these tunnels, perhaps f o d in some w a y by the initiai mtrapment of the liquid phtinun? soivent, a d as tiny “‘waw channels” 01). The Lght beam entering one of these wa?e ctaanaeh will undergo a n extremeiy high number of reflections because of tire small dimension in traveling down the tunnel. Thus, the overall effective number of reflections is very large c o r n p a d to those calculated from the gross geonet~ of the cr)stal. The average width of these tunnels was a b u t 1 x 10-5 cm. It should be noted ai this pomt that t k cspcobabiy do not contain any of the eiectmlysis solution This concept is further substanriatea cv the fact tha: IKS ekctrodes made in the exact same nadnn: exceot that the fiim was applied ~ 7 t h mi! s t r c . h l ~ v e ’ : dimfarto the light path were essentially opaque wth respect tu the solution ohase. At present this theor?. is onlv speculaxion. however, detailed investigations of tiie film construction and optical theory of ti= wave channels is being mvestigaated and w3l be reported in the near future.
difhsion theory well. Further investigauon of other metal surfacese methods of film deposition, characteristic: of ’.:le deposited layer, and optical characteristics of the film aie fi, progress and will be reported at an early date. Also. mtd film deposits on quartz plates are being studied in the ultmviolet region and those on AgCi and KRS-5are being studied in the IR region. Preliminary experiments have shown tbci pallad~umfilms on KRS-5 crystals have a much broader IR wndow (5 to I4 microns) than conducting germanium c r y ta!s (2 to 10 microns) used in previous in situ IRS electlolvslstudies ACKNOWLEDUMKW
The authors thank D. I. Meyer of the Physics Department of The University of Michigan for hs helpfu! suggctions, and John Rosen for taking the electron photoilli’crographs
B. STANLCY PO?+. JAhm s. M n m h LEohi 0. wM!XROhi H A R R Y 13. MARK. jk?.
coNcLLIsToh.‘s
It is felt that t k e results show that it is possible to prodtice suitab!e thin metal surfaces on glass that are optically traxparent as far as 1RS is concerned and that enable onz to study the IRS ektrochemical techniq=e and eiectrode reaction
mechanisms without serious ctmnges of the eieztrotk itself CP the absorbance background during electrolysis. This conciusion is supported by the tact that the data in the vsibie region diffusion of the spectrum were reproducible and fiiied f) D. 4. Meyer, Physics lkpt. University of Michigan. i>F:vate communhon, 1W
Bpartment of Chemistry The University of Michigan Ann Arbor, hilkh
RECEIVED for review J d y 27, 196s. Accepted December ‘2, 1566. Research supported by a grant from The U . S. Anrig Research Oftice, Durham, Contract No. DA-31-124-ARO D-284 Divisior of Fuel Ghemistrj, 153d Natiocal MeelI%, ACS, Aprii 1%7, Miami Beach, Fia
-
Modification Po increase Sensitivity of Barium Chforaniialte Method for Sulfate,
I____ _ _ I
--
0.7 *
0.500r
06.
2
0.5 .
O
0 a
6
7 . L
e
n
04.
a
2a 0 3 02-0
O't
L
0
I
S
3
2
4
5
I
L
8
PH
Figure 1. Absorbance of aqueous chloranilic acid as a function of pH 6; X 10-4Mchloranilic acid
ilate, the color is intensified and the sensitivity is thereby improved. In the procedure described below a very concentrated phosphate buffer is used so that only a few drops are required to adjust the pH to the plateau region indicated in Figure 2. Figure 3 compares calibration curves prepared with and without the addition of the phosphate buffer. As can be seen from these curves, the addition of the phosphate buffer increases the sensitivity by a factor of about four. Additional studi1:s indicated that a minimum of 50 mg of finely ground barium chloranilate and a reaction time of at least 1 hour in an end-over-end agitator are required to obtain maximum absorbance.
