Sample pH value

The standard procedure for a two-point calibration of a pH electrode is to select two buffers that bracket the expected sample pH value, whose difference in pH does not exceed 4 units. One of these buffers must be at pH 7. The pH of the second buffer solution should be within 0.07 pH unit of the manufacturer s stated value. If a larger deviation is noted, examine the electrodes and replace them if they are faulty. Adjust the slope or temperature control to make the pH value observed identical with the value stated. Standardization should be repeated until both buffer solutions for standardization give observed pH values within 0.02 pH unit of the stated value without further adjustment of the controls. 

Disposal and drainage waters are among the few matrices in which free sulfide can be determined. Depending on the sample pH value, the transition and heavy metals present in these samples form sparingly soluble precipitates with sulfide. When the pH value of the sample is changed via dilution, the chemical equilibrium in the sample shifts, which could result in an increased amount of free sulfide. However, difficulties in... 

Figure 2. Dependence of unfolding rate constants on pH. Wild type tailspike protein was prepared in 50 mM Tris, 1.7 mM 2-mercaptoethanol and 2% SDS and adjust to different pH values by 1 N HCl. Thermal unfolding was done at 65°C and followed by SDS-PAGE at about 20°C. Sample pH values shown here have been corrected to 65°C. kj (a) and k2 ( ) shown in log are the thermal unfolding rate constants for the conversions from N to I and from I to M, respectively. The linear lines through the data points are the results of least-square fit to each individual pH phase for both kj and k2 data. The calculated slopes of the fitting lines for kj are -0.46 and 0.35 for the low and high pH phases, respectively and for k2 are -1.9 and 1.1 for the low and high pH phases, respectively.

For serum measurement the sensor system is calibrated with urea dissolved in 0.0185 mol/1 Tris-HCl buffer, pH 7.0 ((3 = 8 mmol/l). In this manner, disturbances caused by individually different sample pH values and variations in the buffer capacity are kept within the noise limits of the measuring system. Comparison with the Berthelot method resulted in the following equation ... 

The imprinted template in the MIP can be easily removed with an acidic solution, leaving cavities complementary to the 3D shape of the template. The approach is fast (total processing time less than 3 h) and has potential for easy mass production. More interestingly, the prepared MIP array exhibited several favorable features such as high specificity, superb tolerance for interference and the applicability to a wide range of sample pH values. 

Because of these influences, a sample pH value cannot be extrapolated to another temperature or dilution. If the pH value of a particular solution is known at 40°C, it is not automatically known at 25 C. The standard buffer solutions were studied at different temperatures and compositions to define their activity, and unless the same is done for a sample, its pH under different conditions is not known because of these variables. 

Secondary standard pH buffer solutions which do not meet the requirements of primary standard solutions but provide coverage of the pH range not covered by primary standards. Used when the pH value of the primary standard is not close to the sample pH value. 

One potentially powerfiil approach to chemical imaging of oxides is to capitalize on the tip-surface interactions caused by the surface charge induced under electrolyte solutions [189]. The sign and the amount of the charge induced on, for example, an oxide surface under an aqueous solution is detenuined by the pH and ionic strength of the solution, as well as by the isoelectric point (lEP) of the sample. At pH values above the lEP, the charge is negative below this value. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Both openings of the flask are stopped with cotton wool and the medium is sterilized by placing it in an autoclave for 30 minutes at 120°C. The flask is then cooled to 29°C to 30°C and a small sample is taken to check the sterility and the pH value which should be approximately 5. 

It is known that polymers may exist in various stationary states, which are defined by the amount and distribution of intermolecular bonds in the sample at definite network structure. The latter is defined by the conditions of storage, exploitation, and production of the network. That is why T values may be different. The highest value is observed in the equilibrium state of the system. In this case it is necessary to point out, that the ph value becomes close to the ph one at n,. 

While certain reservations must be kept in view (i.e. there is not necessarily a correlation between pH and corrosivity, and different samples of the same species of wood show a wide scatter of pH values, which might well be even wider if differences in duration of seasoning were taken into account), the results of vapour corrosion tests nevertheless indicate a general correlation between quoted pH values and the corrosiveness of wood vapours. It may reasonably be concluded that a strongly acid wood, pH less than 4-0, is potentially dangerous, and a less acid wood, pH more than 5 0, is likely to be relatively safe. 

Measurement of Acidity or Alkalinity of Explosives. (Measurement of pH value). These tests consist essentially of extrg the sample of expl with w (either cold in the case of liquids such as NG, or hot for solids such as TNT, Tetryl, PA, etc), and detg the acidity of the resulting extr either by titration, colorimetric methods or by means of a pH meter. These tests are described under the individual compds... 

The pH of several solutions was measured in a hospital laboratory convert each of the following pH values into the molarity of H,CT ions (a) 5.0 (the pH of a urine sample) ... 

Fig. 7.—A Portion of the Aliphatic Region of the Proton-decoupled, C-N.m.r. Spectra (at 22.5 MHz) of Fully Reductively [ C]Methylated Glycophorin A, Asialoglycophorin A, and Deglycosylated Glycophorin A , at pH Values of 4.0, 7.3, and 9.0, Respectively. [Time-domain data were accumulated in 8192 addresses, with a recycle time of 1 s. A digital broadening of 3.0 Hz was applied to the data. All glycophorin samples were —1.2 mAf (in H2O), and required 10,000-20,000 accumulations. (Taken from Ref. 57.)]...

For many situations, a simple total anthocyanin determination is inappropriate because of interference from polymeric anthocyanins, anthocyanin degradation products, or melanoidins from browning reactions. In those cases, the approach has been to measure the absorbance at two different pH values. The differential method measures the absorbance at two pH valnes and rehes on structural transformations of the anthocyanin chromophore as a function of pH. Anthocyanins switch from a saturated bright red-bluish color at pH 1 to colorless at pH 4.5. Conversely, polymeric anthocyanins and others retain their color at pH 4.5. Thus, measurement of anthocyanin samples at pH 1 and 4.5 can remove the interference of other materials that may show absorbance at the A is-max-... 

This concept was introduced in 1948 by Sondheimer and Kertesz who nsed pH values of 2 and 3.4 for analyses of strawberry jams. Since then, the nse of other pH values has been proposed. Fnleki and Francis (1968) used pH 1 and 4.5 bnffers to measure anthocyanin content in cranberries, and modifications of this techniqne have been applied to a wide range of commodities. The absorbance of anthocy-anins at two different pH levels (1 and 4.5) is measured at Kis-max and at 700 mn, which corrects for haze in the sample (Table 6.3.1). The total monomeric anthocy-... 

Amino acids have been found in samples of river water (Lee et al. 2006), and chlorination of glycine may produce cyanogen chloride, via A,A-dichloroglycine. At pH values >6, this is converted into cyanogen chloride, whereas at lower pH values A-chloromethylimine is formed (Na and Olson 2006). 

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