Absorptivities for Infrared Determination of Peroxyacyl Nitrates 8
SIR: The peroxyacyl nitrates are a series of unstable compounds that are formed by photochemical reactions in polluted air. They were first discovered during laboratory studies of these reactions using a specially built, long path infrared spectrophotometer (6). Later they were detected in polluted air (8) and finally the first three members of the series were purified by preparative gas chromatography and characterized by their infrared and mass spectra (3-5). The discovery that these compounds are eye irritating and extremely toxic to plants (4, 6) has prompted great interest in their physiological properties. A method of analysis of these compounds based on gas chromatography with electron capture detection has recently been described (1). Concentrations below 0.01 p.p.m. (v./v.) are detectable in samples of 1or 2-ml. volume. An infrared spectrum of the vapor (in N2, a t 10-cm. path) of one of the first purified samples of peroxyacetyl nitrate (PAN) was published several years ago (4, 6). Since that time additional studies of the absorptivity of this compound as well
Table I.
as of the next two members of the homologous series, peroxypropionyl nitrate (PPN) and peroxybutyryl nitrate (PBN), have been made. It has become apparent that the absorptivities given in the published PAN spectrum are too low by a factor of about two. This is probably because of some error in measurement of the concentration put into the cell since all absorptivities were low b y the same factor. The new measurements are the subject of this paper (Table I). Because these compounds are highly explosive, the determination of their absorptivities presented special problems. Two independent methods were eventually developed. In the first, a pressure-volume technique (PV), a glass bulb of known volume on a vacuum line was filled with a measured pressure of PAN vapor. This was brought to one atmosphere with nitrogen (slowly; rapid compression can cause decomposition or even explosion) and then swept into the long path cell (volume, 640 liters; filled with air at atmospheric pressure). This produced a concentration of a few parts per million
Infrared Absorptivities of the Peroxyacyl Nitrates
(As vapor in one atmosphere of air) a X 10’ (p.p.m.-’m-l) 0
II
PAN CHsCOONOz (Microns) 12.61 120 Meters, 5 Detn. Mean 10.3 by pv Mean dev. 0.5 10.1 10 Cm. using conc. from 120 m.
10.74 1.8
8.60 13.9 0.4 14.3
7.68 11.4 0.2 11.2
7.68
5.76 23.6
5.44 10.0
0
II
PPN CHsCHzCOON02 (Microns) 120 Meters, 5 detn. Mean by PV Mean dev. 120 Meters, liquid, 2 detn. 10-Cm.cell
12.58 11.0 0.7 10.5
9.58 4.9
(8.60)’
4.8
(1.1) 10.1
10.56
5.3
11.7
9.63 5.8 0.5
7.7
7.1
14.8
5.76
23.0
5.44
6.8
0
II
P B N CHaCHzCHpCOONOz 120 Meters, liquid, 3 detn. 10-Cm.cell a
(Microns) 12.58 Mean 11.7 Meandev. 0.3 11.7*
No peak.
* Taken equal to the absorptivity found at 120 meters.
928
ANALYTICAL CHEMISTRY
5.75 34.2
5.44 8.9
which gave sufficient absorption for measurement a t a path length of 120 meters. Early results were erratic, perhaps because of traces of moisture in the PAN. This technique did eventually yield reproducible results for the first two members of the series, PAN and PPN. The vapor pressure of PBN is too low to apply this technique with available equipment. The second technique consisted of filling a lambda pipet (5 HI.) with the liquid, then allowing this to vaporize into the circulating system of the long path cell. This also gave a concentration of about 2 p.p.m. and was quite reproducible. The densities of the liquids were estimated to be 1.2 grams per ml. since they sink in water but float on concentrated potassium iodide solution. Molecular weights were taken according to the formulas given in Table I. This technique was successful for PPN and PBN but a single attempt to apply it to PAN failed because the sample exploded while being vaporized into the cell. For PPN both techniques were applied and satisfactory agreement was obtained. At a path length of 120 meters some of the major absorption bands are obscured by water vapor and carbon dioxide absorptions. To obtain information for these regions, spectra were run in 10-cm. gas cells. These do not constitute independent calibrations since the concentrations were determined from long path cell data. A mixture of PAN in nitrogen was found to contain 7230 p.p.m. by diluting 11% or 200-ml. samples into the long path cell. A 10-cm. gas cell was then flushed with this mixture at atmospheric pressure and the complete spectrum (3 to 15 microns) recorded using a Perkin-Elmer double grating spectrophotometer a t a slit program of 10.00. The absorptivities were in good agreement with those obtained a t 120 meters. With the 120-meter path wide slits (350 microns a t 7.68 microns and 8.60 microns, 800 microns at 12.61 microns) were needed to pass enough energy through the single-beam doublepass (NaCl prism) Perkin-Elmer monochromator. This provided much lower resolution than the double grating spectrophotometer. Since the absorptivities were the same it is concluded that the bands are broad compared to the spectral slit width even at the low resolution and that at one
ACKNOWLEDGMENT
atmosphere total pressure in air or nitrogen, PAN does not depart seriously from the Beer-Lambert relationship. A portion of the 10-cm. cell contents was removed, the remainder pressurized to one atmosphere, and the spectrum rerun to reduce the absorbance of the carbonyl band. Absorbance a t each of the six major bands h,&ddecreased by a factor of five. The absorptivities of PPN and PBN measured in the 10-cm. cell were calculated assuming the value at 12.58 microns to be the same as that found at 120-meter path. Because of its lower vapor pressure the P13N spectrum was weaker than the others and therefore somewhat less accuraiie.
