PH glass

Phetharbital [357-67-5] pH Glass electrode Philips 2P Process Phillips catalysts... 

Ion-selective electrodes can also become sensors (qv) for gases such as carbon dioxide (qv), ammonia (qv), and hydrogen sulfide by isolating the gas in buffered solutions protected from the sample atmosphere by gas-permeable membranes. Typically, pH glass electrodes are used, but electrodes selective to carbonate or sulfide may be more selective. 

Specific-Ion Electrodes In addition to the pH glass electrode specific for hydrogen ions, a number of electrodes that are selective for the measurement of other ions have been developed. This selectivity is obtained through the composition of the electrode membrane (glass, polymer, or liquid-liquid) and the composition of the elec trode. Tbese electrodes are subject to interference from other ions, and the response is a function of the total ionic strength of the solution. However, electrodes have been designed to be highly selective for specific ions, and when properly used, these provide valuable process measurements. 

Example 5-1 Calculate the relative error (in proton concentration) that would occur if the pH of a 1 x 10 2 M NaOH solution were measured with a pH glass electrode ( HNa = 10-10 assuming activity coefficient of 1.0). 

Measurement of pH was performed using a Metrohm model 691 pH meter equipped with a Metrohm combined LL micro pH glass electrode calibrated prior to use with pH = 2 and 9 buffers. The checkers found that adjustment to a lower pH led to product with higher amounts of inorganic impurities. The checkers also found that the use of pH paper results in different pH values as compared to the pH meter. 

Research-type pH meters often show a resolving power ApH = 0.001 and normal meters mostly better than pH = 0.01, so that a pH accuracy of 0.01 can be attained for the combination of a pH meter and a pH glass electrode with temperature correction (preferably automatic). 

It will be clear that once these standards have been generally applied, pH measurements in practice can henceforth be made without any objection by the use of pH glass electrodes instead of the classical hydrogen electrode. 

Glass electrode [see Fig 2.10 (1)J. The pH glass electrode, as the most important representative of the glass electrodes, will be the first subject to be treated, and especially in its application to aqueous solutions. Attached to the stem of high-resistance glass, the electrode proper consists of a pH-sensitive glass bulb that acts as a membrane between an inner reference electrolyte and an outer... 

From the above remarks, it will be clear that the development and manufacture of a properly functioning and sufficiently stable pH glass electrode requires a sophisticated compromise based on research and skill. This may be illustrated as follows ... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

Fig. 2.13. Combined pH glass electrode Metrohm 121 (Courtesy of Metrohm).

Input international BNC sockets for pH, glass or combination electrodes, 4-mm sockets for RE electrode and Pt 100 input at 25° C, current <0.1pA, resistance > 1013Q. 

The lanthanum fluoride electrode (discussed in Section 2.6) is used to determine F ions in neutral and acid media. After the pH-glass electrode, this is the most important of this group of electrodes. 

W. Vonau, J. Gabel, and H. Jahn, Potentiometric all solid-state pH glass sensors. Electrochim. Acta 50, 4981-4987 (2005). 

Here, a pH glass electrode may replace the hydrogen electrode in cells (I) and (II). 

The most popular ISE is a pH glass electrode. However, a great number of ISEs have been developed and widely used in such fields as clinical and environmental analyses. For the use of ISEs in non-aqueous solutions, see Chapter 6. 

Acid-base, redox, precipitation and chelometric titrations are usually dealt with in textbooks on analytical chemistry. The titration curves in these titrations can be obtained potentiometrically by use of appropriate indicator electrodes, i.e. a pH-glass electrode or pH-ISFET for acid-base titrations, a platinum electrode for redox titrations, a silver electrode or ISEs for precipitation titrations, and ISEs for... 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Proton acceptors (bases like amines) H+/pH-glass electrode... 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

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