Anal. Chem. 1982, 5 4 , 49-51
4Q
of the solution, in turn, increases its absorbance. LITERATURE CITED
can add on to the monosubstituted hydroquinone forming a disubstituted hydroquinone (reaction j'). The formation of OH
n
these hydroquinones lowers the amount of quinoids that would otherwise form indophenol. As shown in Figure 1,curve c, dimethyl sulfoxide greatly reduces the absorbance of the 90lution. The mechanism of inhibition of color formation caused by the presence of guanidine and NaNOz in the Berthelot reactions is less readily explained. Possibly the reduction of quinoids by these compounds is involved. Potassium dichromate, manganese sulfate, or sodium sulfate form turbid materials in the Berthelot reactions. The turbidity
(I) Berthelot, MI. Rep. Chlm. Appl. 1859, 1 , 284. (2) Noble, E. D. Anal. Chem. 1955,2 7 , 1413-1416. (3) Scheurer, P. G.;Smith, F. Anal. Chem. 1955,27, 1616-1618. (4) Bolleter, W. T.; Bushman, C. J.; Tldweil, P. W. Anal. Chem. 1961,33, 592-594. (5) Wearne, J. T. Anal. Chem. 1963,35, 327-329. (6) Weichselbaum, T. E.; Hagerty, J. C.; Mark, H. B. Anal. Chem. 1969, 4 1 , 848-850. (7) Weatherburn, M. W. Anal. Chem. 1967,39, 971-974. (8) Fym, R. V. E.; Mllham, P. J. Anal. Chem. 1976,4 8 , 1413-1415. (9) Charney, A. L.; Marbach, E. P. Clin. Chem. (Wlnston Salem, N.C.) 1962,8,130-132. (IO) Horn, D. B.; Squire, C. R. Clin. Chim. Acta 1967, 17, 99-105. (11) Ngo, T. T. h t . J . Blochem. 1975,6 , 863-685. (12) Fleser, L. F.; Fleser, M. "Advanced Organic Chemistry"; Relnhold: New York, 1961;Chapter 26,pp 845-851. (13) Hlkosaka, A. Bull. Chem. SOC.Jpn. 1970,4 3 , 3928-3929. (14) Yamaoka, T.; Nagakura, S. Bull. Chem. SOC. Jpn. 1971, 4 4 , 2971-2975, (15) Moxon, G. ti.; Sllfkin, M. A. J . Chem. Soc., Perkln Trans. 2 1972,9 , 1159-1163. (16) Brand, E.; Brand, F. C. Org. Synfh. 1942,2 2 , 59-60. (17) Brooks, J. U.; Charlton, P. T.; Macey, P. E.; Peak, D. A,; Short, W. F. J . Chem. Soc. 1950, I, 452-459. (18)Windus, W.; Shlldneck, P. R. "Organic Syntheses," Collect Vol. 11; Wlley: New York, 1943;pp 345-347.
RECEIVED for review April 6, 1981. Accepted September 30, 1981.
Wet Ashing of Organic Matter for the Determination of Antimony Slxto BaJo" and Ursula Suter Swlss Federal Institute for Reactor Research, 5303 Wurenlingen, Switzerland
The wet ashing of organic matter for Sb determination was investigated by using u radloactlve tracer technique. Wet ashing with HNO, HCi0, mixtures leads to the formation of insoluble Sb compounds. Ail the Sb remialns in solution when HCi0, H$04 mlxture is ured. The influence of a HNO, the ashing vessel (gialss and Teflon) and the oxidation state of Sb were also studied.
