AIDS Automated Digestion System for Safe Use of Perchloric Acid Matt K. John Research Station, Canada Department of Agriculture, Agassiz, British Columbia, Canada
FORTHE DETERMINATION of almost all elements (except boron and chlorine), wet ashing is superior to dry ashing. Since the temperature does not exceed the boiling point of perchloric acid, 203 "C., the loss of elements through volatilization, which might occur at temperatures commonly used for dry ashing, is prevented. Also, the silica formed in the wet digestion is completely dehydrated and the adsorption of micronutrients negligible ( I ) . However, it is important to be aware of the possibilities of explosion (2). Smith (3,4) has recommended either the prior oxidation by nitric acid, or the use of perchloric acid at concentrations ranging from 58-68 in special apparatus designed to maintain this concentration throughout the digestion. Most plant-nutrition laboratories use the standard time-consuming Kjeldahl method. However, in case of an explosion, the enclosed design of the Kjeldah1 flask presents an increased hazard to the operator. A digestion procedure was developed to satisfy the following criteria : (1) The digestion is carried out in a system enclosed by metal. (2) The digestion flask used has no constrictions. (3) The temperature of the system is precisely controlled to avoid violent oxidation and possible dehydration. (4) The system is automated for mass handling of samples.
1-.30 c . )
amp
e Relay
15 amp
Fume Hood
1PEIn q S ensing Probe
r T h ermometer
n r T a y l o r Tube Alumipm Blocks
I
Heating Unit
I
I
Reloy CONSTRUCTION
The apparatus consists of three aluminum blocks (Globe Foundry Ltd., Burnaby, B. C.) of size 33 X 22 X 7.5 cm., each containing 48 holes, of 2.5-cm diameter, which were machine drilled to a depth of 5.5 cm. The blocks were placed on top of a Lindberg/Hevi-Duty heater (Fisher Scientific Co., Ltd., Vancouver, B. C.) as illustrated in Figure 1. In order to facilitate unattended control of the digestion process, the heating unit (2600 watts) is connected to a relay box, regulated by a Lab-Stat controller (Matheson of Canada, Ltd., Whitby, Ontario). The temperature sensing probe of the Lab-Stat is placed on a laboratory thermometer and the thermometer is then inserted into a hole of 8-mm diameter in one of the aluminum blocks. When the desired temperature is reached, the relay is automatically disconnected to maintain +1 "C. Fume disposal is accomplished by a stainlesssteel hood (Canlab, Toronto, Ontario) made specially for perchloric acid. The fume pipes and hood are periodically flushed with water and fume disposal is achieved by a Venturi system. Both the Lab-Stat, and the relay which controls the fume hood, are regulated by separate 24-hr clocks (T. Eaton Co., Vancouver, B. C.). The clocks are programmed to initiate both the heating unit and the fume hood simultaneously, but the fume hood is allowed to operate for 2 additional hours. (1) C. M. Johnson and A. Ulrich, "Analytical Methods," Calif. Agr. Exp. Sta. Bull., 766, 26 (1959). (2) D. Sweasey, Lab. Pract., 17,915 (1968). ( 3 ) G. F. Smith, Anal. Chim. Acta, 8, 397 (1953). (4) Zbkf., 17, 175 (1957).
Clock
15 amp
r1
T
I 5 amp
Figure 1. Schematic diagram of automated digestion system OPERATION
The digestion of plant material is accomplished by weighing 1 gram of ground material into a graduated 50-ml borosilicate glass Taylor Universal Tube (Fisher Scientific Co.). Ten milliliters of an acid mixture containing 7 volumes of concentrated nitric acid and 3 volumes of 72% perchloric acid is dispensed into the tube. The prepared samples are allowed to stand overnight. The tubes are then placed in the aluminum block for digestion. The rheostat of the heating unit is set (dial 6) so that it requires an hour to reach 70 "C and another 4 hours to reach 203 "C. Finally, the temperature-sensing probe is adjusted to maintain 203 "C. To allow automated operation, the clocks had been programmed to run the LabStat and fume hood for 5 and 7 hours, respectively. Under these conditions, the reaction takes place in a controlled manner without foaming and therefore avoids the loss of excessive nitric oxide fumes, allowing more complete oxidation. The temperature increase is sufficiently gradual that oxidation is completed without violent reactions.
