Water-Containing Jars and Greased Plates for Dustfall Measurements RICHARD
L. POND' and R. R. PAXTON
Department of Chemistry and Chemical Engineering, Stanford
In a comparison of two methods for measuring fall out of dust from the atmosphere, cylindrical glass jars partially filled with distilled water were exposed heside 5 X 10 inch glass plates ooated with Vaseline, on small platforms 18 feet shove an open field. After exposure, the water from the jars was analyzed for total solids, both soluble and insoluble. The grease and dust were removed from the plates, using a petroleum ether-benzene solution, the resultant slurry was filtered through a sintered crucible, and hydrooarbon-insoluble dust was weighed in both an oven-dry and an ignited state. The standard deviation between oonourrently exposed water jars was 12.6%. The comparable value between greased plates was 6.8%. Thus the greased plate procedure gave more reproducible results and, with but one exoeption, slightly higher dustfall rates.
Table I. Reproducibility of Water Jar Method (Ignited dustfall rates) Dustfall, Lb./Aere-Dw Duration. Indi% Date days vidual Av. De". De". Aug. 6 to 11 4.86 0.230 0.225 0.005 2.2 0.220 Aug. 11 to 17 5.72 0.196 0,183 0,013 7.1 0.171 Jan. 30 to Feb. 4 5.04 0,346 0.337 0.009 2.7 0.328 Feb. 22 to 24 2.18 4.171 0,146 0.025 17.1 0.121 Apr. 10 to 13 3.06 0.366 0.343 0,023 6.7 0.320 Standard deviation, 12.%% E*POS"X
The t&al soEds value used to compute the dustfall rete was the sum of the insoluble and soluble residues. This sum, t y ically the i. n to 20 mn. for R week's emomre. wa6 first corrected . .. ~. blank, which generally varied from 2'to 8 mg. Greased Plate Method (2, 3). Rectangles of window glass ~
~~~
~~~
~~
Figure 1. Carrying case for plates
~~
~~
(Figure 1). The plates were set out in rectangular platforms just large enough to hold eight plates. All platforms were equipped with brass wires mounted around their periphery to protect the plates and jars from birds (Figure 2). Exposure platforms were mounted at the top of an 18-foot tower in an open field (Figure 3). Some plates and jars were exposed on s, platform 5 feet high.
left a coating ibout 4 microns thiik
gross area co;ered with grease and dust.
All reported dustfall rates were corrected for the blank and represent net values. The calculated net values are based on the weight of dust collected, length of exposure period, and area of collecting surface (4).
The collected dust was separated from the Vaseline by dissolv-
(Less than !&of dust was s o l u b in this solvent.) The dust was weighed in the Selas crucible in both an oven-dry and an ignited condition. A sample consisted of the dust from four plates having a nominal emosed surfaoe of 200 sauare inches. The blank was the
REPRODUCIRILITY
Water Jar Method. Table I shows five pairs of dustfall rates obtained using water-containing jars, which were exposed cancurrently itt the same level and within 4 feet of each other. The per cent deviation for each pair i s based on the deviation of each jar from the average of the two dustfall rates. Greased Plate Method. Table I1 shows Seven groups of dustfall rates obtained concurrently by the greased plate method. As in Table I, the per cent deviations are based on the average dustfall rate for the group. Deviations ranged from 0.9 to 9.7%,
posure, oven-dry basis). Obtaininn Dust SamDles. Several successive ~ r o u p sof dust-
I
Univenitv, Stanford, Calif.
Present address. California Research Corp., Richmond. Calif,
714
V O L U M E 27, NO. 5, M A Y 1 9 S 5
715
left on the platform marking the edges of the plates. This did not occur in any of the other five tests. A statistical examination of these five sets of data showed that at the 99.9+% confidence level, the greased plate method gives higher dustfall rates. DISCUSSION
Figure 2.
Plate a n d jar platforms
wires guard against bird damage
the standard deviation being 6.8%. These deviations were eompared to those reported in Table I using the statistical F test (5). It was found that the greased plate method has a better precision than the water jar method at the 95% confidence level. Table 111reports data for the same tests as Table 11,but on an oven-dry basis. When oven-dry rather than ignited weights are used, the deviations range from02 to 10.8%, the standard deviation being 7.0%. Simple drying of the residues instead of igniting them resulted in no significant lass of precision.
