United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas NV 89193-3478 Research and Development EPA/600/S4-88/026 Oct. 1988 v°/EPA Project Summary The Determination of pH by Flow Injection Analysis and by Fiber Optrode Analysis Stephen H. Pia, Donna P. Waltman, and Daniel C. Hillman Two new protocols for measuring pH have been developed. The first measures pH colorimetrically using an indicator dye mixture in a flow injection analysis (FIA) procedure. The second measures pH using a fiber optic chemical sensor (FOCS or optrode) specifically developed for pH determinations. The FOCS meth- od measures pH by monitoring the fluorescence of fluorescein deriv- ative bonded to the distal end of a fiber optic cable. The FIA method currently has a precision and accuracy of about ±0.2 pH units and can measure 100 samples/hour. The matrix may affect the precision and accuracy but has not been fully investigated. The FOCS method has a precision of ±0.05 to 0.20 pH units and an accuracy of ±0.1 to 0.6 pH units. The bias is largely due to inadequacy of the calibration model, which needs further development. About 10-60 samples can be anal- yzed. The response time is matrix dependent. It varied from 10 seconds to 7 minutes in the solutions studied, with slowest response in dilute, poorly buffered samples. This Project Summary was developed by EPA's Environmental Monitoring Systems Laboratory, Las Vegas, NV, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction An important parameter determined for acid deposition characterization and monitoring is pH. It has been measured during all of the EPA's Aquatic Effects Research Program Surface Water Surveys in both the field (on-site) and analytical laboratories. Laboratory mea- surements with pH electrodes were generally precise and accurate ( + 0.05 pH units), but proved to be time consuming and labor intensive. When used in the field, electrodes and meters performed marginally (± 0.5 to ±1 pH unit) Future surveys and long-term monitoring projects will require field pH measurements using either in-situ or closed-system techniques. This ne- cessitates the development of new methods for determining pH in the field. In an initial step to develop a field method, this study was undertaken to fully characterize two indicator-based pH methods and check their suitability for field use. The first involves the use of an indicator dye mixture with a pH dependent absorbance in a FIA method. The second method uses an optical fiber coated with fluorescein, the fluorescence of which at 530 nm is pH dependent. Procedure All chemicals were ACS analytical reagent grade or better. Solutions were prepared in ASTM Type II water and were stored in high density polyethylene bottles at 4 degrees centigrade. Real lake water samples were field audit samples from the Eastern Lake Survey. They were originally taken from Big Moose Lake and Bagley Lake and were homogenized and stabilized for use in the survey. The pH and other physical and chemical prop- erties of these real samples were ex- tensively characterized. ------- In the flow injection analysis method, a pH indicator reagent is mixed with sample and injected into a deionized water carrier stream. The absorbance of the effluent is monitored at 555 nm. The pH indicator reagent is a proprietary mixture of pH indicator dyes whose absorbance at 555 nm is proportional to pH. A fiber optic chemical sensor (FOGS or optrode) for pH is constructed by immobilizing fluorescein to the distal end of a fiber optic. Fluorescein is a fluorescent compound which emits light at 530 nm when excited at 485 nm. The intensity of fluorescence at 530 nm is pH dependent. Sample pH is measured by dipping the probe into the sample and monitoring the fluorescence at 530 nm while exciting at 485 nm. The following issues were addressed in evaluating the methods: calibration curves, precision, accuracy, sample analysis rate, matrix effects, and real sample analysis. The basic method analytical characteristics were deter- mined using stable, well characterized pH buffers. The effects of several matrix variables (ionic strength and buffering capacity) on the analytical characteristics were studied using acetate, phosphate, and sulfuric acid solutions. Finally, the performance of the method was tested with two real lake water samples. Results and Discussion The analytical figures of merit of the three methods of pH determination are given in Table 1. The FIA method compares very well with the pH electrode for the routine determination of pH in natural surface waters. It has acceptable precision and bias and has a very high sampling rate. Improvement in both precision and accuracy are possible by improving the FIA hardware (more accurate and precise solution handling, using syringe pumps). Also the linearity and applicable pH range may be increased by modifying the indicator reagent mixture and/or monitoring more than one wavelength. The results of the FOGS pH method were encouraging, but the technique needs more development than the FIA method. The optrodes investigated were prototypes and consequently the analytical characteristics varied from optrode to optrode. The bias was dependent upon the optrode sensitivity and varied from ±0.1 to ±0.6 pH unit. The bias is magnified by the inadequacy of the calibration model. The linear model chosen for the optrode does not explain all of the variation. A polynomial model will be tested when the software becomes available. Another optrode characteristic is a limited lifetime. Optrode sensitivity decreases from day to day. Finally, while the optrodes are generally faster than electrodes, the achievable sampling rate is affected by ionic strength and buffering capacity. Conclusions and Recommendations The FIA method appears to be the more promising of the two new techniques at this time. With further work, a method suitable for long-term, unattended monitoring of pH in surface waters could be developed. The FOGS pH optrode wili require manufacturing changes and further characterizations in order for it to become analytically viable. Potential interferences in the FIA method should be further investigated. For example, the bias for dilute acid standards was larger than anticipated. The bias appeared to be related to the pH or acid used rather than ionic strength. Also, interferences from potential organic components in surface waters should be studied. Finally, a larger set of real samples should be analyzed and compared to pH electrodi data. In the near future, the use o chemically sensitive field effect transis tors (Chemfet) for pH measurement wil be studied. Preliminary indications an that Chemfets will be an excellent tool fo measuring pH directly or as a detector ii a FIA method. Table 1. Comparison of Analytical Figures of Merit of Electrode, FIA, and FOCS pH Methods pH Electrode FIA FOCS Rate (samples/hr) 4-20 pH Range 3-10 Bias (pH Unit) ±0.05 Precision (pH Unit) ±0.05 100 3.6-6.8 ±0.01 ±0.05 to ±0.20 10-60 3.6-7.5 ±0.1 to ±0.6 ±0.02 to±0.2 ------- Stephen H. Pia, Donna P. Waltman, and Daniel C. Hillman are with Lockheed Engineering and Management Services Company, Inc., Las Vegas, NV 89109 Edward M. Heithmar is the EPA Project Officer (see below). The complete report, entitled "The Determination of pH by Flow Injection Analysis and by Fiber Optrode Analysis," (Order No. PB 88-235 502/AS; Cost $74.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency Las Vegas, NV 89193-3478 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 J.S G'->"!CIAL:\:ML' r,,~i;;~;-r-c~Y '-\iTxMj.Tf , J.o.HJJi/'t-;: i" ""'' /:;i •*;- :3 Q ? 5 -!~ '• ' \f t&Ht V* -,, J Official Business Penalty for Private Use $300 EPA/600/S4-88/026 0000329 PS ------- |