary, and the elimination of poisonous, malodorous hydrogen sulfide constitutes an additional advantage. Because of the very slight solubility of many sulfidedes, it would seem that quantitative analytical determinations of many different substances could be based upon the precipitation of the constituents as sulfides. When hydrogen sulfide is used, this prooedure is not practical beautuse sulfides so formed are often partbdly colloidal and not completely separable from the liquid media. in which they are formed. The same sulfides, precipitated by hydrolysis of thioacetamide, are not colloidal; this procedure can be used for quantitative analytical determinations. Questions:
Antimony triohloride is soluble in concentrated hydrochloric acid. When sufficient water is added to this solution a white precipitate, probably SbOC1, is formed. Comparison of the oslculated percentage of antimony in ShOCl with that found by
A354 / Journal o f Chemical Education
analysis of the precipitate would serve to confirm or deny the supposed identity of the precipitate. Is SbOCl the correct formula.for this preoipitate? Type metal, used by printers in Linotype machines, is an alloy of lead, antimony, tin, and sometimes one or two other met&. This allay is melted and kept molten prior to casting type; as a result, some oxidation occurs and the composition of the alloy changes. This change is called "dross deterioration." Obtain a sample of usedtype metal from your local printer. Determine its composition and compare with the original composition of the type metal, as furnished by the type metal supplier. Which component of type metal alloy is preferentially subject to oxidation? Two differentkinds of questions, one relating to a problem that is chemical in nature, and one directing attention to analysis, are given above. Thioacetamide can be used in the analysis of many other compounds and mixtures. From your own experience select a chemical or analytical question which e m be answered by using thioacetamide as an analytical tool and set out to find the answer. Examples of a few more such questions are: Does a chemical change occur when silver chloride is exposed to light and thereby becomes darker in color? How much nickel metal is present in a nickel (51) coin? What is the thickness of the tin coating on a "tin can?" Different types of solder melt at different temperatures; m e these differences in melting points related to differences in composition? Why do sulfides precipitated with thioacetamide coagulate more readily then the same sulfides precipitated with hydrogen sulfide?
