1821
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 500 0 - 10% I ( O H - 4 % P Y R O G A L L O L 0 - 20%KOH-4%
400
b-
20%KOH-8%
PYROGALLOL PYROGALLOL
0 - 10% K O H - 8 % P Y R O G A L L O L
“0
200
400
600
M I C R O G R A M S OF OXYGEN
Figure 4.
Effect of Reagent Concentration
reported. Specific conditions prevailing at each calibration are shown on the figure. DETERMINATION OF OPTIMUM CONCENTRATION OF REAGENTS
Before final calibration, a series of runs was made to determine desirable reagent concentrations. The effects of various concentrations of both the alkali hydroxide and the pyrogallic acid are shown in Figure 4. Excess pyrogallol results in nonlinearity a t higher concentrations of oxygen. Runs were also made with no hydroxide and with sodium carbonate; sensitivity was low,
however. The 4% pyrogallol in 10% potassium hydroxide has proved to be reliable and easily handled. Various stopcock greases were used with no apparent affect upon results. Care must be taken to prevent grease being carried into the reagents, however, as the cloudy fiolutions thus obtained will give spuriously high results. -4 cellulose-base grease was found t o be most satisfactory because of the ease of cleaning the apparatus following a determination. The usual procedure was to place the apparatus, after washing with water, in a muffle furnace and heat to 550’ C. for a short time. Various solvents also effect good cleansing with this type of grease. The bulk of the work a t this laboratory was performed on nitrogen, but oxygen in helium, argon, and hydrogen has also been determined by this method. While the oxygen content of tank carbon dioxide \vas not determined, runs involving the pure gas demonstrated that no spurious colors viere introduced by the presence of this gas. Because of their reaction xith hydroxide, a revision of sampling technique n-ould be required when large percentages of carbon dioxide or any acid gas were present in the sample. I t seems reasonable that a sybtem of “stripping” such as that proposed by Deinum and Dam ( 2 ) and used more recently by Brooks et al. ( 1 ) could be applied to this procedure, making it useful for the determination of dissolved as well as gaseous oxygen. I t is further felt that use of the liquid in question as the solvent for the reagents, or the use of water as a “stripping” agent, might also give satisfactory results, though such an application would certainly require further investigation. LITERATURE CITED (1) Brooks, F. R., Dimbat, M.,Treseder, R. S.,and Lykken, L., A l n a ~ .CHEM., 24, 520 (1952). (2) Deinum, H. W., and Dam, J. W., Anal. Chim. Acta, 3 , 3 5 3 (1949). (3) Hand, P. G. T., J . Chem. Soc.. 123, 2573 (1923). (4) hfellan, I., “Organic Reagents in Inorganic Analysis,” Philadelphia, Pa., Blakiston Co., 1941. (5) Shaw, J. A , IXD.ENG.CHEX.,ANAL.ED., 14, 891 (1942) (6) Tuve, R. L., U. S. Patent 2,440,315 (1948). (7) IVillst%tter, R., and Heiss, H., Ann., 433, 17 (1923). RECEIVED for reriew June 2 , 1952. .4ccepted -4ugust 16, 1952.
Determination of Arsenic in Fruits and Vegetables J. C. BARTLET, MARGARET WOOD, AND R. A. CHAPMAN Food and Drug Laboratories, Department of National Health and Welfare, Ottawa, Canada
M
ANY procedures have been developed for the determination of small quantities of arsenic. The Gutzeit method ( 1 ) has
been in use for many years but its accuracy is limited and reproducibility of stains depends on a number of factors ( 2 , s ) . For microgram quantities of arsenic, Cassil and Wichman (4)have employed a microtitration procedure after distillation of arsine. Arsenic has also been isolated by distillation as arsine (11, 1 4 ) , the trichloride ( 5 , Is), and the pentavalent bromide (9, IO), and by carbon tetrachloride extraction as arsenic xanthate (8). Following isolation arsenic was estimated colorimetrically as the molybdenum blue complex, Hydrazine sulfate has been recommended by a number of investigators (9, 12, 1 4 ) as the reducing agent in the molybdenum blue procedure, although stannous chloride ( 1 6 ) and reduced molybdate ( 1 7 ) have also been used. Preliminary experiments with the bromide distillation procedure (9) gave promising results. However, in the course of the study several points of technique were found to be critical, and these factors were investigated more intensively. The following procedure, which is essentially that of Magnuson and Watson ( 9 ) , was finally adopted.
