Poisoning Titration

Poisoning Titration. The number of active centres on platinum responsible for 

Bond and P. A. Sermon, React. Kinet. Catal. Lett, 1974, 1, 3. 

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Mass transfer problems can be addressed in several simple ways by varying the stirring rate, varying the amount of catalyst, varying the temperature, grinding, and poisoning titration. Several of these are discussed in an article by Roberts.48... 

Dispersion is defined as the fraction of active atoms in surface positions. Higher dispersions are achieved either as monolayers of atomically dispersed material or as very small crystallites. Both exist in practice. In either case, measurement of surface concentration is most easily performed by quantitative probing with selective molecules. There are three common approaches measurement of (I) chemisorption isotherms. (2) reaction titration, and (3) poison titration. 

Poison titration is a convenient way to measure the concentration of active sites. The best procedure is to use a simple pulse reactor, such as that in Fig. 7.26. Pulses of a poisoning agent are injected between reactant pulses. If all the poison adsorbs irreversibly, then activity declines with each pulse. Typical results arc shown in Fig. 7.27, in which hydrogen sulfide poisons metal sites. Extrapolation of the activity curve to zero gives the amount of poison necessary to neutralize the active sites. A knowledge of surface stoichiometry is necessary to proceed further. For example, in Fig. 7.27 the assumed ratio was two nickel for each sulfur. This technique has the potential for innovative application to many systems. 

These are rapid, simple reactors that offer economy of feed and provide protection against process deactivation. A typical scheme is given in Fig. 7.26. The operation is transient so that concentrations change over the surface. Kinetic interpretations are difficult for any but first-order rate equations. Pulse reactors do have utility in poison titration measurements, in determining activities of the fresh surface, and for following surface conditioning. 

Many such poisoning titrations have been performed over the years and the results vary widely with the poison chosen and the catalyst used [377]. Some other examples are listed in Table 7. Clearly, some poisons are more selective than others. In Figures 29 and 30, one can see the typical shape of the titration curve as different types of poisons were added incrementally to a polymerization reaction. The activity, on a relative basis, is plotted against the poison stoichiometry (number of poison molecules added per Cr atom). All of the curves have the classical poisoning profile. Benzene was a mild poison, whereas oxygen was a severe one. 

The knowledge of the colloidal chemistry of gold was used by Kim and Turkevich(106) to prepare monodisperse palladium particles in diameter greater than 75 A. The number of surface atoms determined by electron microscopy was found to be equal to the number of catalytic centers determined by poison titration for the athylene hydrogenation reaction. The surface of the palladium catalyst was homogenous. The velocity of the catalytic reaction was found to be proportional to the number of surface atoms. All surface atoms were active. 

Zinc cyanide [557-21-1] M 117.4, m 800"(dec), d 1.852. It is a POISONOUS white powder which becomes black on standing if Mg(OH)2 and carbonate are not removed in the preparation. Thus wash well with H2O, then well with EtOH, Et20 and dry in air at 50°. Analyse by titrating the cyanide with standard AgN03. Other likely impurities are ZnCl2, MgCl2 and traces of basic zinc cyanide the first two salts can be washed out. It is soluble in aq KCN solns. However, if purified in this way Zn(CN)2 is not reactive in the Gattermann synthesis. For this the salt should contain at least 0.33 mols of KCl or NaCl which will allow the reaction to proceed faster. [J Am Chem Soc 45 2375 1923, 60 1699 1938-, Org Synth Coll Vol III 549 1955.]... 

Determination of calcium. Pipette two 25.0 mL portions of the mixed calcium and magnesium ion solution (not more than 0.01M with respect to either ion) into two separate 250 mL conical flasks and dilute each with about 25 mL of de-ionised water. To the first flask add 4 mL 8 M potassium hydroxide solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 30 mg each of potassium cyanide (Caution poison) and hydroxylammonium chloride and swirl the contents of the flask until the solids dissolve. Add about 50 mg of the HHSNNA indicator mixture and titrate with 0.01 M EDTA until the colour changes from red to blue. Run into the second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1 mL. Now add 4 mL of the potassium hydroxide solution, mix well and complete the titration as with the first sample record the exact volume of EDTA solution used. Perform a blank titration, replacing the sample with de-ionised water. 

Notes. (1) For elementary students, it is sufficient to weigh out accurately about 1.25 g of arsenic(III) oxide, dissolve this in 50 mL of a cool 20 per cent solution of sodium hydroxide, and make up to 250 mL in a graduated flask. Shake well. Measure 25.0 mL of this solution by means of a burette and not with a pipette (caution — the solution is highly poisonous) into a 500 mL conical flask, add 100 mL water, 10 mL pure concentrated hydrochloric acid, one drop potassium iodide solution, and titrate with the permanganate solution to the first permanent pink colour as detailed above. Repeat with two other 25 mL portions of the solution. Successive titrations should agree within 0.1 mL. 

