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Anodic Polarography of Cyanide in Foodstuffs T. J. Farrell' Food and Drug Administration. 1521 W. Pico Boulevard, Los Angeles, CA 90015
R. J. Laub2and E. P. Wadswoflh, Jr. Sari Diego State University, San Diego, CA 92162 Generallv included in anv set of laboratorv . ex~eriments . for untlergraduate-le\,eI instrumental analysis is an exercise that in\dves some form of cnthodic voltammetry. n r o ~ ~ i n c mercnry-electrode (DME) polarography of ktificiai-mixtures of cations is often employed, since the corresponding reduction potentials with very many solvent/supportingelectrolyte systems are known accurately and can be reproduced with relatively simple apparatus. Such experiments serve as a good introduction to voltammetric analysis and also are usually well received by students. However, in an effort to increase student interest in electrochemistry as well as to device laboratory work that involves analyses of common items with which all students are generally familiar, we have refined over the years an alternative experiment in which the amount of cyanide present in foodstuffs is determined by anodic polarography. Cyanide, in the form of glucosidic nitrile, is prevalent in varying amounts in well over 1000 plant species representing some 80 families and 300 genera, including some fungi and bacteria (1). Hydrogen cyanide is also released autolytically by several species following tissue damage. (This is believed to serve as a primitive defense mechanism, which is known as cyanogenesis.) Further, a number of such plants serve as foodstuffs (2). Two of these, cassava and immature lima beans, if consumed in excess, can in fact cause cyanotic poisoning in children depending upon where the crop is raised and the level of maturitv a t which it is harvested. For rxample, 11 ib 1101 uncomnion for lmmattlre limn hrunsgrown in the trooics to contain scvrral hundred .. Dpm hvdrol\vahle . CN- ( I ) , whereas we have abtained values as low as 25 ppm for lima beans originating in Canada. In terms of hydrocyanic acid (HCN), the minimal lethal oral dose for humans is 570 wg per kg of body weight, and the minimal lethal concentrations in air are 120 and 200 mg m-3 for periods of exposure of 1 h and 10 min, respectively (3). Hartung has described the toxic effects of cyanides and nitriles in detail, including methods of ingestion, modes of action, treatments, and physiologic responses to various concentration levels (4). From the standmint of assav. or acid hvdrolv.. enzvmatic . . . sis is generally used to release cyanide from organic matter, where the liberated HCN is trapped in aqueous base. Many methods of analysis in addition to the electrochemical procedure reported here then become applicable, including colorimetry, titrimetry, and various atomic spectroscopic techniques as well as gas, liquid, and ion exchange chromatograp h 6~6 ) .
quantitation with a nomograph of diffusion current (id) vs. molar concentratim. The nomograph is constructed I,\, the stt~dentsfrom data ohtained for stnndard solutions of CN-I. The entire experiment can be completed in 3 h. Foodstuff Sample We have routinely employed dried immature lima beans as the sample type (mature lima beans have little or no measurable cyanide), since these materials are readily available in any market, are inexpensive (students are given the option of purchasing their own variety), and the amount of CN- is sufficiently high to permit accurate quantitation. Hydrolysis Apparatus The apparatus used for the acid hydrolysis of CN- in powdered samples is shown inFigure 1;a two-neck LLflaskisequipped with a condenser, heating mantle, and nitrogen purge line. The exit gas (flowrate of -100 mL is passed via ashart length of tubing to a gas-washing tube through a fritted cylindrical glass tip. Hydrolysis Procedure (The following should he carried out in a webentilated fume hood.) A tared sample of approximately 25 g of powdered (Moulinex hand-held coffee grinder) immature lima beans is added to 200 mL of 1:l vlv phosphoric acid solution. The mixture is then refluxed gently for 1%h while being swept withnitrogen. The liberated HCN is collected as CN- in a 40-mL test tube containing 10.0 mL of 1.75 M KOH. Fallowine of the hvdrolvsis steo. .. eomoletion . , , , . 20.0 mL- of snt~lrntrdburic atrd solut~onis pipettcd into the rollretion tube, the frittrd giaas tip uf the purge linr bring rinved in the process. ~
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Standard Solutions (While most are aware of the considerable toxicity of cyanide, laboratory instructors should take advantage of this experiment to remind students of the proper techniques of handling such substances.) Stock solutions of CN- as well as a blank of KOH are prepared beforehand by the instructor. The solutions consist of KCN or NaCN in 1.75 M KOH and should range in concentration
Experimental In the laboratory, the students carry out acid hydrolysis and collection of the cyanide liberated from a powdered sample of baby lima beans, followed by anodic polarographic
' Present address: Division of Food Chemistry and Technology.
Food and Drug Administration. Washington, DC 20204. Author to whom correspondence and reprint requests should be addressed.
Figure 1. Apparatus for the acid hydrolysis of glucosidlc cyanide in powdered foodstuffs: (1) gas rotameter wilh flow-rate adjustment valve; (2) l-L roundbonom flask; (3) heating mantle connected to a variable transformer: (4) lab jack; (5) condenser: (6) flexible connector tube; (7) gas washing tube with frilled-glass cylinder at tip. immersed in 10.0 mL of 1.75 M KOH. Volume 64
Number 7
July 1987
635
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