Residual titration

In usual practice, the volumetric titrations may be accomplished either by direct titration method e.g., assay of HC1 employing NaOH as the titrant, or by residual titration method e.g., assay of ZnO in which case a known-excess-measured volume of standardised solution of H2S04, more than the actual amount chemically equivalent to ZnO, is added to the sample thereupon, the H2S04 which remain unreacted with ZnO is subsequently titrated (sometimes referred to as back titration or residual titration in the text) employing standardized NaOH solution. 

The two methods, namely direct titration method and residual titration method are briefly discussed as under ... 

Residual titration or back titration is normally employed in the following two situations, namely Case I when a chemical reaction proceeds rather slowly or sluggishly, and... 

In usual practice, the residual titration is accomplished by allowing to dissolve the substance under estimation in an accurately measured quantity of a standard solution of known strength present in excess and subsequently titrating the excess of the latter with another previously standardized solution. A good number of examples of this particular method shall be discussed in subsequent exercises. 

A few typical examples of acidimetric titrations, employing direct titration method (DTM) and residual titration method (RTM) from the Pharmacopoeia of India are described here ... 

Residual titration method is an alternative means of assay of drugs by Aqueous Titrations . Justify the statement with the help of assay of the following pharmaceutical substances ... 

The residual titration method for pharmaceutical substances using potassium permanganate solution are mainly of two categories, namely ... 

Residual Titration Method (Excess of Iodine Titrated with Sodium Thiosulphate)... 

A few other pharmaceutical substances may also be assayed by adopting the residual titration method as shown in Table 7.2. 

Masking and demasking agents, and (iii) Residual titration methods. 

In actual practice, an excess of the standard solution of disodium edetate is added to the sample, pH is adequately adjusted for the residual titration with a metal-ion solution e.g., ZnS04 and employing an appropriate indicator which is sensitive enough to the respective titrant. However, the metal ion under estimation remains firmly complexed with the EDTA and offers little interference with the Zn-EDTA complex formed. It has been established experimentally that bismuth readily yields a highly stable complex which may be titrated conveniently between pH 1 and 2. Bismuth forms a stable complex by reacting with EDTA quantitatively at pH 4.0 and, therefore, dithizone is employed as an indicator to detect the end-point for it has a transition state of colour at pH 4.6. 

Table 9.4 Substances Assayed by Residual Titration with EDTA...

How does residual titration method help in the complexometric btrations Elaborate the assay of the following drugs by this technique ... 

The first of these, utilized by Yoder, McCalip and Seibert,34 and by Balch, Broeg and Ambler,37 provides for the extraction of the aconitic acid from the sample being investigated, usually with diethyl ether, and the subsequent isolation of the acid from the solvent. In dealing with solid samples, e.g. alkaline earth aconitates, evaporator scale, etc., the prescribed procedure is to dissolve the material in aqueous mineral acid and to extract the acid solution exhaustively with ether. The ether extract is then evaporated under reduced pressure, the dried residue titrated with standard alkali and the titratable acid calculated as aconitic acid. In dealing with such solid samples it is often necessary to make an additional determination for oxalic acid which otherwise would be assumed to be aconitic acid.37 The aconitic acid in liquid samples is usually precipitated as the insoluble lead salt which is separated and treated as any other solid sample. In some cases this procedure is unnecessary and the liquid samples are merely acidified with a mineral acid and then extracted with ether.37 This method for the determination of aconitic acid, however, requires a considerable amount of time and is further complicated by the interference of ether-soluble waxes and non-volatile acids. 

The kinetics of AA and N,N -methylene-bis-acrylamide polymerization with varied contents of monomers (up to 15 carrier wt%) on a magnetite surface at different values of emitter power was studied. The analysis of kinetic curves obtained by monomer residue titration in non-aqueous medium (methanol) indicates that complete polymerization of the biocompatible layer of selected mass is reached in 2 min. 

Water Determine as directed for Method lb (Residual Titration) in Method I (Karl Fischer Titrimetric Method) under Water Determination, Appendix I IB. 

Principle See the information in the section entitled Principle under Method la. In the residual titration, add excess Karl Fischer Reagent to the test specimen, allow sufficient time for the reaction to reach completion, and titrate the unconsumed Karl Fischer Reagent with a standard solution of water in a solvent such as methanol. The residual titration procedure is generally applicable and avoids the difficulties that may be encountered in the direct titration of substances from which the bound water is released slowly. 

Standardization of Water Solution for Residual Titration Prepare a Water Solution by diluting 2 mL of pure water to 1000 mL with methanol or another suitable solvent. Standardize this solution by titrating 25.0 mL with the Karl Fischer Reagent, previously standardized as directed under Standardization of the Reagent. Calculate the water content, in milligrams per milliliter, of the Water Solution with the formula... 

Reagent added after introduction of the specimen X is the volume, in milliliters, of standardized Water Solution required to neutralize the unconsumed Karl Fischer Reagent, and R is the ratio V/25 (milliliters of Karl Fischer Reagent m cr of Water Solution), determined from the Standardization of Water Solution for Residual Titration. 

Transfer the quantity of acetylated oil specified in the monograph, and accurately weighed, into a tared 125-mL Erlen-meyer flask, and add 10 mL of neutral alcohol, 10 drops of phenolphthalein TS, and 0.1 N alcoholic potassium hydroxide, dropwise, until a pink endpoint is obtained. If more than 0.20 mL is needed, reject the sample, and wash and test the remaining acetylated oil until its acid content is below this level. Prepare a blank for residual titration (see General Provisions), using the same volume of alcohol and indicator, and add 1 drop of 0.1 /V alkali to produce a pink endpoint. Transfer 25.0 mL of 0.5 N alcoholic potassium hydroxide into each of the flasks, reflux them simultaneously for 1 h, cool, and titrate the contents of each flask with 0.5 N hydrochloric acid to the disappearance of the pink color. Calculate the percentage of Total Alcohols (A) by the equation... 

The theoretical description of the titration curves of proteins and polyprotic acids has been studied since the pioneering work of Linderstrom-Lang [2] and Tan-ford [3]. In the following I will introduce the theory presently used in the calculation and analysis of protein residue titration curves. 

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