Monograph - Metrohm

More documents

Recommendations

Info

Practical thermometric titrimetry 31 Fig. 23: Titration plot of the determination of fluoride with boric acid. 7.5.2. Determination of formaldehyde Formaldehyde can be determined in electroless copper plating solutions by the addition of an excess of sodium sulfite solution and titrating the liberated hydroxyl ion with standard acid. H 2 C=O + SO 3 2– + H 2 O [HO–CH 2 –SO 3– ] + OH – 7.6. Thermometric titration of water The thermometric titration of water using Karl Fischer (KF) reagent is feasible due to the large reaction enthalpy involved (approx. –67 kJ/mol). However, other workers have sought to avoid some of the disadvantages and limitations inherent in the use of the KF reagent. Sadtler and Wilson (ca. 1980) developed a thermometric titration technique relying on the endothermic acid-catalyzed reaction of 2,2-dimethoxypropane (DMP) with water. H + CH 3 C(CH 3 O) 2 CH 3 + H 2 O 2CH 3 OH + (CH 3 ) 2 CO ΔH 0 f (kJ/mol) –459.4 –285.8 2 x –239.2 –239.2 ΔH 0 r = +27.6 kJ/mol The reagent had seen prior use in the dehydration of histological specimens, and in infrared and gas chromatographic methods for the determination of water. DMP is of low toxicity, quite stable, not hygroscopic and only reacts with water in the presence of an acid. The reaction products (acetone and methanol) are relatively benign. Many applications have been demonstrated, including the determination of water in concentrated acids (where the acid itself provides the catalyst for the reaction) (see Figures 24 a and 24 b). Methane sulfonic acid may be used as a catalyst in many applications.
32 Practical thermometric titrimetry Fig. 24 a: Plot of thermometric titration of water in concentrated hydrochloric acid using DMP as titrant. Fig. 24 b: Thermometric determination of moisture in commercial (ultrafine) cobalt oxyhydroxide. An excess of added DMP is back-titrated with a standard solution of water in 2-propanol. Rounding of the endpoint suggests that the reaction might not proceed to stoichiometric equilibrium. Inspection of titration curves shows that, as expected, the reaction is driven closer to stoichiometric equilibrium when water is determined in strong acids. 7.6.1. Advantages of the DMP thermometric method: • DMP is stable and essentially non-hygroscopic, does not react with water until catalyzed with acid. Does not require special conditions in storage • DMP is of low toxicity, reaction products are relatively benign • Analysis can be integrated with other thermometric titration procedures simply by adding an additional buret to the system • Solvent system can be adjusted to suit the sample – a range of anhydrous organic solvents and mixtures thereof can be used • Applicable to a wide range of sample types, particularly those that are acidic or neutral in character. Samples analyzed have ranged from plant oils with moistures in the range of 0.02…0.04% to concentrated HCl with a water content of 67% • Can be useful for analysis of samples where Karl Fischer or other methods have not proven suitable
Page 1 and 2: Monograph Practical thermometric ti
Page 4: Practical thermometric titrimetry 3
Page 7 and 8: Practical thermometric titrimetry 2
Page 9 and 10: Practical thermometric titrimetry I
Page 11 and 12: 10 Practical thermometric titrimetr
Page 31: 30 Practical thermometric titrimetr
Page 35: 34 Practical thermometric titrimetr

Monograph - Metrohm

Create successful ePaper yourself

Delete template?

Save as template?