PH measurement accuracy

The desired results for a practical measurement vary from application to application. In many pH measurements, accuracy within 0.1 pH unit is more than sufficient while 0.01 pH is required in some other more accurate tests. Reaching an accuracy of less than 0.01 pH will be a challenge for most measurement systems. 

NaCl solution. In one set of experiments, the slurry was titrated with 0.1 normal NaOH solution in one cm increments to a pH of about 10. The samples of this set are referred to as the "slurry" samples (titration of the calcined zeolite - NaCl solution slurry). In another set of experiments, the calcined zeolite - NaCl solution slurry was filtered, the filter cake washed with about 100 cnP distilled water, and the combined filtrates were titrated with 0.1 N NaOH solution, again to a pH of about 10. The samples of this latter set are referred to as the "filtrate" samples (the zeolite being removed by filtration prior to titration of the filtrate). In addition to the manual titrations, automated potentiometric titration curves were obtained with a Metrohm E636 Titroprocessor, which has an estimated pH measurement accuracy of 0.001 pH unit and an estimated volumetric addition accuracy of 0.001 cm ... 

The second question concerns the quality of the chemical control, directed more at the chemical analysis proper and its procedure. Important factors here are sufficient specificity and accuracy together with a short analysis time. In connection with accuracy, we can possible consider the quantization of the analytical information obtainable. For instance, from the above example of titration, if we assume for the pH measurement an accuracy of 0.02, an uncertainty remains of 0.04 over a total range of 14.0, which means a gain in information of n1 = 14.0/0.04 = 350 (at least 8 bits) with an accuracy of 5% as a mean for the titration end-point establishment of both acids, the remaining uncertainty of 1% over a range of 2 x 100% means a gain in information of n2 = 200 (at least 7 bits), so that the two-dimensional presentation of this titration represents a quantity of information I = 2log nx n2 = 15 bits at least. 

Accuracy is a measure of how close the result is to the true value while reproducibility or precision is a measure of how close a series of measurements on the same sample are to each other. The accuracy and reproducibility of pH measurements can be highly variable and are dependent on several factors electrode stability (drift and hysteresis), response slope/calibration curve, and accuracy of the pH meters. While some of these factors are determined by the properties of electrodes, some measures can be taken to improve measurement accuracy and reproducibility. 

The pH measurements of the mixed contents were made immediately after taking the soil and liquid samples. The presence of a very large amount of suspended sediment affected the accuracy and reproducibility of these pH measurements. The average values for each container are summarized in Table II. The deposited pesticides, the formulating agents and the decomposition products did not change the pH from that observed for ambient water except for... 

A. Kuseler, V. Baelum, O. Fejerskov, J. Heidmann, Accuracy and precision in vitro of Beetrode microelectrodes used for intraoral pH measurements. Caries Res. 27 (1993) 183-190. 

Therefore, analysts must remember that reliable and accurate pH measurements depend on the quality of the equipment used, the maintenance of the electrodes, the accuracy of the calibration, and the adherence to proper procedures. 

The difference in the conductivity of the calibration buffers and sample can cause a very large error on the sample measurement, due to junction potentials in different environments. Solid samples should be dissolved in purified water. It is necessary that the water be carbon dioxide-free. The presence of dissolved carbon dioxide will cause significant bias in the measurement of samples with low buffering capacity. For pH measurements with an accuracy of 0.01 to 0.1 pH unit, the limiting factor is often the electrochemical system (i.e., the characteristics of the electrodes and the solution in which they are immersed). 

In Table 11-1, the volume of acid added is designated Va. pH is expressed to the 0.01 decimal place, regardless of what is justified by significant figures. We do this for the sake of consistency and also because 0.01 is near the limit of accuracy in pH measurements. 

Experimental data for a titration of the amino acid glycine are given in Figure 13-13. The initial 40.0-mL solution contained 0.190 mmol of glycine plus 0.232 mmol of HC1 to increase the fraction of fully protonated + H3NCH2C02H. Aliquots of 0.490 5 M NaOH were added and the pH was measured after each addition. Volumes and pH are listed in columns A and B beginning in row 16. pH was precise to the 0.001 decimal place, but the accuracy of pH measurement is, at best, 0.02. 

Errors 1 and 2 limit the accuracy of pH measurement with the glass electrode to 0.02 pH unit, at best. Measurement of pH differences between solutions can be accurate to about 0.002 pH unit, but knowledge of the true pH will still be at least an order of magnitude more uncertain. An uncertainty of 0.02 pH unit corresponds to an uncertainty of 5% in J4H.,... 

Accuracy and Interpretation of Measured pH Values. To define the pH scale and pertnil the calibration of pH measurement systems, a scries of reference buffer solutions have been certified hy the U.S. National Institute of Standards and Technology iNIST). The acidity function which is the experimental basis for the assignment of pH. is reproducible within about O.IKl.I pH unit from It) to 40T. However, errors in the standard potential of the cell, in the composition of the buffer materials, and in the preparation of the solutions may raise the uncertainty to 0 005 pH unit. The accuracy of ihe practical scale may he furthei reduced to (I.Ot)X-(l.(ll pH unit as a result of variations in the liquid-junction potential. 

