PH meter calibration

Once the electrodes have been prepared for a given aqueous-organic solvent, the pan determinations can be made at each temperature, either graphically or by direct reading on commercial pH meters calibrated for poH measurements. In this procedure the pH meter is used as a milli-voltmeter. A solution A (10 M HCl in the aqueous-organic solvent considered) is selected as the standard reference solution, its pan being calculated for any temperature according to the Debye-Hilckel formula. After the electrodes have been immersed in this solution, the... 

Change buffer solutions for pH meter calibration frequently they deteriorate from mold growth and contamination. 

Scope This method is suitable to determine the pH of aqueous solutions. While pH meters, calibrated with aqueous solutions, are sometimes used to make measurements in semia-queous solutions or in nonaqueous polar solutions, the value obtained is the apparent pH value only and should not be compared with the pH of aqueous solutions. For nonpolar solutions, pH has no meaning, and pH electrodes may be damaged by direct contact with these solutions. References to the pH of nonpolar solutions or liquids usually indicate the pH of a water extract of the nonpolar liquid or the apparent pH of a mixture of the nonpolar liquid in a polar liquid such as alcohol or alcohol-water mixtures. 

Al(i-OC3H7)3 was dissolved at 60 °C in TPAOH (12 wt % in aqueous solution). The solution was cooled to room temperature, then Si(OC2Hs)4 in C2H5OH was added. As the monophasic clear sol was obtained, the pH value was measured with a Ross Sure-Flow Combination, Orion model 8172, after pH meter calibration with two buffers (pH 7 and 10). 

Once the electrodes have been prepared and calibrated for a given hydroorganic solvent, the paH determinations can be made at the required temperature, either graphically or by direct reading on commercial pH meters calibrated for paH measurement. 

Control of a number of physicochemical factors is critical to achieving maximal precision and accuracy in measured values. These include the choice of e>q)eri-mental method, pH meter calibration, temperature control, solvent composition, ionic strength, absence of atmospheric CO2 contamination, estimation methods for activity coefficients, and chemical stability. 

Temperature dependence equations for pK and pK values show the importance of good temperature control during pH meter calibration as well as in measurements. To a greater or lesser extent, all acid-base reactions vary with temperature. The effect of temperature changes for equilibrium reactions are closely described by the Valentiner-Lannung equation [80-82] ... 

NB No details were given of the pH meter calibration. Fluorometric measurements were used to assess the pJCa values for flie first excited state. Used previously described spectrophotometric titration procedures Cappomacchia A, Casper J and Schulman SG, /. Pharm. ScL, 63, 1272-1276 (1974). 

I = 0.01 (KCl) NB Metal-free solutions of die tetracycline were titrated with N2 atmosphere standard NaOH solution and ttie pH measured. No details were given of the pH meter calibration. Metal stability constants were determined from identical titrations in the presence of varying concentrations of nickel(II), zinc(II) or copper(II) ions. 

Tam KY andTakacs-Novac K, Multi-wavelengfii spectrophotometric determination of acid dissociation constants. Anal. Chim. Acta, 434, 157-167 (2001). NB A new spectrophotometric method was validated by comparison wifii file standard method of Albert and Serjeant, using 10 M solutions in 0.15M KCl. No details of pH meter calibration. In the novel method, operational pH values were converted to p[H] by a multi-parametric equation (ref. 12, Avdeef A). Sample concentrations of 10-100 uM were titrated in a Sirius GLpKa instrument and pH data were collected when drift was <0.02 pH/min. 15-25 wavelengfiis were used for each measurement. Note that errors are for reproducibility only, not accuracy. 

N6 diese values were measured for use with data on the kinetics for hydrolysis of d ese esters. Details given for pH meter calibration in die kinetics section and it is assumed that the same procedures were used for d e pK measurements. An experimental method was used to estimate activity coefficients at I = O.IM. 

NB No details provided of pH meter calibration for eidier method. No demonstration that saturation had been reached in the solubility-pH mediod. Use was not made of the improved data handling procedure for the solubility method proposed by Lewis (1984) based on Zimmermann (see introductory text, refs. 52 and 53). 

It is critical that pH electrodes are calibrated often as the slope of their response curve will change with temperature, as defined by the Nernst equation [2], Calibration should be performed with at least two certified pH calibration buffers that bracket the expected pH of the sample. Calibration is performed as described in the manufacturer s instructions provided with the pH meter. Calibration should be checked by measuring the pH of the first calibration buffer. The result should be no more than 0.02 pH units different from the certified pH of the buffer after adjusted for temperature. pH measurements are best performed when calibration buffers and samples are held at a constant temperature. 

Variability in the liquid junction potential (Section 15-2) was hypothesized to lead to the variability in the pH measurements. Standard buffers used for pH meter calibration typically have ion concentrations of -0.05 M, whereas rainwater samples have ion concentrations two or more orders of magnitude lower. To test the hypothesis that junction potential caused systematic errors, a pure HCl solution with a concentration near 2 X lO"" M was used as a pH calibration standard in place of high ionic strength buffers. The data in panel b were obtained, with good results from all but the first lab. The standard deviation of all 17 measurements was reduced from 0.077 pH unit with the standard buffer to 0.029 pH unit with the HCl standard. It was concluded that junction potential is the cause of most of the variability between labs and that a low ionic strength standard is appropriate for rainwater pH measurements. 

Moore EW. Determination of pH by the glass electrode pH meter calibration for gastric analysis. Gastroenterology 54 501-501, 1968. 

For pATa determinations in heavy water, pD values are obtained using a pH meter calibrated either against standard pD buffers, or against pH buffers and then applying the correction pD = pH + 0.4. 

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