ANALYST, JANUARY 1988, VOL. zyxwvuts 113 zyxwvutsrqp 201 z Effect of Heating Appliance Type on Parr Bomb Temperature Response Performance Paul J. Lechler" and Mario 0. Desilets zyxwvu Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA and Frank J. Cherne Electrical Engineering Department, University of Nevada, Reno, NV 89557, USA Stainless-steel and Teflon acid digestion bombs are designed for the dissolution of relatively inert and/or volatile substances by reaction with a solvent at elevated temperatures and pressures. This is accomplished by heating the loaded bombs, usually in a laboratory drying oven or furnace. It has been demonstrated that because of performance differences between different heating appliances, inside-bomb temperature increases with time can differ markedly even when the various heating devices are set for the same temperature. Because most reactions are temperature dependent, performance differences between heating appliances can significantly affect dissolution times for given applications. The effect of heating appliance type on the dissolution of native sulphur by oxidation with concentrated nitric acid is examined. Keywords: Acid digestion bombs; sulphur; reaction rate; heaiing appliances Acid digestion bombs are generally Teflon-lined, stainless- steel containers which, when sealed, are capable of digesting resistant materials with suitable solvents under elevated temperature and pressure conditions while retaining poten- tially-volatile components. Bombs are widely used in the geochemical community for the dissolution of silicate rocks with hydrofluoric acid mixtures. Recent research at the Nevada Bureau of Mines and Geology has involved digestion of natural, native sulphur with nitric acid in Parr 4749 acid digestion bombs. 1 Preliminary results of these studies pointed to significant differences in the reaction kinetics depending on the type of heating appliance used to heat the bombs. It was therefore difficult to generalise regarding appropriate diges- tion times for given furnace temperatures when compiling the results from these studies. To clarify these bomb temperature responses, an investigation into the effects of two different types of heating appliances was initiated. These pronounced effects are discussed in this paper. Experimental Dissolution reactions which are temperature dependent are related to the inside bomb temperature rather than to the indicated furnace or oven temperature and the two tempera- tures are not the same for the first hour or two of heating. One hour or less is a common length of time in which to attempt sample decomposition with an acid digestion bomb.2 Hence the inside-bomb temperature during actual decomposition is generally significantly lower than intended. In order to evaluate the inside-bomb temperature response to heating in both simple laboratory drying ovens and thermocouple-con- trolled, constant-feedback laboratory furnaces, a series of carefully controlled temperature measurements were per- formed. Parr No. 4749 acid digestion bombs with cited maximum working temperatures of 250 "C and maximum working pressures of 1800 psig were used. The bombs were heated for 2 h at 100 k 2 "C in (1) a Precision Thelco Model 18 laboratory drying oven and (2) a Lindberg Hevi-Duty Type 51232 thermocouple-controlledfurnace while monitoring the inside- bomb temperature. Further temperature measurements were obtained by placing the temperature sensor inside the heating appliance zyxwvu * To whom correspondence should be addressed. and, after the equilibrium temperature (100 zy "C) was reached, the appliance door was opened, a bomb placed inside and the door closed. The furnace or oven temperature response to temporary cooling was thereby measured. Temperature measurements were made using a National Semiconductor LX5700H temperature transducer mounted inside a fully assembled Parr No. 4749 bomb. This transducer is fabricated on a single monolithic chip which includes a temperature sensor, stable voltage reference and operational amplifier. The output of the LX57OOH transducer is directly proportional to the temperature and is 10mVK-1. The transducer is linear from -55 to +125 "C. The temperature readings were recorded using a Hewlett-Packard Model 680 strip-chart recorder scaled to indicate 0-100 "C. Results and Discussion Routine, well established geochemical applications of acid digestion bombs are performed successfully without undue regard for precise time -temperature - reagent constraints. However, frontier applications of such devices to, for example, the digestion of sulphur by oxidation with concen- trated nitric acid, without some estimate of reaction kinetics and product pressures, can result in bomb failure.1 Such occurrences for the Parr No. 4749 bombs, with pressure blow-out orifices, are not very inconvenient or expensive as only the Teflon cups and lids have to be replaced. However, the Parr No. 4745 bombs, which are in much more widespread use in the geochemical community, have no provision for safe overpressuring and their failure would probably be much more dramatic. Obtaining an advance estimate of reaction Oven/Furnace temperature = 100°C 60 I I I 30 60 90 120 Fig. 1. Plots of inside-bombtemperature responses with time after being placed in a 100°C furnace and oven at time zero. Room temperature, 24 "C 0- Ti m e/m i n Published on 01 January 1988. Downloaded by Drexel University on 26/10/2014 12:47:51. View Article Online / Journal Homepage / Table of Contents for this issue