Greased plates have been widely used to measure dustfall rates, particularly in the western United States, where the long dry summers favor their use. Practical experience has shown that the greased plate method has the following advantages: relatively large samples are obtained, handling in the field is simplified (no liquids to spill), individual dust partielerr can be examined microscopically, there is no evaporation, freezing, or mold growth in the solvent, and contamination of the sample (by bird droppings, etc. j is readily detected. This investigation has confirmed these advantages, and has shown that under the conditions of the test, the greased plate method gives more reproducible dustfall measurements than the
Table 11. Reproducibility of Greased Plate Method 1Ienited dustlall rates) .I Dustfall, Lb./Aere-Day Duration, I"& days vidual AT. De-. 0.317 0.003 0.320 14.00 0.314 0.022 0,248 0,227 12.94 0.205 0.221 0.004 6.15 0.217 0.225 0,280 0.011 4.36 0.270 ~~
EXpOs"re
Date Maroh 26 to Apr. 9 May 4-6 GI9 July28toAug.3 Aug. 6 t o 11
0.9 9.7 1.8
Figure 3.
3.9
0.291
Aug. 11 to 17
5.72
Feb. 22 to 24
2.18
0.336 0.324 0.197 0.190 0.216
0.330
0.006
1.8
0.208
0.011 0.018
5.3 8.7 3.8 9.6 9.2 5.8 3.3
0,008
0.020
0.223
Apr. 10 t o 13
%
De".
3.06
0.386 0.450
0.425
0.439
0.039
0.025
0,014 Stsndkrd deviation, 6.8%
For all three sets of data, the per cent deviation tends to he smaller for longer exposures. The one red exception to this trend is the 12.94-day exposure with greased plrttes-the only one that wzm interrupted by a few day3 of rain. COMPARISON O F METHODS
In six measurements of the dustfall rate greased plates were exposed concurrently and beside water jars. I n most instances the test array included a t least two sets of plates and two water jars. Table IV shows the averaged dustfall rate found by eaoh of these two methods. In five out of six cases the average rate found by the greased plate method was higher than that found using water jars. The weather during the test period of this sixth sample was unusual, in that it included several damp, foggy days. When these plates were demounted from the test platform there was water beneath them, and a brown stain was
Pleta and jars e-ed
Test site
on upper and lower pllstforms
Table 111. Reproducibility of Greased Plate Method (Oven-dry basis) Dustfall. Lb./Aore-Day Individual Av. De".
De".
0.451 0.453
0.452
0.001
0.2
12.94
0.371 0.305
0.338
0.033.
g.8
July 28 to Aug. 3
8.15
0.293 0.276
0.284
0.0085
3.0
An=. 6 to 11
4.86
0,390 0.397
0,393
0.0035
0.9
Aug. 11 t o 17
5.72
0.402
0.0075
1.8
Jan. 30 t o Feb. 4
5.04
0.410 0.396 0.250 0.297
0.279
0.030 0.020 0.004
10.8 7.2 1.5
EXD0S"R
Date
Duration. days
Mar. 26 to Apr. 9
14.0
May 4-6 8-19
0.276
0.295
Feb. 22 t o 24
2.18
Apr. 10 to 13
3.06
0,294 0,289 0.289
0.016
5.7
0.301
0.007 0.012 0.012
2.3 4.0 4.0
0.032
10.6
0.718
0.077 0.043 0.034
10.7 6.0 4.7
0.333 0.641
0.761 0.752
%
Standard deviation, 7.0%
716
ANALYTICAL CHEMISTRY ACKNOWLEDGMENT
Table IV. Comparison of Two Methods Exposure Date July 28 to Aug. 3 Aug. 6 to 11 .4ug. 11 to 17 Jan. 30 to Feb. 4 Feb. 22 to 24 Apr. 10 to 13
(Ignited weights) Av. Dustfall, Lb./Acre-Day Duration, Water Greased days Jar plate 0.221 6.15 0.217 0.280 4.86 0.225 5.72 0.183 0.330 (0.181) 5.04 0.337 2.18 0.148 0.208 0.425 3.06 0.343
The authors wish to thank the Dow Chemical Co. for the financial support which made this work possible. LITERATURE CITED
(1) Larson, G. P., and coworkers, 4ir Pollution Control District,
Los Angeles County, “Test Procedures and Methods in Air Pollution Control,” pp. 53-60. (2) Mitchell, J. P., J.Ind.Eng. Chem., 6 , 4 5 4 ff. (1914). (3) Paxton, R. R., Rock Products, 54, No. 2, 114-18; No. 6, 127 ff. (1951).
