CORRESPONDENCE
SIR: Carpenter and coworkers ( I ) have reported that the iodometric titration procedure for residual oxidants in chlorinated seawater can underestimate the true value by threefold or more. Their results indicate that the amperometric standard method also badly underestimates the residual oxidant concentration in chlorinated seawater. Workers a t our laboratory and other marine science facilities were disturbed to read that the accepted method for analysis of residual oxidants in seawater may be greatly in error. During the seawater oxidant analysis, the order of addition of the chemicals is of paramount importance. Our experimental results are in agreement with those of Carpenter et al. ( I ) if we follow the mixing order that they reported. However, when we use a method that differs solely in the order of addition of reagents, the results are an accurate and precise estimate of the residual oxidants in seawater. “Standard Methods” ( 2 ) reports two different mixing orders for residual chlorine analysis in water. With regard to the mixing order, we have included three examples from the literature. Carpenter et al. ( I ) reported, “Adjust the pH of the sample to 4 with acetate buffer, add KI solution and titrate with either sodium thiosulfate or phenylarsine oxide solutions.” “Standard Methods” ( 2 , page 111) recommended under iodometric method, to place the pH buffer in the titration container, add KI, then pour in the sample, mix and titrate. Also, “Standard Methods” ( 2 ,page 116) recommended under amperometric titration method to treat the sample with KI followed by buffer and titrate. All these procedures work properly for fresh water, but we have found that only the mixing order reported by “Standard Methods” ( 2 ,page 111) is reliable for residual oxidant determination in chlorinated seawater. The “mixing order” we have been using consists of adding 2 mL of buffer (2, page 115) and 2 mL of KI solution ( 2 ,page 115) to an empty beaker. Swirl the reagents for a few seconds, then rapidly pour in 200 mL of sample and titrate immediately with phenylarsine oxide, using either the amperometric method (2, page 112-6) or the potentiometric method described by Eppley and coworkers ( 3 ) . To determine the accuracy of our method, both fresh water and seawater were spiked with known concentrations of calcium hypochlorite or sodium hypochlorite. At the 2 ppm chlorine level, the measured levels are within 10%of the predicted level if the analysis is performed immediately before the natural chlorine demand of the seawater has much effect. As a check of our procedure, we have also analyzed spiked seawater samples by the back titration procedure (2,page 382) which Carpenter et al. ( I ) found to be accurate for chlorinated seawater. Over the range of 0.01-1.0 ppm chlorine, these two methods agree within 5%.The precision of both of these procedures is f 1 0 % or better for one SD over the range of 0.010-1.0 ppm oxidant as chlorine. The reagent blank in seawater is less than 0.002 ppm oxidant as chlorine. We conclude that the “Standard Methods” (2, pages 112-6) can be used for seawater if the mixing order we have reported here is used. J. C. Goldman and J. M. Capuzyo (private com-
1088 Environmental Science € Technology i
munication) have had satisfactory results with oxidant analysis of seawater using the same mixing order as we use. Data reported in the past by this laboratory and others using our identical mixing order should not be considered in error due to analytical methods, since this method has been shown to produce consistent and accurate results.
Literature Cited (1) Camenter. J. H.. Moore. C. A.. Macaladv. D. L.. Enuiron. Sci. “
I
Technol., 11 (lo),992-4 (Oct. 1977). (2) American Public Health Assoc.. “Standard Methods for the Examination of Water and Wastewater”, 13th ed., pp 111-6, APHA, Washington, D.C., 1971. (3) Eppley, R. W., Renger, E. H., Williams, P. H., Estuarine Coastal Mar. Sci., 4,147-61 (1976). Based on work performed under Department of Energy Contract E Y -76-C-06-1830.
Eric A. Crecelius G. Roesijadi Thomas 0. Thatcher Battelle-Pacific Northwest Laboratories Marine Research Laboratory Route 5, Box 1000 Sequim, Wash. 98382
SIR: The report of Crecelius et al. that premixing of the buffer and KI solutions in the procedure for the determination of residual oxidants in chlorinated seawater eliminates the errors due to spurious iodate formation suggests that an easily performed change in the procedure will produce accurate results. However, it should be emphasized that in contrast to present practice, particulars of the procedure used by various investigators will need to be reported. The errors described by Carpenter et al. occur with the “standard” amperometric method as described in “Standard Methods” ( I , page 116), EPA Environmental Protection Technology Series ( 2 ) ,and the instructions for commercial amperometric titration equipment ( 3 ) ,which has been widely used. If these errors have been fortuitously avoided by using the premixing procedure or the back titration procedure ( I , page 382), this fact should be specifically noted in research reports.
Literature Cited (1) American Public Health Assoc., “Standard Methods for the Examination of Water and Wastewater”. 13th ed., _DD _ 116,382, APHA,
Washington, D.C., 1971. (2) Manabe, R. M., Envir. Prot. Tech. Series, EPA 660/2-73-039,Nat. Env. Res. Center. USEPA. Corvallis. Ore.. 1974. (3) Wallace and Tierman Div., Pennwalt Corp., Instructions for Amperometric Titrator, 1976.
James H. Carpenter Rosenstiel School of Marine and Atmospheric Science University of Miami 4600 Rickenbacker Causeway Miami, Fla. 33149
0013-936X/78/0912-1088$01 .OO/O @ 1978 American Chemical Society
