Combination pH probe

This experiment is an acid-base titration similar to those performed in Chapters 4 and 5. It is the titration of 0.10 M HC1 with 0.10 M NaOH, as in Experiment 8, but a combination pH probe will be used to monitor the pH during the titration, as in Experiment 10. The pH meter to be used has an RS232 output to interface with a microcomputer. We will use this special feature of the pH meter to feed the pH data directly to a microcomputer in an example of data acquisition by computer, and observe the titration curve traced on the screen in real time. 

FIGURE 14.8 Photographs showing the porous fiber strands on various combination pH probes. The arrow points to the strand in each case. Also notice the glass membranes and the protective plastic sheaths. 

Fig. 12.6 Combination pH probe, (a) Conventional design, (b) Pressurized reference electrode solution. (Courtesy Hach Co.)...

The pH, EC and Fe3+ were used as control parameters. The first two were measured with an Orion probe combined pH/ATC electrode Triode and a conductivity cell DuraProbe ref. 0133030. Fe3+ was determined by molecular absorption (thiocyanate method). Mineralogical composition of the precipitates was determined by X-ray powder diffraction (XRD). Scanning electron microscopy, combined with an energy dispersive system (SEM-EDS), allowed the observation of morphological and compositional aspects of the precipitates. 

In order to use the pH electrode described above, two half-cells (probes) are needed—the pH electrode itself and a reference electrode, either the SCE or the silver-silver chloride electrode—and two connections are made to the pH meter. An alternative is combination pH electrode. This electrode incorporates both the reference probe and pH probe into a single probe and is usually made of epoxy plastic. It is by far the most popular electrode today for measuring pH. The reference portion is a silver-silver chloride reference. A drawing and a photograph of the combination pH electrode is given in Figure 14.7. 

A combination pH electrode consists of a pH electrode and a reference electrode in a single probe. 

Turbidimetric titrations were conducted in three ways. In "Type 1" titrations, a mixture of PDMDAAC (0.04-4.0 g/L) and protein (0.1-10 g/L) were combined at pH 4 in distilled deionized water or dilute (0.05-0.5M) NaCl. The optical probe (2 cm path length) of a Brinkman PC600 probe colorimeter (240 nm), and a combination pH electrode connected to an expanded scale pH meter (Orion 811 or Radiometer pH M26), were both placed in the solution. Titrant (0.50 M NaOH or 0.50 M HC1) was delivered from a 2.0 mL microburet (Gilmont) with gentle stirring. Alternatively, turbidity was monitored while a protein solution was added to PDMDAAC (at constant ionic strength) or vice-versa. Turbidity was reported as 100-96T, which is linearly proportional to the absolute turbidity in the range 80<%T<100. 

PH electrodes from Crison, combination electrodes that contain a built-in temperature probe, a bridge electrolyte chamber and movable sleeve junction from Mettler, and combination pH puncture electrodes with spear-shaped tip from Mettler are some commercially available pH measurement systems for semisolid formulations. 

Combined measuring probe Has a thin-walled glass bulb at its tip which detects H+ ions in solution, and produces a small voltage (around 0.06 V per pH unit), and as such provides a signal related to the concentration of H+ ions which can be converted into a measurement of pH. For simplicity, the pH probe can be... 

Electrokinetic potential of the alumina samples redispersed in 0.001 N KNO3 was measured over a pH range between 2 and 9.5 with a Pen Chem System 300 instrument. The coated alumina powder was ultrasonically dispersed at 0.001 wt% into 300 mL of 0.001 N KNO3 for ca. 15 min and immediately placed under N2 atmosphere. A 50-mL portion was placed onto a titration stirrer under N2 atmosphere that was fitted with a pH probe, a mechanical stirrer, and a port for addition of titrant. A portion of sample was pumped into the S3000 cell, which was fitted into a constant temperature bath set at 25 °C. This sample portion was used to rinse the cell of the previous sample. A second portion of sample was pumped into the cell. The pH was recorded and the zeta potential was measured. Two measurements were taken, one at the front stationary layer and the second at the back stationary layer. The histograms were then combined and averaged. The pH was adjusted with either 0.01 N KOH or 0.01 N HNO3, and measurements were repeated for each desired pH value. Once all the desired pH versus zeta potential data were obtained, the data were transferred to an IBM PC and plotted. Estimates of isoelectric points (IEPs) were made from the plots obtained. 

Two approaches have been adopted for pC02 sensing, namely (1) wet sensors and (2) plastic (solid-state) sensors. A wet sensor consists of a pH-sensitive probe dissolved in aqueous bicarbonate buffer solution, which is separated from the gaseous or liquid test medium by a gas-permeable membrane [9]. In plastic sensors, a polar pH-probe is immobilised in a thin polymer film. The probe is usually ion-paired with a lipophilic base such as a tetra-alkyl ammonium hydroxide. This ion-pair combination facilitates compatibility between the two components, whilst simultaneously eliminating the need for aqueous buffers due to the associated water of hydration. This makes it possible for these sensors to maintain their sensitivity to CO2, although they may show some humidity dependence. The requisites of a CO2 probe are a suitable a significant change in absorption or emission characteristics upon... 

A typical pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The measuring part of the electrode, the glass bulb at the bottom of the pH probe (Figure 4.8), is coated both inside and out with a 10nm layer of a hydrated gel. These two layers are separated by a layer of dry glass and the potential is created by the equilibrium in H"" ions across the membrane. 

Combination electrodes have increased in use and are a consoHdation of the glass and reference electrodes in a single probe, usually in a concentric arrangement, with the reference electrode compartment surrounding the pH sensor. The advantages of combination electrodes include the convenience of... 

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