Precipitation Reactions and Titrations

Some chapters have been reordered to better tie together related topics. The chapter on basic tools and operations in anal3dical chemistry has been moved to the front of the text (Chapter 2), at the suggestion of a number of users. Different sections of it can be assigned by your instructor as needed for the laboratory. The chapter on gravimetric analysis and precipitation equilibria (Chapter 10) has been moved to just before the one on precipitation reactions and titrations, which better aligns these related topics. 

PART III Classical Methods of Analysis 311 Chapter 12 Gravimetric Methods of Analysis 314 Chapter 13 Titrimetric Methods Precipitation Titrimetry 337 Chapter 14 Principles of Neutralization Titrations 368 Chapter 15 Titration Curves for Complex Acid/Base Systems 395 Chapter 16 Applications of Neutralization Titrations 428 Chapter 17 Complexation Reactions and Titrations 449... 

Finally, we will apply our knowledge of the mole method from Chapter 3 to the three types of reactions studied here. We will see how gravimetric analysis is used to study precipitation reactions, and the titration technique is used to study acid-base and redox reactions. (4.6, 4.7, and 4.8)... 

Briefly describe (a) half-equation method of balancing redox equations (b) disproportionation reaction (c) titration (d) standardization of a solution. Explain the important distinctions between (a) a strong electrolyte and strong acid (b) an oxidizing agent and reducing agent (c) precipitation reactions and neutralization reactions (d) half-reaction and overall reaction. 

Titrimetric methods have been developed using acid-base, complexation, redox, and precipitation reactions. Acid-base titrations use a strong acid or strong base as a titrant. The most common titrant for a complexation titration is EDTA. Because of their... 

Representative Examples of Coulometric Titrations Using Acid-Base, Complexation, and Precipitation Reactions... 

Analysis for Poly(Ethylene Oxide). Another special analytical method takes advantage of the fact that poly(ethylene oxide) forms a water-insoluble association compound with poly(acryhc acid). This reaction can be used in the analysis of the concentration of poly(ethylene oxide) in a dilute aqueous solution. Ereshly prepared poly(acryhc acid) is added to a solution of unknown poly(ethylene oxide) concentration. A precipitate forms, and its concentration can be measured turbidimetricaHy. Using appropriate caUbration standards, the precipitate concentration can then be converted to concentration of poly(ethylene oxide). The optimum resin concentration in the unknown sample is 0.2—0.4 ppm. Therefore, it is necessary to dilute more concentrated solutions to this range before analysis (97). Low concentrations of poly(ethylene oxide) in water may also be determined by viscometry (98) or by complexation with KI and then titration with Na2S202 (99). 

The majority of potentiometric titrations involve chemical reactions which can be classified as (a) neutralisation reactions, (b) oxidation-reduction reactions, (c) precipitation reactions or (d) complexation reactions, and for each of these different types of reaction, certain general principles can be enunciated. 

Bromo-4,5,6-trichlorophenol. 2,3,4-Trichlorophenol (20 grams, 0.10 mole) was dissolved in 30 ml of HO Ac and titrated with bromine (18 grams, 0.11 mole) dissolved in HOAC-H2O (30 ml each). The last few drops of bromine caused the characteristic color of bromine to persist. The product was precipitated by adding water to the reaction mixture. The filtered product was recrystallized from methanol-water solvent yielding 27 grams (98% yield) of white crystals. 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

We next investigated the dealkylation of S-b-tBM with TMSI. Unlike the reaction with S-b-MM, it required only 4 hr at room temperature to completely cleave the t-butyl ester. Work-up under acidic conditions gave S-b-MA which was virtually identical by NMR, IR, GPC, and titration with that just described above. Likewise, neutralization with KOH resulted in quantitative conversion to S-b-MA.K. Although the initially formed product of the reaction of alkyl esters with TMSI is presumably the trimethylsilyl ester (1 7 ), we were not able to isolate or characterize this copolymer. It is known that trimethysilyl methacrylate and its polymers spontaneously hydrolyze even in moist air (19). Any traces of water in the methanol used to precipitate the reaction mixture would thus preclude isolation of the intermediate trimethylsislyl ester. 

Precipitation titrations are typified by the titration of chloride with silver or vice versa. In this case, interferences with the precipitation reaction may occur because of components in the soil, and the soil itself may interfere with detection of the end point. Thus, complexation reactions are rarely applied directly to soil however, they can be applied to soil extracts. Common environmental titration methods described in the United States Environmental Protection Agency (USEPA) methods are summarized in Table 10.1 [1,2],... 

In this chapter, you learned about solutions and how to use molarity to express the concentration of solutions. You also learned about electrolytes and nonelectrolytes. Using a set of solubility rules allows you to predict whether or not precipitation will occur if two solutions are mixed. You examined the properties of acids and bases and the neutralization reactions that occur between them. You then learned about redox reactions and how to use an activity table to predict redox reactions. You learned about writing net ionic equations. Finally, you learned how to use the technique of titrations to determine the concentration of an acid or base solution. 

In general, titrations governed by precipitation reactions do not really constitute an appreciable number in volumetric determinations in comparison to either redox or acid-base reactions. The interaction between silver-nitrate and sodium chloride in solutions result into the precipitation of silver chloride as shown below ... 

For analysis by titration with Fehling s solution, the sample is treated with lead acetate to precipitate protein and fat, filtered, and the filtrate titrated with alkaline CuS04, while heating. The reactions involved are summarized in Figure 2.37. 

The constant depends on the reference electrode used with the fluoride electrode. A 20.00-mL solution containing 0.060 0 M NaF was titrated with 0.020 0 M La(N03)3 to precipitate LaF3. Just considering the precipitation reaction, fill in the following table and plot the expected titration curve, assuming that the constant is —20.0 mV. 

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