PH-sensor

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Fouling of the pH sensor may occur in solutions containing surface-active constituents that coat the electrode surface and may result in sluggish response and drift of the pH reading. Prolonged measurements in blood, sludges, and various industrial process materials and wastes can cause such drift. Therefore, it is necessary to clean the membrane mechanically or chemically at intervals that are consistent with the magnitude of the effect and the precision of the results requited. 

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... 

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

The new approach for development of pH sensor with wide acidity range (2.5 M H SO - pH 5.5) based on the use of Congo Red and Benzopurpurin 4B immobilized in polyamido- or arachidic acid nanosized sensitive matrix will be demonstrated. 

NOVEL OPTICAL pH SENSORS BASED ON CATECHOL AZO DYE DERIVATIVES... 

GPR4 19q13.2-q13.3 cAMP t (Gs) pH sensor of endothelial cells... 

Yang LV, Radu CG, Roy M et al (2007) Vascular abnormalities in mice deficient for the G Protein-coupled receptor GPR4 that functions as a pH sensor. Mol Cell Biol 27 1334-1347... 

Apart from the advantage of very rapid response, ISFETs can be easily constructed as real microprobes, although their construction requires considerable skill. At present the ISFET as a pH sensor is less selective than the... 

One of the interesting features in the structure-photophysical property relationship of fluorescein is that the quantum yield of fluorescein increases under the basic condition. Therefore, many of fluorescein derivatives have been used as pH sensors to measure intracellular pH due to their pH-responding photophysical property [53]. Although fluorescein itself is slightly fluorescent in alcoholic solutions, the addition of alkali (pH > 8) to the fluorescein solution produces the very intense fluorescent alkali salt. The salt form of fluorescein... 

Whitaker JE, Haugland RP, Prendergast FG (1991) Spectral and photophysical studies of benzo[c]xanthenes dyes dual emission pH sensors. Anal Biochem 194 330-344... 

Flanson GT, McAnaney TB, Park ES, Rendell ME, Yarbrough DK, Chu S, Xi L, Boxer SG, Montrose MH, Remington SJ (2002) Green fluorescent protein variants as ratiometric dual emission pH sensors. 1. Structural characterization and preliminary application. Biochemistry 41 15477-15488... 

Most probably, the first - but non-fiberoptic - sensors for continuous use where those for pH and for oxygen. It has been known for decades that cellulosic paper can be soaked with pH indicator dyes to give pH indicator strips which, however, leached and thus were of the "single-use" type. The respective research and development is not easily traced back since it is not well documented in the public literature. However, in the 1970s, indicator strips became available where they pH indicator dye was covalently linked to the cellulose matrix, usually via vinylsulfonyl groups. These "nonbleeding" test strips allowed a distinctly improved and continuous pH measurement, initially by visual inspection. In the late 1980 s instruments were made available that enabled the color (more precisely the reflectance) of such sensor strips to be quantified and related to pH. Respective instruments are based on the use of LEDs and are small enough to be useful for field tests in that they can be even hand-held. This simple and low cost detection system is still superior to many of the complicated, if not expensive optical pH sensors that have been described in the past 20 years. 

Other very early work includes that of Boisde at the CEA9,10. A scheme of the experimental set-up used by his group is given in Fig. 1. The fiber optic pH sensor described by Peterson et al.11 in 1980 was another milestone. A 2-volume book that appeared in 1991 gives an account of the early work on fiber optic chemical sensors and biosensors12 up to about 1989. Earlier reviews include those of Kirkbright et al.13, Borman14, and Hirschfeld et al.15. 

Zhang and Seitz somewhat later described a sensor for carbon dioxide that is based on measurement of fluorescence60. It was prepared by covering a pH sensor based on fluorescence with a C02-permeable membrane and contacting the pH-sensitive membrane with a reservoir of hydrogen carbonate. As CO2 diffuses across the membrane it causes a change in pH which is measured via the change in fluorescence from the base form of the... 

As stated above, the beginning of optical pH sensor technology remains hidden. What is nowadays refered to as a sensor layer was formerly mostly refered to as a test strip, a dry reagent chemistry, or an immobilized reagent. 

As the potential of optical fiber probes for pH measurements was rapidly recognized, several other articles appeared within a few years75 83. Most were reflectance-based, and Seitz reported the first fluorescent pH sensors84, 78. The article by Janata85 on whether pH optical sensors can really measure pH is another "must" in the early literature since it points to aspects hardly addressed in pH sensor work. 

Figure 4. First fiber optic pH sensor for in vivo use.

Petersons pH probe also was modified in order to give a miniature fiber optic sensor potentially suitable for glucose measurements90. Kopelman et al.91 developed a fiber-optic pH nanosensor for physiological measurements using a dual-emission sensitive dye. The performance of a pH sensor was reported92. An unclad fiber was dip-coated with a thin layer of porous cladding within which a pH-sensitive dye was entrapped. The fundamental... 

S. Goldstein et al. 1980 Miniature fiber optic pH sensor for blood... 

L.A. Saari, W.R. Seitz 1982 pH sensor based on immobilized fluorescein... 

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