APPARATUS
Either a Cenco 250-natt variable heatcr or a Precision Scientific 750-watt Ful-Kontrol heater equipped with pipe clay rings or an asbestos ring was employed. The area of the bottom of the flask exposed to the heat was approximately 4.5 cm. in diameter. The distillation apparatus as described by Magnuson and Watson (9) was used. In all, three traps were employed; trap 1 was exactly as described by Magnuson and Watson ( 9 ) , while traps 2 and 3 had no baffle in the lower chamber. All the traps were purchased from the Scientific Glass Apparatus Co. REAGENTS
Potassium Bromide Solution, 30% u~eight/volume. Dissolve 30 grams of reagent grade potassium bromide in water without heating. Filter as soon as it is dissolved through a coarse filter paper and dilute to 100 ml. Molybdenum Color Reagent. A4dd 10 ml. of concentrated sulfuric acid to 40 ml. of water, cool, dissolve in this solution 1.0 gram of ammonium molybdate, and dilute to 100 ml. Hydrazine Sulfate Solution, 0.05% weight/volume in water. Standard Pentavalent Arsenic Solution. Dissolve 1.5 grams of arsenic pentoxide in 100 ml. of -V sodium hydroxide, add 600 ml. of distilled water, neutralize with 100 ml. of 1 N hydrochloric
ANALYTICAL CHEMISTRY
1822 Although organic insecticides have largely replaced arsenical sprays in the fruit industry, appreciable amounts of lead arsenate are still used for apples and a few other fruits. Therefore, a sensitive and accurate method is required for the determination of arsenic on these products. The bromide distillation method of Magnuson and Watson has been adapted for the determination of arsenic spray residues in fruits and vegetables. The 4-minute distillation time of the original method was found to be i n s u 5 cient under certain conditions, and the procedure is acid, and dilute to 1000 ml. Place three 25-ml. aliquots in glasssto pered flasks, and add 25 ml. of concentrated hydrochloric acid a n 8 5 0 ml. of 10% potassium iodide solution t o each. Make simultaneous blank determinations in triplicate. Allow the flasks to stand in the dark for 2 hours, then titrate the free iodine with 0.1 N sodium thiosulfate. One milliliter of 0.1 AV sodium thiosulfate is equivalent to 3.75 mg. of arsenic. Make appropriate dilutions from the stock solution to give a solution containing approximately 5 micrograms of arsenic per ml. Cupferron Solution, 57, weight/volume aqueous solution. Prepare fresh daily. PROCEDURE FOR ARSENIC DISTILLATION AND DETERMINATION
Digest a 5- to 50-gram sample with nitric acid and a known amount of sulfuric acid until all organic matter is destroyed, being careful to maintain an excess of nitric acid until the digestion is complete. Perchloric acid may be used to speed up the destruction of the organic material, but if the sample contains vegetable matter, the solution should be fumed until no precipitate of potassium perchlorate forms on cooling. Transfer the sample or an aliquot containing not more than 30 micrograms of arsenic and 5 ml. of sulfuric acid to a twonecked EO-ml. distilling flask. Add a few glass beads and sufficient concentrated sulfuric acid to bring the total volume of sulfuric acid to 5 ml. Heat until strong sulfuric acid fumes appear. Cool and add 5.0 ml. of water to the sample solution. Add 2.0 ml. of potassium bromide solution to the dropping funnel which is placed in the outer neck of the flask. Put the trap in the other neck and place on a hot electric heater. When water has begun to condense in the trap, add 3.0 ml. of water through the top of the trap and attach the condenser. Blow in the potassium bromide solution from the dropping funnel, followed by 2 ml. of water as a rinse. Continue the distillation until thin fumes appear in the lower chamber of the trap. These fumes should appear between 4 and 9 minutes after the addition of the potassium bromide. Disconnect the apparatus and transfer the distillate through the top of the trap t o a 28-1111. beaker. Transfer from the beaker to a 25-ml. volumetric flask. Rinse the trap and the