(2-6) as a basis for the interpretation of spectroscopic data pertaining to vanadium-oxide-based catalysts, and hence as a starting-point for discussion of the behavior of these catalysts in practical applications, it seems of interest to make our findings known.
GIANFRANCO FABBRI PIETROBARALDI
t
w
0
z
Figure 1. Infrared spectra of vanadium-oxides
2 c
fz
d e
Istituto di Chimica Fisica Laboratorio di Chimica Fisica Applicata Universith di Modena 41 100 Modena. Italv RECEIVED for review December 6, 1971. Accepted February 7,1972.
x
I
(2) S. Yoshida and K. Tarama, Bull. Inst. Chem. Res. Kyoto Uniu., 47, 246 (1969). (3) K. Tarama, S. Teranishi, and S. Yoshida, ibid.,46, 185 (1968). (4) G. Blyholder and M. C. Allen, Inorg. Chem., 9, 302 (1970). ( 5 ) Y. Kera, S. Teratani, and K. Hirota, Bull. Chem. SOC.Jup., 40, 2458 (1967). (6) Y. Kera and K. Hirota, J . Phys. Chem., 73, 3973 (1969).
AIDS FOR ANALYTICAL CHEMISTS Determination of Fluoride in Vegetation Using the Specific Ion Electrode R. L. Baker Research Department, Chemetals Division, Diamond Shamrock Chemical Co., 711 Pittman Road, Baltimore, Md.
SOLUBLE FLUORIDES and atmospheres containing fluorine compounds cause damage to and, in severe cases, destruction of plants and plant tissue. Until the present, the Willard and Winter distillation ( I , 2) and modifications thereof and subsequent titration or colorimetric determination have been used extensively for fluoride analyses in vegetation. This procedure is somewhat time-consuming and requires a great deal of apparatus if a number of determinations are to be run at once. I have developed a method for the determination of fluoride combining a NaOH fusion with the technique of standard addition and employing the fluoride specific ion electrode for measurement of the fluoride ion. Baumann (3) and others have described various standard addition techniques using the fluoride ion electrode. The technique by its very nature cancels out the effects caused by differing pH and ionic strengths encountered between standards and actual samples. With these effects eliminated, the observed potential changes can be related directly to concentration rather than ionic activity. The calibration curve, as it were, is prepared in the sample solution. (1) H. H. Willard and 0. B. Winter, IND.ENG.CHEM., ANAL.ED.,5 7 (1933). (2) Association of Official Agricultural Chemists, “Official Methods of Analysis,” 1965 Ed., p 360, sec. 24.029. (3) E. W. Baumann, Anal. Chem. Acta., 42, 127 (1968). 1326
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EXPERIMENTAL
Apparatus. The Orion 9409A Fluoride Electrode with a Leeds & Northrup Calomel reference electrode and a Leeds & Northrup Model 7403 meter were used in this work. A supply of 50-ml plastic beakers is necessary. Reagents. FLUORIDE STANDARD SOLUTION.A 1 OOO-pg/rnl F- solution is prepared by adding 2.211 grams of NaF to water and diluting to 1.00 liter in a volumetric flask. SODIUM HYDROXIDE SOLUTION.Six hundred seventy grams of NaOH pellets are dissolved in water and diluted to 1 liter. A blank consisting of 10.0 ml of this solution should be taken and carried through the entire procedure. If F- is found, a correction can then be made in the final results. I found none in my solutions. Procedure: A 25- to 30-gram sample of vegetation is collected at the sampling site and sealed into a plastic bag. The sample is then refrigerated and analyzed as soon thereafter as possible. In the laboratory the sample is cut into -1-inch lengths and mixed to produce a sample as nearly uniform as possible. Ten grams of the sample is weighed to the nearest 0.01 gram and set to dry at 110 “C for 24 hours. At the end of that time, the sample is reweighed and the dry sample weight calculated. Another portion of the sample (5-10 grams depending on the amount of F- expected) is weighed to the nearest 0.01 gram and transferred to a 50-mI Ni crucible. Ten milliliters (6.7 grams) of NaOH is added to the crucible and the sample is dried for 2 hours at 150 “C. At the end of the drying time, the sample is transferred to a 550 “C muffle furnace and the sample fused for 2 hours. After fusion, the
Table I. Comparison of Results Obtained by the Fusion and AOAC. Methods on Actual Samples Fusion AOAC method, method, Sample No. PPm PPm 11.8 12.5 1
e
7
2 3 6
165.8 868.0
166.2 902.0
a Association of Official Agricultural Chemists recommended procedure (2).
~LM.F' 6
aliquot size, the dry sample weight, etc., to give ppm F- in the sample on a dry weight basis.
4
3
RESULTS AND DISCUSSION
2
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0
1
2
3 GR.
4
5
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NAOH
Figure 1. NaOH strength us. pmole of F- recovered at various F- levels in actual samples sample is cooled and 25 ml of HzO is added to dissolve the melt. Heat is applied. It is sometimes necessary to break up the melt with a stainless steel spatula. After the melt is completely dissolved, it is transferred quantitatively to a 50-ml volumetric flask and diluted to the mark. The solution is then filtered through a Whatman No. 41 paper into a plastic beaker and aliquots of this are taken for analysis. The aliquot is transferred to another plastic beaker; a small amount of H20, a stirring bar, and calibrated pH electrodes are inserted. The pH is adjusted to 5.2 with acetic acid. This should be added drop-wise to prevent loss of sample upon rapid evolution of COz. After the pH adjustment is made, the sides of the beaker are washed down and the F- electrode is inserted. The potential is allowed to equilibrate for 10 minutes with the sample being stirred. A reading is taken after 10 minutes and a known concentration of F- is added. Usually no more than 0.10 ml of 1000 pg/ml F- solution was necessary. (If it takes more F- than this, cut down the size of the original sample aliquot.) Enough F- should be added to give a AE 2 40 mV. The F- in the sample beaker is calculated as follows : X = moles F
where
X
=
AE A
= =
moles F- added (1000 pg F-/ml = 52.6 X mole F/ml) change in mV reading upon addition of F59 mV at 25 "C
A derivation of this can be found in Baumann's paper. Appropriate calculations are made, taking into consideration
The effect of NaOH concentration in the crucible upon Frecovery is shown in Figure 1. The NaOH concentration seems to make little difference in F- recovery below the 2pmole level but above this level a definite trend is observed. Yu and Miller (4) pointed out that some of the F- present in injured plant tissue is there as organic fluoride-citrates and acetates. The data shown in Figure 1 tend to substantiate this. For high F- concentrations a high caustic strength is necessary to break down and hold the F- during sample preparation. Table I shows typical results obtained on 3 actual vegetation samples of differing F- concentration when run by this method and when run by the AOAC standard F- method (2). The only modification to the distillation procedure was that after distillation, the F- in the distillate was determined with the fluoride electrode by standard addition using a total ionic strength buffer as a background. The accuracy and precision of the method is +lOz. Because of the nature of the sample and sampling conditions, wider variations may result. I did not study the effect of fusion temperature or the effect of interfering ions because of the excellent agreement between the distillation and fusion method. The distillation method removes all interferences while the fusion standard addition method effectively cancels them out. CONCLUSIONS This method is less time-consuming than the standard distillation technique in that it does not require constant attention of the analyst and a minimum amount of reaction apparatus is necessary. ACKNOWLEDGMENT Thanks are extended to J. Y. Welsh for his help and comments during this work. RECEIVED for review July 29, 1971. Accepted January 11, 1972. (4) M. Yu and G. W. Miller, Envir. Sci. Technol., 4, 492 (1970).
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