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After digestion, the Taylor tubes are removed and allowed to cool. A 5-ml volume of 1N HC1 is added to dissolve any calcium sulfate precipitate that may exist and the volume is then made up to 25 ml. To dissolve the solids, the samples are warmed and aliquots taken for various elemental determinations. After the blocks have cooled for 2 hours, the next digestion can be initiated, allowing for 2 digestions in a 24-hour period, resulting in the preparation of 288 samples per day. CONCLUSIONS
This method has been tested in the Agassiz Soils Laboratory during the last four years. The digestion of over 5000 samples, with varying types of plant materials, allowed comparison with the Kjeldahl digestion method, and no significant differences in elemental determination were found. However, it is important to note that while the digestion system was experimental, using various acid mixtures, one explosion occurred. Although the Taylor tubes were shattered to a distance of 2 meters, the explosion caused no damage to property or operator. It resulted when an insufficient amount of nitric-perchloric acid mixture was used without predigestion. The material consisted of 1 gram of silage corn and 4 cc of acid mixture. After an hour of heating, the material appeared paste-like. Then a spark was seen, immediately followed by
an explosion. The thermometer in the aluminum block recorded 140 “C, a temperature well below the boiling point of perchloric acid (203 “C)which is normally considered the critical temperature for violent reactions. The possible cause of the explosion was the formation of an alcoholic ester, and this, and similar reasons, have been given by Deiss (5) and Sweasey (2). However, use of the same material, with a higher amount of acid mixture, resulted in normal digestion. Although the use of aluminum blocks and a temperature controller are helpful in reducing the dangers of explosion, it is recommended that the following precautions be taken : (1) Predigest the samples overnight at room temperature. (2) Samples containing alcohols, glycerols, or esters should be presoaked with acid mixture for longer periods of time and digested at low initial temperatures. (3) Use a minimum 10 ml of acid mixture to 1 gram of plant material. RECEIVED for review August 16, 1971. Accepted September 15, 1971. Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the named product. ( 5 ) E. Deiss, Z . A n d . Chem., 107, 8 (1936).
Conversational Text Editor for a Laboratory Minicomputer Herbert P. Yule’ Activation Analysis Section, Analytical Chemistry DiGision, National Bureau of Standards, Washington, D.C. 20234 To UTILIZE an available Varian 620i minicomputer for data acquisition and manipulation and for result computation ( I ) for the NBS Activation Analysis Section, considerable programming in the assembly language, DAS, is required. To facilitate the preparation of source program tapes, the debugging of these tapes, and updating of programs, a conversational editing program is reported here for other 620i users who may wish to acquire copies of the program. Modification of source program tapes may be performed under control of a program which permits reading the source tape line by line and allows the operator the options of inserting, modifying, or deleting lines of program text. As each line is completed, it is punched into a new source tape. This procedure is rather cumbersome since modifications must be made in order of appearance in the text, and errors made during source tape editing may necessitate repeating the source tape modification procedure. An alternate method of editing text is to store it in core and modify the text as desired without regard to the order in which modifications are made, and without reading and punching tapes. This “conversational” method is more convenient and much quicker to use. In programming for our system, a complete source program is stored on a rotating drum memory and read by a subroutine which has been appended to the DAS assembler. No source program is punched until a permanent copy is needed. An additional advantage of the program is that it was purposely written to accept commands very similar to those of a widely available commercial conversational text editor ( 2 ) ; anyone familiar with this time sharing 1 Present address, NUS Corporation, 4 Research Place, Rockville, Md. 20850
(1) Nat. Bur. Stand. ( U . S . ) ,Tech. Note 548, Dec. 1970, pp 124-139.
(2) Tymshare Manuals Reference Series, “Editor,” Tyrnshare, Inc., Palo Alto, Calif., July 1969. 430
service can quickly learn to operate the presently described text editor. Frequent use of this service was made during preparation of our text editor. The present program is configured to our hardware: 16 bit, 8192 word memory, 30720 word rotating drum memory, high speed paper tape reader, and model ASR33TZ Teletype. However, modifications to other configurations are possible. Program size, including the drum driver, is about 3300 words (octal), and the pointer table occupies locations 200-206 (octal). Remaining core may be used for text. Text may be entered from the keyboard, tape reader, or drum. As text is entered from the keyboard, it may be edited by deleting characters or lines. Existing text may be appended via the keyboard or tape reader. Text stored in core may be listed, punched, or transferred to the drum. Lines of text may be addressed by line number, and search routines permit quick and ready determination of line numbers. Addressed lines may be modified, deleted, or listed, and text may be inserted before an addressed line. The line address is easily incremented or decremented. Copies of the source program and operating instructions are available from the author. The program is relatively new, and cannot be guaranteed as bug free. Nevertheless, recent experience with the program has shown it to work well and be a great aid in programming the computer.
RECEIVED for review June 28, 1971. Accepted September 22, 1971. In order to specify procedures adequately, it has been necessary to identify commercial materials in this report. In no case does such identification imply recommendation or endorsement of the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