+
+ +
The wet ashing of organic matter for trace element determination is a well-established technique (1-5). Problems have been met, however, in the determination of Sb in organic matter after wet ashing with a HN03 + HC104 mixture; part of the Ski remains fixed on the walls of the glass vessel used for this step. Only one observation of such "losses" has been found in the literature ( 6 ) . The purpose of the ]present investigation was to look for a reliable method for the wet ashing of organic matter for Sb determination. The element, marked with the appropriate y-emitting isotope, waa studied in the "carrier-free" to 100 bg range. The oxidation state after wet ashing was studied by liquid-liquid extraction with zinc diethyldithiocarbamate. EXPERIMENTAL SECTION Reagents. The acids used were HN03 (65%),HC104 (60%), and HzSO4 (95-97%). The 122J"Sb(III)scdutions were prepared from neutron-irradiated high-purity metallic Sb by dissolving it in hot HzS04. The solutions used for the experiments were 1.0 mg Sb(III)/mL in 5 M lJ2S04and 50.10 nig Sb(II1)mL in 1.5 M
HzS04.The "carrier-free" 126Sb(III+ V) solution was in 4 M HC1. The solution of Cr(II1) (10 mg/mL) was prepared by dissolving M HN03. The solution of zinc diethyldiC I ( N O ~ ) ~ . ~in H ~1O thiocarbamate (Zn(DDC)J was 1.7 X lo9 M in CHC13 (7). The organic matter for the wet ashing experiments were "fat-free" milk powder and wheat flour. The dry-ashing residues (700 "C) were '7.5% and 0.42% in weight, respectively. Apparatus. Some wet ashings were made in 100-mL Pyrex conical flasks on a hot plate (380 X 180 mm; 1400 W). Others were carried out in Teflon tubes (outer diameter, 23 mm; height, 170 mm; wall, 1 mm thick) in a temperature programmable aluminum heating block with holes (diameter, 27 mm; depth, 120 mm) type RNS2HR4 (Gebr. Liebisch, Bielefeld, GFR). Glass fiber Whatman GF/B filters (32 mm) in "chimney" holders were wed. The recovery measurements and the liquid-liquid extractions were made as described in ref 8. Counting was done with a well-type NaI (Tl) crystal. Wet-Ashing Experiments. Organic matter (1 g), marked Sb(II1) (100 pL), Cr(II1) solution (100 pL, only in some experimenta), and the acid mixture ("OB + €IC104or "OB + HCIOl + H2S04)were poured into the conical flasks or the Teflon tubes. Cr(II1) was employed as an indicator of total oxidation of the organic matter. Its color changes from green (Cr(II1))to orange @(VI)) near the boiling point (203 "C) of the azeotropic mixture of HC104-Hz0 (72.5% HC104). The conical flasks were placed on the hot plate, the surface temperature of which was raised from 160 to 260 "C over 1-2 h. The Teflon tubes were introduced into the aluminum heating block. The following heating program was used: 1 h from 20 to 150 "C, 7 h at 150 "C, and 1 h from 150 to 230 "C. This was followed by 0.5 h (experiments with "OB + HC104mixture) or 2 h (experiments with HN03 + HCIOl + HzSO4 mixture) at 230
0003-2700/82/0354-0049$01.25/00 1981 American Chemical Society
50
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
‘
t
w
I
0
2
m
’t
5
WHEAT FLOUR
0.5 v)
9 10‘
k--+---f
1
1
/‘i LiU NO ORGANIC MATTER
11
10’
M 20 a
-I (3
92
I
MILK POWDER (g)
Figure 2. Sb adsorption on glass during the wet ashlng of milk powder (no repllcates): Sb added, 100 pg; the same conditions as in Figure
1.
102
Sb ADDED(,ug)
Flgure 1. Sb adsorption on glass during the wet ashing (mean f l standard deviation; number of replicates): acids used, 10 mL of HNO, 10 mL of HCi04; weight of the flnal residue of HCIO,, 6-10 g.
+
OC. The final solutions (unless otherwise stated) were 72.5% HCIOl (experiments with HNOa -k HClO,) or concentrated H2S04 (experiments with HNOa + HC104 + H2S04). They were not taken to dryness. Twenty milliliters of water was added to the solutions from the wet ashing. Colorless, clear solutions were always obtained. They were boiled for 10 min, cooled, filtered, diluted with water to 100 mL, and finally extracted with ZII(DDC)~(30 mL, 2 min shaking time). Sb(II1) is quantitatively extracted under the conditions used, whereas Sb(V) does not extract (9). The activity of aliquots of the organic and the aqueous phases, as well as the webashing vessel and the filter, was measured and compared with the appropriate standard. If Sb was adsorbed onto the walls of the wet-ashing vessel, 20 mL of 1 M NaOH was added and the solution boiled for 10 min. This was followed by cooling and dilution with 12 mL of 5 M H2S04,made up to 100 mL with water and extracted with Zn(DDC)2as above. The activity was measured as described in the preceding paragraph.