It is desirable to avoid errors in pH measurement, which limit the accuracy of the above NMR pH titration. If an NMR titration is applied to a mixture of two acids, HA and HA, each of whose chemical shifts, 6 and S, follows Equation (13), it is possible to eliminate pH from the two equations and replace it with n, the number of equivalents of titrant added. Thus Ellison and Robinson obtained Equation (14), where A = 6-6, A = 6 -6, A° = <5° -S °, and R = KJK1 20 Moreover, it is not necessary to prepare solutions of exact molarity, because n can be evaluated more readily as (6-6 )/(6° -6 ), from the variation of the chemical shift of HA during the titration. When R is near 1, this is approximately a parabolic dependence of A on n. The titration of a mixture of formic acid and 1802-formic acid permitted the evaluation of the lsO IE on acidity from the 13C NMR chemical shifts 6 and S of the carboxyl carbons. In practice, this involved fitting to the three parameters A-, A°, and R. This same equation was used with 31P chemical shifts to evaluate the lsO IE on the acidity of phosphoric acid and alkyl phosphates.21... 

In most applications, the use of a high-accuracy PS for pH measurement will not be justified if a traceable secondary standard of sufficient accuracy is available. It is therefore recommended to derive secondary pH standards, pH(SS), from the pH(PS) buffer solutions. 

Considering the definition presented, the accuracy of pH measurements of the skin is questionable. There are two inconsistencies emerging from the definition of pH. First, the skin, especially the epidermis, is not a diluted aqueous solution. Moreover, various residues located on the skin surface may influence the readings, if conducted by devices not designed for existence of many different substances. 

Last but not least is the e.s.r. method discussed in Section 5. The accuracy of the e.s.r. method itself is very high, and the overall accuracy is limited by that of the pH measurements, whereas in the previous techniques, the determination of other parameters is less accurate than the pH determinations. As mentioned above, the e.s.r. 

Several recent NMR studies have alloHed the identification of about tnenty silicate oligomers and quantitative or semi-quantitative estimates of their respective concentrations. In spite of the precautions used by some authors, the measured concentrations and in particular the experimental monomer concentration (used as a basis in our calculations) are probably not alaays very accurate expecial-ly in the case of the less alkaline or the more concentrated (in silica) solutions. The use of an inert silicon-containing internal standard could perhaps be useful. To this lack of accuracy is added the uncertainty of the pH measurement in such strongly alkaline solutions. 

While the above approach works very well at temperatures below 100 °C, it is difficult to apply the IUPAC recommendations at temperatures above 100 °C when a high-temperature system should be pressurized. Definitely, at temperatures below 300 °C the HECC represented by (25) can be employed for pH measurements in the solutions where the half-reaction of the Pt(H2) electrode is a reversible process. Accuracy of the measurements could be 0.01 pH units and is mainly limited from estimating the diffusion potential in Eq. (26). 

Direct Photometric Methods. Absorption of ultraviolet (UV) light at 200 to 225 nm and 270 to 290 nm is used to measure the protein content of biological samples. Absorption of UV light at 280 nm depends chiefly on the aromatic rings of tyrosine and tiyptophan at pH 8. Accuracy and specificity suffer from an uneven distribution of these amino acids among individual proteins in a mixture and from the presence in body fluids of free tyrosine and tryptophan, uric... 

The elements of good quality assurance of blood gas and pH measurements include proper maintenance of the instrument, use of control materials, verifying electrode linearity, checking barometer accuracy, and accurately measuring temperature. 

The spectrophotometric method is very well suited to the measurement of pH. The accuracy naturally is greater than when comparisons are made with the eye. The method may be... 

A procedure is presented for estimation of uncertainty in measurement of the pK(a> of a weak acid by potentiometric titration. The procedure is based on the ISO GUM. The core of the procedure is a mathematical model that involves 40 input parameters. A novel approach is used for taking into account the purity of the acid, the impurities are not treated as inert compounds only, and their possible acidic dissociation is also taken into account. Application to an example of practical pK(a> determination is presented. Altogether, 67 different sources of uncertainty are identified and quantified within the example. The relative importance of different uncertainty sources is discussed. The most important source of uncertainty (with the experimental set-up of the example) is the uncertainty of the pH measurement followed by the accuracy of the burette and the uncertainty of weighing. The procedure gives uncertainty separately for each point of the titration curve. The uncertainty depends on the amount of the titrant added, being lowest in the central part of the titration curve. The possibilities of reducing the uncertainty and interpreting the drift of the pKJa) values obtained from the same curve are discussed. 

G. K. McMiUan, Understand some basic truths of pH measurement, Chein. Eng. Prog. 87 (10), 30-7 (1991). (Why actual accuracy of pH electrodes in most industrial applications falls far short of the level expected). 

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