Method of performing in situ calibrated potentiometric pH measurements
A device for the precise and accurate potentiometric pH measurements in situ. Embodiments of a potentiometric device according to the invention consist of one or more glass pH-sensitive electrodes connected to a potentiometer. A key feature of the device is that, rather than being calibrated conventionally with buffers, it can be calibrated with an in situ device that measures pH spectrophotometrically. Spectrophotometric pH measurements obtained via sulfonephthalein absorbance measurements are inherently calibrated (do not require buffers). Thus, devices according to the invention allow for continuous potentiometric pH measurements with occasional spectrophotometric calibrations. The spectrophotometric calibration device consists of a spectrophotometer with associated pumps for combining a sulfonephthalein pH indicator with the aqueous medium whose pH is to be measured. The device will record potentiometric pH measurements for an extended period of time until the spectrophotometric device is autonomously activated for another calibration. In this manner precise and accurate pH measurements can be obtained continuously in the environment, and the low energy expenditure of the potentiometric device provides excellent endurance. Also provided is a method and associated devices for spectrophotometrically determining the salinity of an aqueous medium.
1. A method of performing in situ continuous autonomous potentiometric pH measurements for extended periods of time comprising the steps of:
recording a plurality of potentiometric pH measurements using a potentiometric pH measurement device;
autonomously activating a spectrophotometric calibration system positioned within the device wherein the system is activated by an event trigger or a defined time interval wherein the event trigger is a sudden pH change outside the typical deviations of pH recordings;
calibrating the potentiometric pH measurement device comprising the steps of:
obtaining spectrophotometric pH measurements from a spectrophotometer located within the device comprising the steps of:
sampling an aqueous medium;
collecting data representative of temperature and salt concentration of the sampled aqueous medium;
combining a sulfonephthalein indicator and a first portion of the sampled aqueous medium;
delivering the combined medium to an optical cell;
obtaining an absorbance spectrum from the combined medium;
calculating solution pH of the sampled aqueous medium utilizing the optical absorbance ratios at multiple wavelengths, the temperature, and the salt concentration;
obtaining a potentiometric pH measurement of a second portion of the aqueous medium substantially contemporaneously with the spectrophotometric pH measurements;
calculating the deviation between the potentiometric pH measurement and the spectrophotometric pH measurement; and
adjusting the potentiometric pH measurement device using the calculated deviation, wherein the adjusting calibrates the potentiometric pH measurement device; and
recording the calibrated potentiometric pH measurements for an extended period of time until an event trigger or preset time interval autonomously activates the spectrophotometric calibration system to calibrate the device.
2. The method of claim 1 wherein measurement frequency of the potentiometric pH measurement is 60 Hz or below.