Stoichiometry titration

Until the second equivalence point, we can obtain the analytical concentration of HM and from the titration stoichiometry. At 25.01 mL, the values are calculated as... 

Although there are reports involving the use of hydrogen alone,161 the surface composition of supported Pt-Ru bimetallic catalysts are more commonly measured using a selective titration method.162-164 The titration stoichiometry of the reaction between chemisorbed oxygen and gaseous CO is different for the two metals the ratio of surface metal/02/C0/C02 is 1/0.5/2/1 for Pt and 1/1/1/0.3 on Ru.164 These ratios are independent of surface composition and the concentration of Ru and Pt in the surface can be calculated from the equations ... 

This technique provided three advantages it increased sensitivity by 3-fold, it eliminated a concern about oxygen contamination, and it was more applicable than H2 chemisorption to used catalysts, which might have some contamination. The small amoimts of H2O formed were adsorbed by the support or by the walls of the cell if T < 273 K [27,28]. Subsequent studies indicated that for very highly dispersed Pt catalysts this titration stoichiometry could change from Oad Had Ht tr = 1 1 3 (Eq. 3.9) to 1 2 4 [60]. Wilson and Hall then showed that chemical equation 3.6 for large Pt crystallites shifts to chemical equation 3.5 for small Pt particles, which then gives a 1 2 4 ratio [51], i.e., the titration reaction becomes... 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Knowing the stoichiometry of the titration reaction(s), we can calculate the moles of analyte. 

Almost any chemical reaction can serve as a titrimetric method provided that three conditions are met. The first condition is that all reactions involving the titrant and analyte must be of known stoichiometry. If this is not the case, then the moles of titrant used in reaching the end point cannot tell us how much analyte is in our sample. Second, the titration reaction must occur rapidly. If we add titrant at a rate that is faster than the reaction s rate, then the end point will exceed the equivalence point by a significant amount. Finally, a suitable method must be available for determining the end point with an acceptable level of accuracy. These are significant limitations and, for this reason, several titration strategies are commonly used. 

This reaction occurs quickly and is of known stoichiometry. A titrant of SCN is easily prepared using KSCN. To indicate the titration s end point we add a small amount of Fe + to the solution containing the analyte. The formation of the red-colored Fe(SCN) + complex signals the end point. This is an example of a direct titration since the titrant reacts with the analyte. 

The equivalence point of a complexation titration occurs when stoichiometri-cally equivalent amounts of analyte and titrant have reacted. For titrations involving metal ions and EDTA, the equivalence point occurs when Cm and Cedxa are equal and may be located visually by looking for the titration curve s inflection point. 

This is simplified for titrations involving EDTA where the stoichiometry is always 1 1 regardless of how many electron pairs are involved in the formation of the metal-ligand complex. 

At the equivalence point, the titration reaction s stoichiometry requires that... 

Where Is the Equivalence Point In discussing acid-base titrations and com-plexometric titrations, we noted that the equivalence point is almost identical with the inflection point located in the sharply rising part of the titration curve. If you look back at Figures 9.8 and 9.28, you will see that for acid-base and com-plexometric titrations the inflection point is also in the middle of the titration curve s sharp rise (we call this a symmetrical equivalence point). This makes it relatively easy to find the equivalence point when you sketch these titration curves. When the stoichiometry of a redox titration is symmetrical (one mole analyte per mole of titrant), then the equivalence point also is symmetrical. If the stoichiometry is not symmetrical, then the equivalence point will lie closer to the top or bottom of the titration curve s sharp rise. In this case the equivalence point is said to be asymmetrical. Example 9.12 shows how to calculate the equivalence point potential in this situation. 

By now you are familiar with our approach to calculating titration curves. The first task is to calculate the volume of Ag+ needed to reach the equivalence point. The stoichiometry of the reaction requires that... 

Data from the spectrophotometric titrations of Fe + with SCN , and of Cu + with EDTA are used to determine the stoichiometry of the resulting complexes using the method of continuous variations. 

Consistent with this, dissolution of KF increases the conductivity and KIFe can be isolated on removal of the solvent. Likewise NOF affords [NO]+[IF6] . Antimony compounds yield ISbFio, i-2. [IF4]+[SbF6], which can be titrated with KSbFfi. However, the milder fluorinating power of IF5 frequently enables partially fluorinated adducts to be isolated and in some of these the iodine is partly oxygenated. Complete structural identification of the products has not yet been established in all cases but typical stoichiometries are as follows ... 

The discussion of acid-base titrations in Chapter 4 focused on stoichiometry. Here, the emphasis is on the equilibrium principles that apply to the acid-base reactions involved. It is convenient to distinguish between titrations involving—... 

