1 in the Chemical Laboratory I
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Edited by N O R M A N V. STEERE, 140 Melbourne Ave., S:E. Minneopolis, Minn. 554 14
CXIV. Perchloric Acid Digestion Without a Perchloric Hood
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H. G. Griffin and M. B. Hocking,* Department of Chemistry, University of Victoria, Victoria, B.C., Canada. V8 W ZYZ A standard procedure for dissolution of organic material prior to analytical trace metal determinations is to bail the sample in a mixture of concentrated nitric and sulfuric acids, with subsequent additions of nitric as required. This method, which entirely avoids the use of perchloric acid, would not completely dissolve samples of tobacco being tested. A survey of the literature revealed that successful completion of the wet ashing could probably be achieved either with perchloric acid following a nitric/sulfurie acid mixture preliminary digestion (1, 2). or with simply a mixture of nitric and perchloric acids under controlled temperature conditions (3-6). To sidestep the risks of formation of anhydrous perchloric acid (7, 81 and ineidentally to reduce the risk of trace metal losses (11, i t was decided to avoid the former system entirely. Use of the nitric/perchloric acid digestion mixture required access to a perchloric hood (9)for safe application, which was not available a t the time. Hence, a system was devised to enable perchloric digestions to be carried out in a completely self-contained manner, with negligible risk of pressure build-up. The design, essentially a much-modified distillation unit, (See Figure) incorporates
* Correspondence should be addressed to this author.
a regular long-necked bomsilicate glass Kjeldahl flask with a thermometer pocket fused into the vessel reaching close to the bottom. Thus the stage of the digestion can be readily followed by temperature while adequate reflux length is provided by the neck of the digestion flask to avoid entrainment loss of components during digestion. In order to obviate any risk of ground glass surfaces or glass-to-glass frietion triggering the decomposition of any transitory organic perchlorates (lo), the distilling head was connected to the top of the Kjeldahl via a large polished glass, spherical joint separated by a perchlaric resistant TeflonQ-covered silicone rubber O-ring (41125 Rotulex spherical joint?). Addition of any further required reagents during the digestion was by removal of a hollow, machined teflon plug fitted to a fire-polished glass opening, again designed to avoid glass to glass friction. The efficient, large bore water condenser was blown integrally with the head to provide easy vapour or gas (primarily NO*) flow from the digestion flask, and to avoid any risks of leaks from unnecessary joints. The condenser fed into a double-sphere antisuckback drip tube discharging into 350400 ml of cold water. The whole apparatus was mounted just above a large highwalled enameled steel tray which could retain all materials in the event of a rup-
ture, and behind a portahle safety screen pius armoured glass fume hood door in a 'egular fume hood. f Manufactured by Sovirel Glass Co., 90-92 rue Baudin. 92 Levallois- Perret, France. A polished glass conical joint. 241 40, made by Quickfit and Quartz, was found to be equally safe when used with a 2 mil. thick "Teflon" sleeve separating the two surfaces. Alignment was somewhat more awkward during assembly and disassembly with the tapered joint. (Continued on page A290)
Volorne51. Number5. May 1974
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A289
Safety
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Using this apparatus, with concern for the safety precautions for perchloric acid use conveniently summarized by Muse (a), has enabled completion of more than 140 digestions uneventfully and with complete containment of perchloric acidlwater azwtrape vapours. Four gram or smaller samples of plant material were digested by first moistening with 10 ml of distilled water. Then 20 ml af cold concentrated nitric acid was added fallowed hy a premix of 20 ml of 70% perchloric acid with 20 ml of concentrated nitric acid. An electric heating mantle was applied for digestion monitored by the thermometer a t the pocket (by eye, or sensing probe (5)). When the temperature reached 200"C, normally in about 1 hour, digestion was complete. Any further chemical treatment of the digest required for particular analyses was carried out in the original digestion flask through the Teflon%toppered top closure. This system was convenient in use requiring very little additional handling than the more straightforward procedure using a perchloric hood, and provided a means of conducting perchloric digestions without hazard, while all precautions (8) are rigorously followed. The periodic dis-
A290
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Journal of Chemical Education
mantling and removal of apparatus required for flush-down of a perehloric hood probably near equates in manipulative time that required for the use of the containment digestion apparatus. But perhaps most importantly, the digestion method described outlines a safe and readily arranged alternative for the oceasional requirement of perchloric acid digestion, when the desirable perchloric hood is not available.
LITERATURE CITED
(11 Remington, R. E., Coulson, E. J.. and von Kolnitz. H.. Indust. and E n s Chem.. Anal. Ed.. 6. 280 (19341. Of&. AmTTic. Chem. X X (2). 172 (21 Csisil. C.C.. k . (1965). (3) Kahsne,E.. 2. AnaLChem.. 111.14 (1937). (41 Smith. G. F.. Analyst.80.25 119551. (51 John. M. K., Anal. Chem.. 44, 429 (19721; and reis. cited therein. agenkopi. G. K.. Neurnsnn, D. R., and W d r i i i ,
(81 M & L. A , J. Chem. Ed.,49.A463 (19721. (9) "Handbmk of Laboratmy Saiefy." 2nd ed., N. v. Steer., Ed., The Chemical Rubber Publishing Co.. Cleveland. 1971, psee269. (101 Muse, L.A.. Chem.snd Eng.Newr. 51.29 (19731.