beaker with two or three 2-ml. portions of water and combine with the distillate. Add 2.0 ml. of ammonium molybdate solution and 2.0 ml. of hydrazine sulfate solution, mix thoroughly, and dilute to the mark. Heat for 10 minutes in a boiling water bath and cool. Measure the absorbance in a suitable photoelectric colorimeter or spectrophotometer against distilled water. Carry a blank on all reagents through the entire procedure. The amount of arsenic in this blank should be determined from the calibration curve and this amount subtracted from the arsenic recovered from samples. Calibration Curve. Add aliquots of the standard arsenic solution to 25-ml. volumetric flasks to cover the range of 5 to 30 micrograms. A reagent blank should also be carried through the rocedure. Bdd 3 ml. of 1.0 AV hydrochloric acid to each flask. bevelop the color as described for the distilled samples and read the absorbance against distilled water. The wave length of maximum absorption of the molybdenum blue color is reported as 840 mp (15), although wave lengths as low as 660 mp have been used (5). However, during this investigation a red-sensitive phototube was not available for the Beckman Model B spectrophotometer which was employed for all absorbance measurements. Therefore a 50-mm. cell and a wave length of 740 mp was used. Under these conditions the maximum sensitivity of the method was not utilized. If a Beckman Model DU epectrophotometer or similar instrument is
modified by distilling to the appearance of thin fumes. Tin in canned fruits interfered with the determination by distilling and precipitating as metastannic acid which adsorbed a portion of the arsenic. This interference can be overcome by extracting the tin with cupferron and chloroform before distillation. The method has been applied to canned fruit which contained appreciable amounts of tin, and to fresh fruits and vegetables. The arsenic content of representative samples of fruits and vegetables is given.
available, satisfactory sensitivity can be obtained employing a 10-mm. cell a t 840 mp. JVith the Beckman Model B the curve deviated slightly from Beer’s lam, probably owing to stray light effects and the fact that the blue-sensitive phototube was used outside its normally useful wavelength range. However, the calibration curve obtained was reproducible. PROCEDURE IN PRESENCE OF TIN
Transfer an aliquot of the digested sample solution to a 5O-ml. beaker and evaporate to sulfuric acid fumes. Cool, transfer the sample to a 60-ml. separatory funnel, and dilute so that the final acidity is approximately 10% sulfuric acid by volume. Cool and add 2 ml. of the cupferron solution and 10 ml. of chloroform. Shake vigorously for 2 minutes. hlloiv the chloroform to settle, drain off the chloroform immediately, and discard. Add a further 10 ml. of chloroform, shake as before, and discard. Transfer the aqueous layer through the top of the separatory funnel into a 2-necked distillation flask. Add sufficient sulfuric acid to bring the total volume of sulfuric acid to 5 ml. and evaporate to sulfuric acid fumes. Carry out the distillation and arsenic determination as described in the preceding section. DISCUSSION AND RESULTS
Effect of Distillation Conditions.
Magnuson and Watson
(9) recommend a 4-minute distillation. When this procedure was
followed, it was found that the amount of water added was critical. Table I s h o w representative recoveries obtained when 19.3 micrograms of arsenic were distilled in the presence of varying amounts of water with trap 1. A Ful-Kontrol heater with a dial graduated from 0 to 100 was used to obtain these data. The highest consistent recoveries were obtained when 4.0 ml. of water were added before the start of the distillation rather than the 5 ml. specified in the original procedure. However, low recoveries were obtained with trap 2 with amounts of arsenic over 13 micrograms unless the heater setting was advanced and 5 or 6 ml. of water were added. Under these conditions about 4.5 mrq. of acid were
Table I.
