Micro-titrations

Despite affecting conversion, mass transfer is known to impact enantio- and regioselectivity for many reactions [63]. For this reason, conventional micro-titration apparatus, typically employed in combinatorial chemistry of single-phase reactions, also often suffer from insufficient mixing when dealing with multi-phases [63, 66]. 

Alimarin, I. P., and M. N. Petrikova Complexometric Ultra-Micro Titration of Nickel with Use of a Mercury Electrode. Zh. Analit. Khim. (Russ.) 21, 1257 (1966) J. Anal. Chem. USSR (English Transl.) 21, 1114 (1966). 

Thus, K, = K hKj and AH, = AH h + AH,. Micro-titration-based calorimetric experiments carried out during the sorption of amino acids on strong-acid cation-exchange resin in aqueous solutions at pH = pi have indicated that AH, 0 [15, 32], On this basis AH h = -AHj. The AH, values of nine amino acids determined by these microtitration calorimetric experiments [33], are also listed in Table 3-It is known that... 

A sample of the hydrogel carbamate (50-80 mg) was dissolved (or slurried) in 3 mL DMF. The solution was titrated in a micro-titration vessel equipped with a 5.5 mm combination electrode. 

J. Jakmunee, L. Pathimapomlert, S.K. Hartwell, K. Grudpan, Novel approach for mono-segmented flow micro-titration with sequential injection using a lab-on-valve system a model study for the assay of acidity in fruit juices, Analyst 130 (2005) 299. 

PLA has been widely studied for use in medical applications because of its bioresorbability and biocompatible properties in the human body. The main reported examples on medical or biomedical products are fracture fixation devices like screws, sutures, delivery systems and micro-titration plates [8]. 

Although commercial apparatus is available for micro-titrations, much of this is fragile or hard to clean. The apparatus shown in Fig. 3.1 can be put together from easily accessible materials plus a miniature glass electrode. With its aid as little as 0 5 mg. of a substance, dissolved in 0 5 ml. of water, can readily be titrated. In this way, ionization constants can be used to help identify small fractions isolated in chromatography, and the solution remaining after titration can be used for ultraviolet spectroscopy. 

The potentiometric micro detection of all aminophenol isomers can be done by titration in two-phase chloroform-water medium (100), or by reaction with iodates or periodates, and the back-titration of excess unreacted compound using a silver amalgam and SCE electrode combination (101). Microamounts of 2-aminophenol can be detected by potentiometric titration with cupric ions using a copper-ion-selective electrode the 3- and... 

The simple electrical circuit shown in Fig. 16.15(h) is suitable for this procedure. The voltage applied to the titration cell is supplied by two 1.5 V dry cells and is controlled by the potential divider R (a 50-100 ohm variable resistance) it can be measured on the digital voltmeter V. The current flowing is read on the micro-ammeter M. 

Procedure. Place 25.0 mL of the thiosulphate solution in the titration cell. Set the applied voltage to zero with respect to the S.C.E. after connecting the rotating platinum micro-electrode to the polarising unit. Adjust the range of the micro-ammeter. Titrate with the standard 0.005 M iodine solution in the usual manner. 

Dilute solutions of antimony(III) and arsenic(III) (ca 0.0005M) may be titrated with standard 0.002 M potassium bromate in a supporting electrolyte of 1M hydrochloric acid containing 0.05 M potassium bromide. The two electrodes are a rotating platinum micro-electrode and an S.C.E. the former is polarised to +0.2 volt. A reversed L-type of titration graph is obtained. 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

Procedure. Pipette 25.0 mL of the thiosulphate solution into the titration cell e.g. a 150mL Pyrex beaker. Insert two similar platinum wire or foil electrodes into the cell and connect to the apparatus of Fig. 16.17. Apply 0.10 volt across the electrodes. Adjust the range of the micro-ammeter to obtain full-scale deflection for a current of 10-25 milliamperes. Stir the solution with a magnetic stirrer. Add the iodine solution from a 5 mL semimicro burette slowly in the usual manner and read the current (galvanometer deflection) after each addition of the titrant. When the current begins to increase, stop the addition then add the titrant by small increments of 0.05 or 0.10 mL. Plot the titration graph, evaluate the end point, and calculate the concentration of the thiosulphate solution. It will be found that the current is fairly constant until the end point is approached and increases rapidly beyond. 

This reaction serves as a known model reaction to characterize mass transfer efficiency in micro reactors [5]. As it is a very fast reaction, solely mass transfer can be analyzed. The analysis can be done simply by titration and the reactants are inexpensive and not toxic (although caustic). 

Micro amounts of sulfur in polymer are usually determined by oxygen flask combustion, sodium peroxide fusion in a metal bomb followed by titration [30], pyroluminescence [36] or ICP-AES. An oxygen flask combustion photometric titration procedure capable of determining total sulfur in polymers in amounts down to 50 ppm was reported. The repeatability of the sulfur determination in polyolefins in the oxygen flask is 40% at 50 ppm level, improving to 2% at the 1 % level [21]. Crompton [31] has also combined Schoniger flask combustion with a colorimetric procedure for the determination of phosphorous in polymers in various concentration ranges (0.01 to 2%, 2 to 13%). 

Berka et al [59] described an accurate and reproducible coulometric method, with chlorine electrogenerated at the anode, for the determination of micro quantities of primaquine phosphate. Titration was carried out in an anode compartment with a supporting electrolyte of 0.5 M sulfuric acid-0.2 M sodium chloride and methyl orange as indicator. One coulomb was equivalent to 1.18 mg of primaquine phosphate. The coefficients of variation for 0.02-0.5 mg of primaquine phosphate were 1-5%. Excipients did not interfere. 

Zhan [61] reported the use of an oscillopolarographic method for the determination of primaquine phosphate and other drugs in pure form and in pharmaceutical preparations. The sample solution was mixed with potassium bromide and 6 M hydrochloric acid and the mixture was titrated with 0.1 M sodium nitrite. A micro platinum electrode and a platinum electrode were used as indicators and reference electrodes, respectively. The mean recoveries were 96.88-99.88%. Results agreed well with those obtained by the Chinese Pharmacopoeia method. 

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