RESULTS AND DISCUSSION Wet Ashing with H N 0 3 HC104. Wet ashing ending in a HC104 solution show apparent “losses”of Sb. This losses are of two kinds: (a) adsorption on the walls of the webashing vessel (glass); (b) formation of sparingly soluble Sb compounds (glass and Teflon). Glass Vessels. Sb is adsorbed on the walls of the glass vessels. The results are given in Figure 1. The percentage adsorbed increases with the amount of Sb present in the system and is strongly dependent on the type of organic matter used. In the absence of organic matter, adsorption is still important. The fraction of Sb adsorbed increases linearly with the amount of organic matter (Figure 2). Experiments made in the presence of Cr(II1) showed that if the heating was stopped before the orange color has appeared (formation of Cr(VI)), no Sb was adsorbed on the walls. The amount of Sb adsorbed increases with the heating time after the appearance of Cr(V1) (Figure 3). Adsorption also increases if the water boiling stage is omitted. Gorsuch (10) finds no adsorption of Sb on the glass walls after wet ashing of cocoa with HN03 HCIOI. He interrupts the heating right at the moment he reaches the 72.5% HCIOI. This agrees with the present findings (Figure 3, 0 min). However, this technique is not suitable for routine analysis. The boiling step with NaOH is very effective for the total desorption of Sb from the walls. However, in experiments
+
+
: I
X
(&)/NO
ORGANIC MATTER
1
-
TIME (min)
Flgure 3. Sb adsorption on glass during the wet ashing (no replicates) (in the x axis the heating time after the oxidation of Cr(II1) to Cr(V1)): acids used, as in Figure 1; Sb added, 100 pg. (A) The boiling step w’kh 20 mL of water was omitted.
made without organic matter and 100 pg of Sb, the NaOH failed to desorb the Sb even after two washings. Total desorption was obtained by heating with concentrated H2S04 for some minutes. The percentage of Sb retained in the filter is proportional to that adsorbed on the walls. About 40% was found in an experiment with 10 mL of HN03 + 10 mL of HClO, and 100 pg of Sb and without organic matter. No Sb was found in the filter (51%)by filtering synthetic Sb solutions (Sb(II1) or Sb(V)) with the same HClO, concentration as that coming from the wet ashing. It is therefore clear that the Sb retained corresponds to a solid phase and not to an adsorption phenomena. No Sb was extracted with Zn(DD(& from the solutions coming from the wet ashing or from the desorption with NaOH or H2S04. It is concluded that after wet washing all the Sb is in the Sb(V) state. Heating Sb(II1) in diluted HC104 in an open system yields Sb(V) in the neighborhood of the 72.5% azeotrope (bp 203 “C). If the heating is continued, the antimonic acid could be dehydrated into SbzOs. The adsorption of Sbz05by glass could be similar to that of SiOz after dehydrating silicic acid. Nonadsorbed SbzOs is retained in the filter. Teflon Vessels. No Sb was adsorbed on the walls (50.1%) in the experiments made in the range “carrier free”-100 pg of Sb, irrespective of the presence or absence or organic matter. The wet ashings were made with 10 mL of HN03 + 5 mL of HC10, and the final solution was 2-3 g of 72.5% HClO,. Sb was retained in large amounts on the filter (20-80%) in the experiments with 100 pg of Sb. This holds true with or without organic matter. With “carrier-free” Sb, the per-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982 - 5 1
\
MILK POWDER
X
N O ORGANiC\ MATT ER
-_
,
'xLX 2
2 WELiGTH OF THE H C l O 4 RESIDUE ( g )
3
Flgure 4. Sb retention in 'the filter after wet ashing in Teflon tubes (no replicates): acids used, 10 mL of HN03 + 5 rnL of HCIO,; Sb added, "carrier free".
H2SOk IN THE HCLOb+ H2S04 MIXTURE ( r n l )
centage retained was iamaller. It increases as the weight of the final residue in HC104 decreases (Figure 4). Practically no Sb (50.8%) was found in the filter after heating 10 mL of HCI04, 1 mg of Cr(III), and 100 pg of Sb a t 190 OC, just to produce the orange color of Cr(V1). If the heating was prolongated beyond this point, the percentage of Sb retained was found to be proportional to the corresponding heating time. At 230 "C, the ]percentagesretained were larger. In a previous paper (8),all the Sb (Sb(II1) or Sb(V)) in the range 1-1000 pg was fiound to be in true solution. This was as Sb(V) after heating mixtures of H F +. HC104 to persistent white fumes of HC104,in Teflon beakers. The HC104 temperature remained 5160 O C for a hot plate temperature of 250 "C because of the poor thermal conductivity of Teflon. This temperature was high enough to oxidize Sb(II1) to Sb(V), but the Sb(V) oxides were easily rehydrated after diluting and boiling with water. As in the wet ashing in glass, no Ski was extracted with Zn(DDC)2,showing that all the Sb in solution was in the Sb(V) state. Variable amounb of Sb (up to 30%) were found in the interphase of the organic and aqueous phases. Probably the colloidal Sb going through the filter was coagulated by shaking with the organic phase. Wet Ashing with HN03 + HC104-I- H2SO4. Practically no Sb was adsorbed on glass (