Using a burette or a pipette with a safety pump (this is necessary owing to the poisonous properties of the solution) measure out 25.0 mL of the arsenite solution into a 250 mL conical flask, add 25-50 mL of water, 5g of sodium hydrogencarbonate, and 2 mL of starch solution. Swirl the solution carefully until the hydrogencarbonate has dissolved. Then titrate slowly with the iodine solution, contained in a burette, to the first blue colour. 

After acidification with H3PO3 the extremely poisonous HCN and H2S formed may be evaporated by refluxing under a fume hood subsequently the thiocyanate can be titrated [10, 11]. The degradation of the polysulfide in the reactions at Eqs. (35) and (36) results in discoloration of the solutions. 

These results, coupled with the chemisorption titrations, provide evidence that a new phase of palladium is formed with tin under hydrogen that is more resistant to sulfur poisoning. ... 

Iodimetric titration, 23 670-671, 676 Iodination, 22 274 Iodination reaction, 9 281 23 649 Iodine (I), 24 353-380. See also Blend iodine value 131I isotope analytical methods for, 24 367-368 catalyst poison, 5 257t... 

True and pseudo-cholinesterase. The above serum preparations contained both the true and pseudo- cholinesterases of Mendel and Rudney.1 The effect of di-isopropyl phosphorofluoridate on these components was examined separately by means of the specific substrates described by Mendel, Mundel and Rudney,2 using the titration method described above. Phosphorofluoridate (5 x 10 8m) gave an inhibition of 57 per cent of the activity towards 00045m acetylcholine, 30 per cent of the activity towards 0-0005 m acetyl-/ methyl-choline, and 40 per cent of that towards 0-005 m benzoylcholine, after incubating the enzyme with the poison for 5 min. Thus in these experiments there appeared to be no appreciable difference in sensitivity of the true and pseudo-cholinesterases of horse serum to phosphorofluoridates. 

The poison was found to be a white hygroscopic solid very soluble in water, partly soluble in methanol and ethanol, but insoluble in most nonpolar solvents such as ethyl and petroleum ethers. It showed no absorption in the ultraviolet and titration showed two pK values at 8.2 and 11.5. The optical rotation was about 130. The molecular formula was found to be... 

Sodium thiosulfate is a common analytical reagent used in iodometric titration to analyze chlorine, bromine, and sulfide. Other uses are in bleaching paper pulp, bleaching straw, ivory, and bones, for removing chlorine from solutions, silver extraction from its ores, a mordant in dyeing and printing textiles, and as an antidote to cyanide poisoning. 

Procedure Weigh a 10,00-g sample on a tared 4 watch glass and brush up all spilled crysts immediately, because they are very poisonous. Transfer the sample to a 4Q0 m beaker contg ca 150 ml distd w neutral to phpht and stir until completely dissolved. Titrate rapidly to colorless end-print with 0.1N sulfuric acid. Avoid overrunning the end point, which causes evoln of the very poisonous gas HNs... 

Oxalic acid is a poisonous compound but an excellent reducing agent that is used to remove rust stains and to reduce chemicals in the laboratory. Suppose that 25.00 mL of a solution of oxalic acid, H2C204 (4), is titrated with 0.100 M NaOH(aq) and that the stoichiometric point is reached when 38.0 mL of the solution of base is added. To find the molarity of the oxalic acid solution, we proceed as follows ... 

An apparently straightforward method for the determination of number and strength of acid sites consists of the determination of the amount of base required to poison catalytic activity for a model reaction. By means of plots of activity versus amount of added base, the number of acid sites is obtained from the threshold amount of base required to remove catalytic activity acid strength is gauged from the slope of the titration curve. This method can therefore be called a catalytic titration. 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

Fig. 12. Titration with poison of a catalyst surface (50). (Reprinted with permission of Elsevier/North-Holland Biomedical Press.)...

Br0nsted acid sites depends on the structure of the amine. Chemisorption data for amorphous oxides (54) show that 2,6-dimethylpyridine (which contains methyl groups that hinder coordination of the nitrogen atom with Lewis acids) is a more selective reagent for the determination of Br0nsted acidity than an unhindered amine such as pyridine. Jacobs and Heylen (44) arrived at a similar conclusion on the basis of an infrared study of amines chemisorbed on Y zeolite. They also found that the poisoning effectiveness of 2,6-dimethylpyridine is much greater than that of pyridine for the catalytic titration of Y zeolite. 

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