entering the column and must be tapped into place before the liquid from the slurry has descended into the funnel stem. The columns should be stored in distilled water when not in use to prevent loss of water by evaporation and the subsequent entry of air. The flow rate of the column should be about 1 ml per minute. Place an aliquot of the sample not exceeding 20 ml and containing no more than 0.1-meq total electrolyte in the resin column and collect the eluate in a SO-ml volumetric flask. After the sample has passed through the column, wash the column twice with 2-ml portions of water. The columns are regenerated after each use by washing with 10 ml of 3M HCL, 5 ml of H20, 10 ml of 1 M N H 4 0 H , and 5 ml of HzO in succession. REACTION WITH BARIUMCHLORANILATE. To the flask containing the combined sample and water washings from the column add 25 ml of ethanol and 1 ml of the 25% NH,CI solution and dilute to volume. Add approximately 75 mg of barium chloranilate and mix in an end-over-end agitator
EXPERI~NT AL
Apparatus. Absorbance measurements were made with an Elko Model 11 Electrophotometer utilizing the S-53 filter and a I-cm cell. Reagents. A 25z ammonium chloride solution was prepared by dissolving 25 grams of NH4C1 in water and diluting to 100 ml. Bariurr chloranilate was finely ground. Ethanol used was 95 %. Phosphate buffer was prepared by dissolving 13.6 grams KH2P04 in 103 grams 8 5 % H3P04and diluting to 100ml. Procedure. ION EXCHANGE SEPARATION OF INTERFERING CATIONS. Many cations interfere by precipitation with chloranilate-cg., (:a+2, Pb+2,Cu+*,Zn+2,Al+3, Fef3. These cations are easily separated from sulfate by ion exchange. The following procedure is used. Prepare ion exchange columns by placing: a glass wool plug in the tip of the stem of a 50-mm diamet1:r borosilicate glass funnel with a 150-mm stem. Pour a slurry of Dowex A l , 50- to 100-mesh resin in the ammonium form into the funnel to fill the stem to within 1 cm of the tip. The volume of the resin in the column is about 2 ml. Lighily tap a cotton wool plug into the stem above the resin column. The cotton plug prevents air from
0300
S G , P eq
Figure 3. Sulfate calibration curves with and without phosphate b a e r
Table 1. Composition of Waters and Recovery of Sulfate
Sample
Ca+
Mgc2 2.0
1
2.0
2 3 4
10.0
2.0
14.0
4.0
...
...
Composition, rneqliter . __ Na+ Kf c114.0 2.9 4.0 6.0 2.0 12.0 1 .o
1.o
6.0
2.0
18.0
...
Sulfate recovered,
HCOa-
SO,-'
...
16.0
...
8.0 2.0
6.0
2.0
...
rneq/liter 15.7 7.8 2.0 1.95
Error, yi -1.9 -2.8
... -2.5
VOL 39, NO. 6, MAY 1967
689
Table 11. Sulfate Determinations for Various Waters Samp!e SO., meqbter Std dev Municipal water 0.755,0.748,0.725 0.037 Supply, Bahia Blanca 0.730,O.725,O.730, 0.730 3.82,4.06,4.Oo,3.89, 3.82,3.90,3.90
Colorado River
0.089
for 1 hour. Filter about 25 ml of the suspension through a good quality filter paper which retains fine particles. Add 5 drops of the phosphate buffer, mix, and measure the absorbance at 530 mp. Determine the sulfate concentration with a calibration curve prepared by carrying aliquots of the standard KzS04solution and a distilled water blank through the entire procedure. Because of the nature of the equilibrium, the amount of chloranilate in solution due to the solubility of barium chloranilate decreases as the initial sulfate in solution increases. As a result, the calibration curve is not linear. Therefore, the absorbance of the blank should not be subtracted from the absorbance of the standards or samples. The total absorbance of the standards should be plotted against sulfate concentration, and the sulfate concentration of the unknowns should be determined by comparing their total absorbance with the calibration curve.
these waters and the recovery of sulfate is presented in Table I. In all cases the recovery of sulfate was within 3 of the added amount. The reproducibility of the method was tested by carrying aliquots of two water samples through the procedure seven times. Each determination was made on a different day. One sample was taken from the municipal water supply of the city of Bahia Blanca and the other was taken from the Colorado River near its mouth in the southern tip of the Province of Buenos Aires. The individual determinations and their standard deviations are presented in Table 11. The procedure presented in this work improves the sensitivity of the original procedure of Bertolacini and Barney without the necessity of access to an ultraviolet spectrophotometer. It should prove of value for the determination of sulfate in water samples and similar aqueous solutions.
R.M. CARLSON' R. A. ROSELL W. VALLEJOS
Instituto de Edafologia e Hidrologia Universidad Nacional del Sur Ave. Alem 925 Bahia Blanca, Argentina 1 Present address, Department of Pomology, University of California, Davis.
DISCUSSION
This procedure was tested on four water samples of known sulfate concentrations which had been prepared by dissolving reagent grade salts in distilled water. The composition of
RECEIVED for review November 14,1966. Accepted February 24, 1967.
Correction Applications of Signal-to-Noise Theory in Molecular Luminescence Spectrometry In this article by P. A. St. John, W. J. McCarthy, and J. D. Winefordner [ANAL.CHEM. 38, 1828 (1966)] the following corrections should be made: 1. On page 1829, Equation 5 should read as below
Pe,
=
P.&s
=
P.b*QhW' __-4rAAX'
3. On page 1832, Equation 22 should read as below
6%
*
ANALYTICAL CHEMISTRY
4. The constant Ks* defined under Equation 23 should be
I"k&.f&~
2. On page 1830, Equation 7 should read as below
.
w,=
5. It should be noted thatf3 is equal to& X fs in Appendix 11. 6. The source intensities, I", used to calculate the values of Cminin Table I were calculated from blackbody theory by assuming that the xenon arc plasma had a temperature of 6OOO" K and an emissivity of 0.06.
