PH-sensitive indicators

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

Note that we can consider the potential of a solutionin the same way as we do the pH of a solution. The potential is defined by the presence of an oxidized and a reduced form of a redox couple, just as the pH is defined by the presence of an acid and its conjugated base. The potential is there regardless of whether we measure it or not, and therefore does not rely on contact with any electrodes. The potential of a solution should be no more mysterious than its pH, except that we happen to have a sense for acidity in our taste buds, but not one for potential. Still, we can imagine the pH of soil or blood without tasting it, and it should be no different for its potential. Of course, any direct measurement of the potential of a solution requires electrodes, but the potential can also be calculated from the concentrations of the redox partners when these are otherwise known or measurable, just as the pH can be calculated from, say, the spectrum of a solution containing pH-sensitive indicator dyes. 

One of the earliest such POC tests developed was the home pregnancy test. It is based on a qualitative immunological antibody assay. It relies on the determination of human chorionic gonadotrophin (hCG) levels in urine, which gives rise to a coloured band if positive. The device is very small and completely disposable. A quality control check is built in where the rest of the urine sample continues along the dipstick and interacts with a pH-sensitive indicator to show that the test has been performed correctly. 

Following the initial feasibility studies of Lubbers and Opitz, Cardiovascular Devices (GDI, USA) developed a GasStat extracorporeal system suitable for continuous online monitoring of blood gases ex vivo during cardiopulmonary bypass operations. The system consists of a disposable plastic sensor connected inline with a blood loop through a fiber optic cable. Permeable membranes separate the flowing blood from the system chemistry. The C02-sensitive indicator consists of a fine emulsion of a bicarbonate buffer in a two-component silicone. The pH-sensitive indicator is a cellulose material to which hydroxypyrene trisulfonate (HPTS) is bonded covalently. The 02-sensitive chemistry is... 

Figure 3.14. Light, carbonated surface zone and dark non-carbonated core in a cut concrete beam the colour difference is caused by a pH-sensitive indicator phenolphthalein that has been applied to the cut surface. In the description of the penetration and reaction of gases into porous materials such as concrete, ideal gases are normally assumed.

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

Fig. 16-4 pH sensitivity to SO4- and NH4. Model calculations of expected pH of cloud water or rainwater for cloud liquid water content of 0.5 g/m. 100 pptv SO2, 330 ppmv CO2, and NO3. The abscissa shows the assumed input of aerosol sulfate in fig/m and the ordinate shows the calculated equilibrium pH. Each line corresponds to the indicated amoimt of total NH3 + NH4 in imits of fig/m of cloudy air. Solid lines are at 278 K, dashed ones are at 298 K. The familiar shape of titration curves is evident, with a steep drop in pH as the anion concentration increases due to increased input of H2SO4. (From Charlson, R. J., C. H. Twohy and P. K. Quinn, Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Atmosphere." NATO Advanced Research Workshop Acid Deposition Processes at High Elevation Sites, Sept. 1986. Edinburgh, Scotland.)... 

Figure 2. Liposomes with a specific character a), temperature-sensitive liposome b). target-sensitive liposome c). pH-sensitive liposome. Closed triangles and rectangles in lipid bilayer indicate amphiphiles which change the liposome s hydration with pH changes.

Rao et al. [140] described a study using a commercial pH sensitive radio-telemetry capsule (RTC) to evaluate small bowel and colonic transit time in athletes with gastrointestinal symptoms. The RTC (type 7006 Remote Control Systems, London, UK) consists of a glass electrode with an integral reference cap and battery. RF transmissions from the capsule are detected by a solid-state receiver worn on the belt of the patient. The recorder samples the pH from the capsule at 6 second intervals for a period of 24 hours. They used pH changes as an indication of the pH capsule s movement. A sharp rise in pH from around pH 2 to pH 6 indicates that the capsule has moved into the duodenum from the stomach. Then the pH progressively rises to a plateau around pH 8, which indicates that the capsule has moved into the terminal ileum. Another commercially available wireless pH sensor (Bravo) from Medtronic Inc. has been used to measure esophageal pH for a period of 48 hours and it will be discussed later. 

The electrodes used in the above studies were double-barreled glass pH sensitive microelectrodes, and the spatial retinal pH profile was recorded by withdrawing the microelectrode tip at a rate of 1 //m/s or lOOpm/step across the retina in vivo or in vitro. In a typical retina pH profile (Fig. 10.9), measured in cat retina by the microelectrode, started from the choroids (Ph = 7.41, at distance Ojum). The pH steadily decreased to a minimum value (a maximum [H+] concentration) in the proximal portion of the outer nuclear layer (pH = 7.14 at —140 jum), then increased to —7.28 (at —310 pm) at the vitreous retinal border. The peak [H+] concentration in this layer indicated that a net production of proton occurred across the avascular outer retina [76],... 

Figure 6.6 Anthrocyanidins impart colour to many natural substances, such as strawberries and cherries. The choice of side chains can cause a huge change in the anthrocyanidin s colour. If the side chain is pH sensitive then the anthrocyanidin acts as an acid-base indicator structures of an anthrocyanidin at three pHs (red in high acidity and low pH, blue in low acidity and high pH and mauve in inter-midiate pHs)...

These workers examined the mechanism of this reaction in detail (258). Burst kinetics were observed suggestive of the formation of an initial species from the catalyst precursor with subsequent slow turnover. The reaction was found to be pH sensitive, with a break point at pH 7.4, indicating a change in mechanism under these conditions. This pH corresponds to the expected value for secondary Cu(II) alkoxides. Based on this evidence, a formulated mechanism was advanced for this reaction, illustrated in Scheme 29. 

Certain water-soluble dyes are directly adsorbed onto cotton that has not been pretreated with a mordant (section 3.2.1). The first dye in which this phenomenon was observed was Congo red (4.57 Cl Direct Red 28), discovered in 1884 by Bottiger. The extreme pH sensitivity of this dye now restricts its use to that of an indicator, but it deserves mention as the forerunner of the direct dye class. 

Many commercially important polymers are produced via emulsion polymerization. This is also one of the most common methods to produce dye-doped beads. A dye is added to the mixture of monomers prior to initiating the polymerization and is either noncovalently entrapped or is copolymerized. The second method ensures that no leaching will occur from the particle but requires modification of the dye (typically by providing it with a double bond). This method is most common for preparation of pH-sensitive beads where a pH indicator is entrapped inside cross-linked polyacrylamide particles. The size of the beads can be tuned over a wide range so that preparation of both nano- and microbeads is possible. Despite thorough washing the surfactants are rather difficult to remove completely and their traces can influence the performance of some biological systems. 

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