Pond, R. L., “Development of Coated Plate Method for the Measurement of Dustfall Rate” h1.S. thesis in chemical engineering, Stanford University, 1954. (5) Youden, W. J. “Statistical Methods for Chemists,” pp. 20-3, Wiley, New York, 1951. (4)
water-jar method. Finally, in the absence of rain or damp fog, the greased plate is consistently more efficient in collecting and holding the dust that falls from the air. For measurement of dustfall over fairly short periods of rain-free weather, the greased plate method has definite advantages over the water-jar method. END
RECEIVED for review June 28, 1961. Acrepted Fehruary 16, 1955.
OF SYMPOSIUM
Other papers in this symposium are published in the M a y issue of lndurtrial and Engineering Chemistry
Principles of Precision Colorimetry A General Approach to Photoelectric Spectrophotometry CHARLES N. REILLEY and CRAYTON M. CRAWFORD University o f North Carolina, Chapel Hill, N. C. This paper is an inquiry into the effect of slit w-idth, sensitivity, and dark-current knob settings on spectrophotometric precision, with a view toward the development of improved high-precision methods. An expression for the relative error is derived, and the conditions for its minimization are discussed under assumptions sufficiently general both to include and extend previous techniques. Four methods are distinguished, of which two are new. One of the new methods gives the best precision obtainable, using two reference solutions and other conditions selected to make this statement true. The selection procedure is described. The other new method is applicable to trace analysis and represents a compromise when solutions sufficiently concentrated to permit optimum conditions are not available. Both methods promise in their respective applications substantial improvement in precision over former methods at small extra cost in time and effort.
T
HE analyst using a photoelectric spectrophotometer has several instrumental controls at his disposal, which he may adjust as his needs dictate. It has long been recognized that best results are obtained with the wave-length knob set at an absorption maximum, assuming no interferences. The slit-width control setting is not critical so long as the light is sufficiently monochromatic that no appreciable apparent negative deviation from Beer’s law occurs. The sensitivity and dark-current knobs are primarily adjustments to fit the amplified photocell output to the limits of the scale on which this output is to be measured. The present paper is an inquiry into what effect the slit-width, sensitivity, and dark-current settings have upon the precision of concentration measurement under various modes of operation. As a result of this inquiry new precision methods are developed and the spheres of usefulness of both new and old methods are made clearer. CLASSIFICATION OF METHODS
Photoelectric methods of spectrophotometry usually have in common the adjustment of the instrument to read first 0 and
then 100, under specified conditions. Bccording to the choice of these conditions, four methods of operation may be distinguished: Previously Described Methods. I. “ORDINARY”METHOD. The instrument is set to read 0 with the photocell in darkness and to read 100 when exposed to light which has passed through pure solvent. 11. “TRANSJ.fITTANCE-RATI0” METHOD. The instrument is set to read 0 with the photocell in darkness and to read 100 when exposed to light which has passed through a reference solution somewhat more dilute than the sample. Proposed Methods. 111. “TRACE AKALYSIS”METHOD.The instrument is set to read 100 when exposed to light which has passed through pure solvent and to read 0 when exposed to light which has passed through a reference solution somewhat more concentrated than the sample. IV. “GENERAL” METHOD.This is potentially the most precise of all. The instrument is set for both 0 and 100 using reference solutions. INSTRUMENT READIRG-CONCENTRATION RELATIONSHIP
For any of these four methods an experimental calibration curve of reading us. concentration must be determined, because a given system may not conform to theory, and in any case the constants involved are not otherwise known. The theoretical curves are of interest, however, as predictions of, and first approximations to, actual behavior. The relation to be expected between instrument reading and concentration of absorbent when both the 0 reference and the 100 reference are allowed to vary, which is the relation for Method IV, will therefore be derived and will of course include as special cases the well-known relations of Methods I and I1 and also that of the proposed Method 111. Several ordinarily well justified assumptions are necessary. First it is assumed that the instrument reading is a linear function of the light issuing from the sample. This means that one may write an equation of the form:
R =kI = hk,I
+ k‘ + k’
(1)
where R is the instrument dial reading, k is the sensitivity, I
