A pH-sensitive detector for flow-through potentiometry based on a graphite/quinhydrone composite electrode was applied for flow-injection analysis (FIA) titrations. Hydrochloric acid and acetic acid were titrated by injection of samples into a sodium hydroxide carrier solution. System conditions were optimised by variation of flow rate, injection volume and titrant concentration. The parameters sampling frequency, residence time and dispersion coefficient were determined. The evaluation of peak width (time between the two inflection points on each side of the peak), peak area and slope of the raising edge of the peak lead were studied with respect to their use for calibration. Hydrochloric acid and acetic acid could be titrated down to a concentration of 2 x 10(-4) mol L(-1) using 150-microL injection volumes, which is almost ten times lower than can be achieved using colour indicators and a spectrophotometric detection.
A combination pH electrode that can be assembled by the student is described. It consists of a glass holder and two sensors in the form of rubber stoppers that contain quinhydrone and graphite. The combination electrode is suitable to teach potentiometric measurements, pH measurements, and the interplay of acid–base and redox equilibria. The electrode meets highest safety standards and overcomes the troubles associated with the use of the conventional quinhydrone electrode. Compared with commercial combination glass electrodes, the combination quinhydrone electrode has the didactic advantage that the sensor and the reference electrodes are both visible and not preassembled.
A new pH sensitive detector for flow-through potentiometry was developed on the basis of a graphite/quinhydrone composite electrode. The detector was constructed of two polymethylmethacrylate plates between which a 0.1 mm thick sensitive layer is situated. After drilling a hole through the plates, a ring of the sensitive layer is exposed to the solution stream. A conventional calomel reference electrode was placed downstream following the flow-through sensor. The response behavior in different electrolyte solutions was investigated. The detector shows a Nernstian behavior for injections of hydrochloric acid into a KCl background solution as well as for the steady state response in concentration step experiments using hydrochloric acid and buffer solutions. The response time is sufficiently short for FIA applications (T 95 between 3 and 5.4 s). The influences of flow rate and injection volume on the detector signal are discussed.
A new pH-probe was developed for in situ determination of soil pH. It consists of a stainless steel tube with a plastic inset containing the indicator electrode, the reference electrode and a temperature sensor at the end of the tube. The indicator electrode is a quinhydrone composite electrode that does not need to be calibrated, because it acquires almost the theoretical predicted potential and has a constant formal potential and slope under all fabrication conditions. The pH-probe has a low standard deviation (AE 4 mV or 0.07 pH units). The response time is short (5 s). To characterize its function the soil pH-probe was used to analyse pond and arable soil samples. The results were compared with those obtained with a conventional combined glass electrode. To evaluate the results, measurements were performed (i) in natural wet soil samples (in situ conditions), (ii) after drying and moistening the soil samples (moistened samples) and (iii) after drying the soil samples and mixing with bidistilled water (soil solutions; generally accepted method in laboratories). The minimum water content required to obtain stable potentials in soil samples was 10%. The influence of S 2À , NO À 3 and Fe 3þ as naturally available reducing and oxidising agents on the potential response of the pH-probe was investigated. All the obtained results demonstrate that the developed pH-probe is a powerful tool to measure the pH of a soil sample under in situ conditions without a calibration step.
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