Anal. Chem. 1984, 56, 1973-1975
LITERATURE CITED (1) "1980 Annual Book of ASTM Standards", Part 12; ASTM: Philadelphla, PA, 1960; Method E350-80. (2) Bernas, Bedrich Anal. Chem. 1968, 40. 1682-1686. (3) Harstein, A. M.; Freedman. R. W.; Platter, D. W. Anal. Chem. 1973, 45. 6 1 1 4 1 4 . (4) Sihnetziei, C. C.; Nava, D. F. Earth Panef. Sci. Lett. 1971, 1 1 , 345-350.
1973
(5) Nelson, G.; Smith, D. L. Proc, SOC.Anal. Chem. 1972 (Aug), 168. (6) Savolainen, A. M. Ph.D. Thesis, Department of Chemistry, Providence College, Provldence, R I , 1980, (7) Miller, Myran H.; Eastwood, DeLyle; Hendrick, Martha S. Spectrochlm. Acta, Part 8 1984, 398, 13-56.
RECEIVED for review March 29,1984. Accepted May 14,1984.
CORRESPONDENCE Residual Ozone Determination by Flow Injection Analysis Sir: For the past 80 years free chlorine has been utilized for the disinfection of wastewater and drinking water in the United States. Recently, the concerns about the chlorinated byproducts produced during the chlorination process (1-4) have renewed interests in alternatives to free chlorine disinfection. Ozonation has been shown (5,6)to be a cost-effective disinfectant which exhibits the additional capability of removing taste, odor, and color-producing compounds. Before any disinfectant can replace free chlorine as the standard method, several important criteria must be met. They include: easy generation of the disinfecting species, production of few, if any, undesirable byproducts, and an easily measured residual (7). Currently, there are a minimum of eight commonly used methods for the determination of residual ozone. A recent report (8)indicates that the determination of residual ozone by the indigo method appears to be the method of choice. Bader and Hoigne' (9) have described a manual method and an automated method (10) for ozone determination based on indigo in the ranges of 0.014.1 mg/L, 0.05 to 0.5 mg/L, and over 0.3 mg/L. In their study the concentration of ozone was related to the difference in absorbance between the sample and the indigo reagent by the equation
A 100 fb v
[Os]= - where A is the difference in absorbance between a blank and a sample, f = 0.42 cm-' per 1mg/L of ozone and is the sensitivity of the determination, b is the cell path length in cm, and loo/ V is a dilution factor, where V is the volume in mL of the sampled used. per 100 mL final volume. Here we describe an automated system for ozone determination based upon the indigo method which incorporates the advantages of the flow injection analysis (FIA) technique (11). We report a comparison of this system to the manual method in terms of detection limits, linear working range, sampling frequency, and interferences.
EXPERIMENTAL SECTION Apparatus. A diagram of the FIA manifold used in this study is presented in Figure 1. The indigo reagent was pumped through 0.5-mm Teflon tubing with a Tecator, Inc., Model 5020 flow injection analyzer at a 1mL/min flow rate. The aqueous ozone sample volume was 100 rL. The decolorization of the indigo reagent was measured at 600 nm by using an Isco V4 UV-VIS spectrometer fitted with a 0.5 cm path length flow cell. Manual UV measurements of ozone concentration were performed on a Hewlett-Packard Model 8450A spectrophotometer at 259 nm using a single stoppered 5.00 cm path length quartz cuvette. For the calculation of ozone concentrations a molar absorptivity of 3300 M-l cm-I (12) was used. (It should be noted that the molar absorptivity of 2900 M-' is not considered to be 0003-2700/84/0356-1973$01.50/0
a "true value", rather as the value which corresponds with the conventional iodide method. Recently, however, Hart (12) has published a molar absorptivity of 3300 M-' cm-' at 260 nm for aqueous ozone which we believe to be the preferred value (S).) Reagents. All chemicals used in this study were analytical grade. Water from a Model D3600 Barnstead Nanopure double still system with UV attachment was used to prepare all solutions. An indigo reagent stock solution was prepared by adding 770 mg of potassium indigo trisulfonate (Riedel-deHaen AG, Hannover, BRD) to a 1-L volumetric flask containing approximately 500 mL of water and 1 mL of concentrated phosphoric acid, stirring, and diluting to 1L. Reagent solutions, 77 mg/L and 15 mg/L, were prepared by adding 100 and 20 mL, respectively, of the stock solution to a 1-L volumetric flask containing 10 g of sodium dihydrogen phosphate and 7 mL of phosphoric acid and diluting to the mark with water. Mn(VI1) standards were prepared by appropriate dilution of a 1000 ppm stock solution of potassium permanganate. Chlorine standard solutions were prepared by diluting a 6 mg/L stock solution which was made by bubbling C12 through 1 L of chlorine-demand-free water (13). The chlorine content was determined by the DPD/ferrous titrimetric method (14).The titer of the chlorine stock solution was checked in triplicate before and after completion of the FIA testing and was found to differ by less than 3%. Ozone Samples. Ozone was generated by passing oxygen gas through an Ozone Research and Equipment Corp. (OREC, Phoenix, AZ) Model 03V9-0 ozonator. Aqueous ozone samples were prepared by bubbling the ozone into a 3-L contractor filled with the prepared water. Procedure, Due to the volatility and rapid decomposition of ozone in solution the preparation of standard ozone solutions cannot be readily performed (8, 15, 16). For this reason comparative techniques must be used to verify the relative accuracy of a new method of determining ozone. In this study direct ultraviolet detection was chosen as the comparative method. Samples of the ozonated water were removed from the contactor through a sampling stopcock directly into a 5.00 cm path length quartz cuvette which was immediately placed into the UV-VIS spectrophotometer for direct UV measurement. triplicate FIA samples were removed from the cuvette by the peristaltic sampling pump of the FIA instrument for introduction into the indigo reagent carrier stream. Triplicate blank solutions were injected after every nine sample injections to correct for any base line fluctuation.
RESULTS AND DISCUSSION In commercial water purification facilities the measurement range of residual ozone is typically between 0.05 and 5 mg/L. Two concentrations of indigo reagent were investigated for detection limits, sensitivity, and linear ranges in this region. The single-channel FIA manifold, shown in Figure 1, had a dispersion coefficient of 1.8 and an injection frequency of 120 injections/h. The results are summarized in Table I. 0 1984 American Chemlcal Society
1974
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984 I N D I G O B L U E RESPONSE T O P E R O X I D E
INDIGO BLUE
1.0
W
0.010
0
z
W
L Y
W
P
I
C
Schematic dlagram of FIA manifold used in the determination of residual aqueous ozone. The indigo reagent was pumped with a peristaltlc pump, P. A 100-pL aqueous ozone sample was injected, I, into the reagent stream and passed through a 60Gm mixlng coil, C. The decolorized indigo blue reagent was detected at 600 nm by a UV-Vis spectrophotometer, D. Figure 1.
b-i
0
0.006
W 0
2
0.004
m
E 0 Ln
m
