Analyst, April 1996, Vol. 121 (559-561) zyx 559 z Paraformaldehyde as an End-point Indicator in Hydrolytic Thermometric Titration of Metal Ions and Iodine zyxwvu Julio Cesar B. Fernandesa, Luiz M. Aleixw, Graciliano de Oliveira Neton and Oswaldo E. S. Godinho"* zyxwvutsrq a Instituto de Quimica, Universidade Estadual de Campinas, Campinas, Brazil b Centro de Tecnologia, Universidade Federal do Maranhiio, Av. dos Portugueses, SIN, Campus Universitcirio do Bacangu, 65080-040, Scio Luis, MA, Brazil The use of paraformaldehyde (PF) as an end-point indicator in hydrolytic titrations involving precipitation, complexation and redox reactions was investigated. The decrease in temperature caused by depolymerization of PF, catalysed by hydroxyl ions produced by hydrolysis of excess of titrant, was employed to locate the end-point of the titration. The results of titrations of alkaline earth metals with carbonate, several metal ions with EDTA and iodine with arsenite are presented. Keywords: Hydrolytic thermometric titrution; catalytic end-point detection; paraformaldehyde; precipitation, complexometric and redox titrations Introduction In hydrolytic titrations, after the determinative reaction is complete, the titrant undergoes hydrolysis, leading to an abrupt change in pH. In some recent papers on hydrolytic titrations, involving and complexation2 reactions, the titration was monitored potentiometrically1.2 or with visual indicators.2 A method involving a combination of turbidimetric and catalytic thermometric titrimetry using acetaldehyde as an end-point indicator for the determination of metal salts, in the presence of mineral acids, has also been described.3 In an earlier paper,4 it was shown that paraformaldehyde (PF) may be employed as an end-point indicator in the titration of acidic substances in aqueous solutions by thermometric titration with catalytic end-point detection. The decrease in temperature due to the depolymerization of PF, catalysed by hydroxyl ions, was employed to locate the end-point of the titration. In this work, PF was employed as an end-point indicator in hydrolytic titrations involving precipitation, complexation and redox reactions. The titration of alkaline earth metals with carbonate and some metals with EDTA are presented as examples of precipitation and complexometric reactions, re- spectively. Finally, the titration of iodine with arsenite is taken as an example of titration based on redox reactions. In all instances the titrant undergoes alkaline hydrolysis according to the general equation An- zyxwvutsrqp + HOH HA("-')- + OH- where An- represents CO32-, EDTA4- or As02-. In this way, the hydroxyl ions generated catalyse the reaction of depolymer- ization of PF: OH- HO(CH20),H = nCH20 + H20 * To whom correspondence should be addressed. Experimental Reagents Ethylenediaminetetracetic acid disodium salt dihydrate, sodium carbonate and arsenic(m) oxide were of analytical-reagent grade. Tetraneutralized EDTA solution was prepared by neutrali- zation of an aqueous solution of the desired amount of the disodium salt. The pH was adjusted to 10.5-10.8 with sodium hydroxide solution.2 This solution was standardized by a conventional proced~re.~ Sodium carbonate solution was prepared by the dissolution of the desired amount of the salt, dried at 270°C. Sodium arsenite solution was prepared by dissolution of the desired amount of arsenic(m) oxide (primary standard) in sodium hydroxide solution in such way that the final concentra- tion of NaOH was three times that of the NaAs02. Paraformaldehyde was reduced to less than 100 mesh particle size. Ethanol-water (1 + I) should not be used until it has attained thermal equilibrium because of the heat of mixing. Apparatus A motor-driven micrometer syringe, as described by Greenhow and Spencer,6 was used to introduce the titrant at a constant delivery rate of 0.13 ml min-1. The temperature changes were detected by means of a thermistor placed in one arm of a Wheatstone bridge and were recorded on a strip chart, as described elsewhere? The titrations were performed in a 25 ml unsilvered Dewar flask and the solutions were stirred with a magnetic stirrer during the titrations. Procedure After the desired amount of sample had been dissolved in either 10 ml of water or 10 ml of ethanol-water (1 + l), 0.34-0.36 g of paraformaldehyde was added and the titration was carried out with vigorous stirring. The end-point was taken as the point where a tangent drawn to the main temperature decrease curve intersects the titration curve at its highest value. The volumes of titrant used in the titration of blank solutions were subtracted from those corre- sponding to the titration of the different samples. Results and Discussion Precipitation Titration The titrations of Ca", Sr" and Ba" with sodium carbonate were taken as examples of precipitation titrations. Initially, it was Published on 01 January 1996. Downloaded by Instituto Municipal de Ensino Superior de Sao Caetano do Sul on 16/12/2013 20:20:21. View Article Online / Journal Homepage / Table of Contents for this issue