Effect of Amount of Water on Recovery of Arfienic
Water in Distillation Flask, MI. 2
3
4 5 6
a
Arsenic Recoverya, % Heater a t 78 Heater a t 70 80.5 83.0 97.0 88.0 99.0 97.3
7 10 Distillation time 4 minutes, trap 1.
85.5 72.5 20 5
90.6 88.0 86.5 36
Table 11. Effect of Amount of Water on Acidity of Distillates Water in Distillation Flask, bI1. 5 6
a
Aciditya, Meq. Heater a t 70 Heater a t 82
7 8 9 Distillation time 4 minutes, trap 2 .
3.3 0.6
4.2 3.2 3.2 2.8 0.1
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
1823
dietilled, and when the modified molybdate reagent of Maren (IO) was used, low results were obtained owing to high activity. Magnuson and Watson (9) reported that 1.0 to 5.0 meq. of distilled acid was the permissible range for color development, while Maren ( I O ) stated that a t least 2.1 meq. was necessary. Magnuson and Watson (9) found that between 2.5 ana 4.0 meq. of hydrobromic acid were distilled. Maren ( I O ) , on the other hand, reported widely varying results with the acidity sometimes as low as 1.3 meq. The results of an experiment to determine the effect of the volume of water on the acid distilled are shown in Table 11. These data indicate that the acidity is dependent on the amount of water added and the temperature of the heater if the distillation is stopped at the end of 4 minutes. I t is probable that the low acidities reported by Maren ( I O ) were due to either an excess of water or insufficient heat.
Table III.
Recovery of Arsenic with Varying Amounts of Watera
\Vat e r in Distillation Flask, 311. 5 6 7 8 9 a
Time of Distillation hIin. Sec.
4 3
5 6 8
Acidity , Meq.
Recovery,
4.25 4.4 4.28 4.3 4.18
100.5 99.1 99.6 98.5 99.1
18 0 28 8 4
70
in this case it coprecipitated a portion of the arsenic. Recoveries of arsenic distilled in the presence of varying amounts of tin are shown in Table V. As may be seen, as little as 10 micrograms of tin interferes seriously with the determination of arsenic. At this level the precipitate of metastannic acid was barely visible in the distillate. Furman et al. ( 6 ) have reported that, after precipitation with cupferron (ammonium salt of phenylnitrosohydroxplamine), tin cupferrate may be extracted quantitatively from an acid solution with chloroform, while arsenic is not removed in either the tri- or pentavalent state. Table VI shows the efficiency of removal of the tin by the procedure as outlined. When the acidity was between 2.5% v./v. sulfuric acid and 15% v./v. the extraction resulted in almost quantitative removal of the tin in every case. Table VI1 gives the recoveries obtained when acidic solutions containing tin and arsenic were extracted and the arsenic was determined by distillation. I t is evident from these data that satisfactory recoveries can be obtained by this procedure.
Recovery of 19.3 Micrograms of Arsenic Distilled in Presence of Tin
Table V.
Sn Present,
Distilled to fumes, trap 2 , heater a t 82. ~
Table IV. Heater Setting 70
~
Recovery of -4rsenic with Varying Heater Settings
Water in Distillation Flask, bI1. Trap 2
1 2 3
81'
~~
1 2
3
Timen 3Iin. See.
bcidity, Meq.
Recovery, 98.5 100.5 100.0 100.7
98.7 98.5 99.3
5 8 8 8
6 16 10 11
15 19 16
3.3 3.4 3.3 3.35
8 8 8
14 6
26 8 49
3.8 4.3 4.0
7
11
%
y
Apparent As Recovered. y
10 50 100 200 500
154,. 20 , 1 37 . 66 7.5, 6.6=
8.5,10.O5 3 5a
a Intensity of blue color decreased with increasing amounts of tin but turbidity which developed resulted in a higher absorbance.
Table VI.
Efficiency of Extraction of Tin with Cupferron Sn in Acid Layera,
H2S04,
Sn Added, y
70 %,./Vu
Y