John Locke of Scott Research Laboratories and Eugene Cardiff of the University of California at Riverside assisted with the preparation, purification, and measurement of many of these sa ples.
8
LITERATURE CITED
(1) Darley, E. F., Kettner, K. A., SteDhens, E. R., ANAL.CHEM.35, 589-91 (1963 j. (2) Scott, W. E., Stephens, E. R., Hanst, P. L., Doerr, R. C., Proc. A P l 37 111, 171-83 (1957). (3) Stephens, E. R., “Symposium on
Chemical Reactions in the Lower and Upper Atmosphere;” Interscience, New
York. 1961. (4). Stephens, E. R., Darley, E. F.,
Taylor, 0. C., Scott, W. E., Intern. J . Air Water Pollution 4 (1/2),79-100 (1961). (5) Stephens, E. R., Darley, E. F., Taylor, 0. C., Scott, W. E., Proc. A P l 40 111, 325-38 (1960). (6) Stephens, E. R., Hanst, P. L., Doerr, R. C., Scott, W. E., Znd. Eng. Chern. 48, 1498 (1956).
EDGAR R. STEPHENS Scott Research Laboratories, Inc. P. 0. Box 2416 San Bernardino, Calif. Air Pollution Research Center University of California Riverside, Calif.’ 1 Address to which requests for reprints should be mailed. This work supported in part by the Air and Water Conservation Committee of the American Petroleum Institute.
Use of Glass Reference Electrode in Potentiometric Titration of Halides with Silver Nitrate in LiN03-KN03 Eutectic Melt SIR: The use of glass reference electrode in fused salts has been demonstrated ( 2 ) . It has also been shown that the silver halides are sparsely soluble in fused alkali metal nitrates (1, 4). Therefore, during the potentiometric titrations of halides with silver ion, sharp end points are observed on account of precipitation reactions. The purpose of this investigation was to show the use of glass reference electrode in the potentiometric determination of a mixture of halides in a molten salt solution such as the Li-K nitrate eutectic.
tion a s well as reagents were described previously (4). Procedure. Essentially the same procedure was used as previously reFor the potentiometric ported (4). determination of halides, the procedure consisted of dissolving appropriate single or mixed halides in the melt. The titration with silver nitrate was then carried out. The e.m.f. of the cell was measured after each addition of AgN03. I n an entirely analogous manner the silver ion added first as silver nitrate was titrated with various halides. All measurements reported in this study were carried out at 165.0’ C.
EXPERIMENTAL
RESULTS AND DISCUSSION
Apparatus and Reagents. The electrolytic cell consisted of a 250-ml. borosilicate glass beaker. Other details of the cell and electrode prepara-
A typical titration curve for a halide mixture in molten nitrate solution is shown in Figure 1. Sharp potential changes a t the equivalence point were
Table 1. Typical Titration Results of Single Halides in Li-K Nitrate Melt
Taken, Halide CI Br I
Found, mg.
me. 137.8 97.2 117.9
136.4 96.6 118.5
observed in all cases. The order of the observed change is I->Br->Cl-. Examples of some experimental results obtained by the use of the present method for melts containing single and mixed halides are given in Table I and Table 11, respectively. The results presented in Tables I and I1 are averages of at least three separate titrations. In cases of sinele halides the relative error as calculated from e.m.f. data is 0.8%. For mixed halides in the melt the relative error is 1%. In carrying out the titration in the reverse manner, Ag(1) was determined in a series of nine runs (three for each halide) a t various initial silver ion concentrations. The relative error is less than 1% over a concentration range of 50 mg. to 1.9 grams per 1000 grams of melt. This is not surprising, however, since the silver electrode has been shown to be reversible in these melts (1, 4). As is well known in aqueous solution (3).if an anion forms a snarselv soluble compound with the metal, such as the silver halides in the present case, the metal can serve as an anion indicator electrode also. That this is so may be
-
,500
b Figure 1. Titration of a halide mixture with silver nitrate in Li-K nitrate melt First inflection indicotes equivalence point for I-, 2nd Br; and 3rd CI-
400
->
300
5
200
LL
I
100
W
0
-100 -200[
0
I
I
100
200
I 300
I 400
Weight of AgNO, added. rng
, 500
I
VOL. 36, NO. 4, APRIL 1964
929