However, it is best to use the full chemical equation when working with titrations to ensure the correct stoichiometry. For example, if hydrochloric acid is used to neutralize Ca(OH)2, we must take into account the fact that each formula unit of Ca(OH)2 provides two OH ions ... 

We can predict the pH at any point in the titration of a polyprotic acid with a strong base by using the reaction stoichiometry to recognize what stage we have reached in the titration. We then identify the principal solute species at that point and the principal proton transfer equilibrium that determines the pH. 

The chemical compositions of the samples, obtained from chemical analyses are reported in Table 1. In order to check the chemical analyses, the mother and washing liquors were collected, analysed and their acidity was titrated. In all cases, the alkaline cations were detected only as traces. The acidimetric titration allowed us to determine the HPA amount remaining in the solution. On the other hand, the samples separated after precipitation and washings were weighted in order to calculate the precipitate yields. The results are reported in table 1 where the samples are designated as MxY (M being the alkaline or ammonium cation, Y the heteroatom, x the stoichiometry deduced from chemical analyses. 

This method is primarily based on measurement of the electrical conductance of a solution from which, by previous calibration, the analyte concentration can be derived. The technique can be used if desired to follow a chemical reaction, e.g., for kinetic analysis or a reaction going to completion (e.g., a titration), as in the latter instance, which is a conductometric titration, the stoichiometry of the reaction forms the basis of the analysis and the conductometry, as a mere sensor, does not need calibration but is only required to be sufficiently selective. 

By plotting i versus the ratio R = (CHX)t/(CB)t during the titration, they determined simultaneously the extent of acid-base interaction, the stoichiometry of that interaction and the degree of association of the acid-base adduct. Fig. 4.13 shows hypothetical titration curves line ABC corresponds to the interaction between B and HX as monomers without further reaction between BHX and HX, and the subsequent occurrence of the latter reaction to a small extent is indicated by the line ABC and to the full extent by line ABDE, when no more HX can react with BHX HX line AFDE arises when formation of BHX HX starts right away in the case of previous partial dimerization of B, the various lines will begin at A instead of A. 

Spontaneous self assembly of a dinuclear triple helical complex is observed with linked bis-[4,5]-pineno-2,2 -bipyridines. Studies by electrospray mass spectrometry, CD and NMR determined that the major species in solution was a complex of Zn L = 2 3 stoichiometry with a triple helical structure and an enantiomerically pure homochiral configuration at the metal centers. The preference for the formation of one of the possible stereoisomers over the other is of interest.265 Another binuclear triple helical complex is formed from zinc addition to bis[5-(l-methyl-2-(6-methyl-2 -pyridyl)benzimidazolyl)]methane. Spectrophotometric titrations with a zinc solution... 

The substitution-inert character of the metal(III) ion in the second transition series has already been discussed in 2.3. However, interesting behavior has been reported by Kasahara et al. [23], who found that a p-diketone coordinated to the central Ru(III) could easily be replaced by an acetonitrile with the aid of a strong acid. When the reaction was conducted in acetonitrile, its stoichiometry was confirmed by means of spectrophotometrie titration as follows ... 

By correlating the observed spectral changes with the concentrations of added cycloamylose, dissociation constants of the cycloamylose-substrate adducts may be calculated (Rossotti and Rossotti, 1961). Values of the dissociation constants determined in this manner for a variety of complexes are presented in Table II. In most cases, stoichiometries of the complexes have been shown to be 1 1 from the presence of distinct isosbestic points in the spectrophotometric titrations. In a few cases, additional spectral perturbations are observed as the cycloamylose concentration is increased, indicating more complex modes of association. Methyl orange, for example,... 

Chemisorption measurements (Quantachrome Instruments, ChemBET 3000) were conducted in order to determine the metal (Co) dispersion. Therefore, the nanomaterial catalysts were reduced under a hydrogen flow (10% H2 in Ar) at 633 K for 3 h. The samples were then flushed with helium for another hour at the same temperature in order to remove the weakly adsorbed hydrogen. Chemisorption was carried out by applying a pulse-titration method with carbon monoxide as adsorbing agent at 77 K. The calculation of the dispersion is based on a molar adsorption stoichiometry of CO to Co of 1. 

Such an electrochemical arrangement can also be used to transport oxygen from one electrode to the other by the imposition of an externally applied potential. This technique, known as coulometric titration , has been used to prepare flowing gas mixtures of oxygen/argon with a controlled oxygen partial pressure, to vary the non-stoichiometry of oxides, to study the thermodynamics of dilute oxygen solutions in metals, and to measure the kinetics of metal oxidation, as examples. 

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