Wise, Edward Nelson2025-07-302025-07-301953-08-31https://hdl.handle.net/1808/36120Ph. D. University of Kansas, Chemistry 1953The chemical analyst who wishes to quantitatively estimate a constituent of a sample has recourse to a number of methods which may be grouped for discussion under three main headings: 1. gravimetric, 2. volumetric, and 3. utilization of physical properties. The classical gravimetric method involves the separation of the constituent in a weighable form, usually by chemical precipitation. Sometimes the constituent is precipitated as a part of a chemical compound of known composition, and sometimes as a part of a compound which may readily be converted to one of known composition. The method commonly involves precipitation, filtration, washing, drying, occasionally igniting, and weighing, making the procedure a time consuming one. When the method is applied with care, it is capable of ye1ld1ng results of high precision and accuracy. In the classical volumetric method, the constituent is chemically reacted with a measured volume of a solution of reagent of known concentration. If this reaction is carried just to the point where the amount of reagent added is chemically equivalent to the amount of constituent present, then the amount of that constituent can be calculated. An accurate determination of the equivalence point of the titration is required, and this is commonly afforded by the use of a chemical indicator or by electrometric means. A chemical indicator indicates, by color change or by the formation of a precipitate, a point in the titration called an endpoint, which is either the equivalence point or can be related to the equivalence point. The electrometric method involves the insertion of electrodes in the solution being titrated, and the observation of the potential of that electrode pair, or of the current flowing between them. An abrupt change in the potential or current affords the indication of equivalence. The basic principles of coulometric analysis were stated by Szebelledy and Somogyi in 1938 (45). These investigators quantitatively determined acids (46, 47), bases (50), hydrazine (49), hydroxylamine (51), and thiocyanate ion (48). In each instance potassium bromide was added to furnish indication of completion of the reaction by the color of Br2 liberated following the desired coulornetric reaction. This liberated Br2 was subsequently determined by the addition of KI, followed by titration with standardized Na2s203. The quantity of electricity which had been consumed by the desired reaction, plus that quantity which had liberated Br2, was determined from the gain in weight of a silver coulometer which had been placed in the electrical circuit. The amount of electricity required to liberate the determined excess of Br2 was subtracted from the total amount of electricity, thus obtaining the assay of the unknown quantity of substance in terms of a known quantity of electricity. Assuming 100% current efficiency, which must be experimentally verified for each type of reaction, the passage of 96,500 international coulombs of electricity indicates the titration of one gram equivalent of reacted substance. The problems of coulometric analysis include 1. the selection of an electrolyte and of a reagent generation cell which will assure production of the required reagent with a known and constant current efficiency, 2. the development of a constant current source which will automatically compensate for variations in the resistance of the generation cell, will be compact and insensitive to vibration, and will be all-electronic in design with provision for variations in line voltage supply, 3. the selection of a synchronous timing device to measure the time of electrolysis and 4. the selection of a satisfactory method of endpoint detection which will be compatible with the titration system, and which will permit sighting-in on the true endpoint by incremental additions near the endpoint. A number of electrolytes from which a reagent may be generated with 100% current efficiency are reported in the literature. The generation cells reported have usually consisted of a single vessel with the active generating electrode contacting the solution directly, and the companion generating; electrode enclosed in a small glass tube contacting the solution through a sintered glass bottom which provides electrical contact but which prevents mixing of the electrolyte surrounding the companion electrode with the main solution. This separation of anolyte and catholyte 1s usually necessary, for the product of electrolysis at the companion electrode is quite often chemically incompatible with the product formed at the reagent generating electrode. The cell devised by DeFord and associates (14) for the generation of the reagent outside of the titration vessel has merit in that it effectively separates the anolyte and catholyte, and also prevents electrical interaction of the generating and indicating electrodes, but it does present time-lag and dilution problems, for the reagent generated is swept from the generating cell into the titration vessel by a continuous flow of electrolyte, and there is a small time lag between reagent generation and its delivery to the titration vessel. In semi-micro and micro analytical titrations, to which coulometric analysis 1s especially adapted, both the 1nclue1on of a relatively bulky isolated electrode and the alternative of continuous dilution are undesirable. Constant-current sources reported in the literature have inherent disadvantages, such as the use of a vibration-sensitive galvanometer (37), the use of a non-standard source of supply (13), the use of a constant voltage source with dropping resistors (41), or the use of batteries as reference potentials (40, 41). A desirable source would not have these features, but would automatically compensate for variations in the resistance of the titration cell and would include automatic compensation for normal variations in the supply voltage. Several synchronous timing clocks with electrically operated start-and-stop clutches are available. These depend on the constancy of the supply line frequency for their timing accuracy. If the local supply is not sufficiently frequency-stable, 60-cycle frequency standards in either tuning fork or crystal controlled models are available. The problem of endpoint detection has received considerable attention, as shown in the above literature survey. This interest has been due, in part, to the inherent difficulties associated with electrometric detection methods. Inconsistent electrode behavior has caused trouble, and has occasionally necessitated unusual care in the handling of electrodes. It has sometimes been necessary to store the electrodes under controlled conditions between titrat1ons, and to pretreat the electrodes before a titration. In automatic coulometric determinations, electrical coupling between the generating circuit and the indicating circuit may prove troublesome. The use of photometric detection would eliminate these troubles associated with sensing electrodes, and would also reduce the number of electrodes present in the titration vessel, The latter advantage is of especial importance in semi-micro and micro analysis, because of the small size of the titration vessel employed. Photometric detection is limited to those systems which give a suitable optical indication of equivalence, such as by the formation of a colored species, like iodine, following the attainment of equivalence, or to those systems to which a chemical indicator may be added to produce a change in the optical density of the solution to provide the desired indication of equivalence. Other changes in the optical density of the solution, such as may be brought about by dilution or by the formation of bubbles, a precipitate, or other complicating absorbing species, require special attention. Their effect may be minimized by differential photometric detection employing two phototubes with filters of different spectral absorption characteristics in front of each phototube. Proper location of the generating electrodes with respect to the beam of light passing through the titration vessel will permit the generated reagent to be swept directly into the beam, thereby causing a desirable anticipation of the true endpoint, with momentary halting of generation, and then incremental additions until the true endpoint is attained.This item is protected by copyright and unless otherwise specified the copyright of this thesis/dissertation is held by the author.DissertationopenAccess