Field measurement of alkalinity and pH

Abstract

The behavior of electrometric pH equipment under field conditions departs from the behavior predicted from Nernst's law. The response is a linear function of pH, and hence measured pH values may be corrected to true pH if the instrument is calibrated with two reference solutions for each measurement. Alkalinity titrations may also be made in terms of true pH. Standard methods, such as colorimetric titrations, were rejected as unreliable or too cumbersome for rapid field use. The true pH of the end point of the alkalinity titration as a function of temperature, ionic strength, and total alkalinity has been calculated. Total alkalinity in potable waters is the most important factor influencing the end point pH, which varies from 5.38 (0° C, 5 ppm (parts per million) HCOs) to 4.32 (300 ppm HCO3,35° C), for the ranges of variables considered. With proper precautions, the pH may be determined to ±0.02 pH and the alkalinity to ±0.6 ppm HCOs for many naturally occurring bodies of fresh water. INTRODUCTION Accurate pH data were necessary in a detailed study of the carbonate chemistry of a stream (Birch Creek, Inyo National Forest, Inyo county, Calif.). Study of the methods and data showed that individual measurements of pH were as much as 0.5 pH unit in error. The large (0.5 pH) errors occurred when the sample had a pH considerably different from that of the reference (buffer) solutions. Errors of this magnitude cannot be tolerated when the pH meter is used to determine the end point of the alkalinity titration. MATERIALS AND METHODS The meter used throughout was a Beckman (Beckman Instrument Co., Fullerton, Calif.) model N pH meter. Keference electrodes were 5-inch fiber-wick saturated calomel electrodes having 30-inch leads, Beckman Instrument Co. No. 39170. Glass electrodes were 5-inch general purpose glass electrodes having 30-inch leads, Beckman Instrument Co. No. 40498. Buffer solutions were obtained from the sources listed in table 1. HI H2 GEOCHEMISTRY OF WATER TABLE 1. Suppliers of buffer solutions pH (25° C) Supplier 4.00±. 01 _________ _______ Beckman Instrument Co. 7.00±.02__ ______________ Do. 8.00±.02______________ Fisher Scientific Co. The pH of each buffer is plotted against temperature in figure 1. All electrodes were tested in the laboratory before use in the field. The test consisted of calibrating the instrument at pH=8.00 at 25° C and measuring the pH of the pH4 buffer. The pH8 buffer has a pH of 8.00±.02 at 25° C, and the pH4 has a pH of 4.00±.01. The observed pH value of the pH4 buffer (pH£) was in the range 4.00 to 4.03 for all pairs of electrodes. The glass and the calomel electrodes were transported to the field in the manufacturer's original packings. The glass electrodes were transported dry and the calomel electrodes were transported with the working end covered by a rubber cap filled with a saturated KC1 solution. No evidence of physical damage to the electrodes was observed when the electrodes were unpacked for use in the field. All glass electrodes were soaked in water for at least two days before use. The pH meter was turned on 1 hour before measurements were made. The meter was left on during each sampling period (24-36 hours). The electrodes were immersed 2 to 3 inches in the stream at all times, except during sampling. For temperature control all buffers and the standard (0.01639Ar H2SO4) acid for alkalinity titrations were kept in plastic bottles immersed in the stream, except during actual use. The following steps were found necessary for accurate pH and alkalinity determinations: 1. Measure water temperature with the thermometer in shade. 2. Set temperature compensator on meter to water temperature. 3. Wash electrodes in a stream of buffer whose nominal pH is near that of the water. 4. Immerse electrodes 1 inch in the buffer. 5. Balance and set meter at the nearest integral value of the buffer pH. 6. Rinse electrodes in water. 7. Collect water sample in beaker. 8. Measure the pH of the sample. 9. Pipet 50 ml of sample into dry beaker. 10. Titrate with standard acid and record pH with each increment of acid in the range from pH 5 to pH 4. 11. Rinse electrodes in water. 12. Wash electrodes in a stream of pH4 buffer. 13. Measure pH of pH4 buffer. FIELD MEASUREMENT OF ALKALINITY AND pH H3