End point determination

A 0.2521-g sample of an unknown weak acid is titrated with a 0.1005 M solution of NaOH, requiring 42.68 mL to reach the phenolphthalein end point. Determine the compound s equivalent weight. Which of the following compounds is most likely to be the unknown weak acid ... 

The precision of the end point signal depends on the method used to locate the end point and the shape of the titration curve. With a visual indicator, the precision of the end point signal is usually between +0.03 mb and 0.10 mb. End points determined by direct monitoring often can be determined with a greater precision. 

A 250.0-mg sample of an organic weak acid was dissolved in an appropriate solvent and titrated with 0.0556 M NaOH, requiring 32.58 ml to reach the end point. Determine the compound s equivalent weight. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

A suitable functional group is assayed in the same sample. In general chemistry and many polymer applications, this is merely the titration of acid groups with a base, or vice versa. Note that only volumetric glassware and a method for end point determination are required to do this. 

Prepare an approximately 0.1 M silver nitrate solution. Place 0.1169 g of dry sodium chloride in the beaker, add 100 mL of water, and stir until dissolved. Use a silver wire electrode (or a silver-plated platinum wire), and a silver-silver chloride or a saturated calomel reference electrode separated from the solution by a potassium nitrate-agar bridge (see below). Titrate the sodium chloride solution with the silver nitrate solution following the general procedure described in Experiment 1 it is important to have efficient stirring and to wait long enough after each addition of titrant for the e.m.f. to become steady. Continue the titration 5 mL beyond the end point. Determine the end point and thence the molarity of the silver nitrate solution. 

Lippi et. al (87) and Dirstine (88) circumvented titration by converting the liberated fatty acids into copper salts, which after extraction in chloroform are reacted with diethyldithio-carbamate to form a colored complex which is measured photometrically. While the end point appears to be more sensitive than the pH end point determination, the advantages are outweighed by the additional steps of solvent extraction, centrifugation and incomplete extraction when low concentrations of copper salts are present. Other substrates used for the measurement of lipase activity have been tributyrin ( ), phenyl laurate (90), p-nit ro-pheny1-stearate and 3-naphthyl laurate (91). It has been shown that these substrates are hydrolyzed by esterases and thus lack specificity for lipase. Studies on patients with pancreatitis indicate olive oil emulsion is definitely superior to water soluble esters as substrates for measuring serum lipase activity. 

During the 1970 s and early 1980 s a large number of test methods were developed to measure the toxic potency of the smoke produced from burning materials. The ones most widely used are in refs. 29-32. These tests differ in several respects the conditions under which the material is burnt, the characteristics of the air flow (i.e. static or dynamic), the type of method used to evaluate smoke toxicity (i.e. analytical or bioassay), the animal model used for bioassay tests, and the end point determined. As a consequence of all these differences the tests result in a tremendous variation of ranking for the smoke of various materials. A case in point was made in a study of the toxic potency of 14 materials by two methods [33]. It showed (Table I) that the material ranked most toxic by one of the protocols used was ranked least toxic by the other protocol Although neither of these protocols is in common use in the late 1980 s, it illustrates some of the shortcomings associated with small scale toxic potency of smoke tests. 

More recently, a colorimetric-based LAL procedure has been devised. This entails addition to the LAL reagent of a short peptide, susceptible to hydrolysis by the LAL clotting enzyme. This synthetic peptide contains a chromogenic tag (usually para-nitroaniline, pNA) which is released free into solution by the clotting enzyme. This allows spectrophotometric analysis of the test sample, facilitating more accurate end-point determination. 

Derivative plots such as that shown in Figure 4.4 can greatly increase the accuracy of the end point determination, provided that a sufficient amount of data is obtained. We need to note that the rate of change of emf with volume V is often very large near the equivalence point, and so it is easy to miss some of the data. 

P. Frake, D. Greenhalgh, S.M. Grierson, J.M. Hempenstall and D.R. Rudd, Process control and end-point determination of a flnid bed granulation by apphcation of near infra-red spectroscopy, Int. J. Pharm., 151, 75-80... 

M. Whitaker, G.R. Baker, J. Westrup, PA. Gonlding, D.R. Rndd, R.M. Belchamber and M.P Collins, Apphcation of aconstic emission to the monitoring and end point determination of a high shear granulation process, Int. J. Pharm., 205, 79-91 (2000). 

For pharmacological, toxicological and transport studies it is of utmost importance to assess not only the viability but also the functionality of the liver slices. This is essential both for end-point determination of toxic cell damage, and to assess the quality of the tissue during in-... 

Selected entries from Methods in Enzymology [vol, page(s)] Median effective dose, 235, 29, 31, 33 determination of median infectious dose, 235, 29-39 median lethal dose, 235, 29 moving average interpolation, 235, 32-34, 36-37, 39 probit analysis, 235, 31-36, 39 Reed-Muench method, 235, 30, 33-35 staircase method, 235, 33, 37-39 median response, 235, 29 dose range for animal experiments, 235, 29 50% end-point determination, 235, 29-30. 

Frake P, Greenhlgh D, Grierson SM, Hempenstall JM, Rudd DR. Process control and end-point determination of fluid-bed granulation by application of near-infrared spectroscopy. Int J Pharm 1997 151 75-80. 

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