An Interpretation of the End Point Error in the Potentiometric Titration of Bromide and Chloride Mixtures with Si Iver Nitrate RICHARD C. BOWERS, LAURA HSU,' and JAMES A. GOLDMAN Department o f Chemistry, Northwestern University, Evanston, 111.
b Factors influencing the bromide end point error in the titration of bromide and chloride mixtures have been investigated. Adding sufficienl electrolyte to flocculate silver bromide as it i s formed reduces error to a minimum. With equimolar amounts of bromide and chloride this minimum error i s between 1.2 and 1.5%. Under nonflocculating conditions, the bromide end point error i s dependent upon the rate of addition of silver nitrate and the ratio of bromide to chloride originally present. More chloride than that corresponding to ideal homogeneous mixed crystal formation i s incorporated into the silver bromide. This extra chloride exchanges only slowly with the bromide ion in solution as long as the silver bromide remains colloidal. Upon flocculation of the chloride-rich colloid, rapid exchange occurs and distribution of chloride and bromide between solid and solution approaches that expected for ideal homogeneous mixed crystals. The bromide end point error under nonflocculating conditions i s interpreted semiquantitatively b y assuming the ratio of chloride to bromide adsorbed on the surface of the colloid to be proportional to the relative concentrations of these ions in solution, and that this ratio governs the relative amounts of these ions precipitated at any stage of the titration.
C
ERROR can be incurred in the potentiometric titration of halide mixtures-for example, in the titration of bromide and chloride mixtures, positive errors of from 1 to 20%, depending upon the conditions, have been observed for the bromide end point. This error has generally been attributed to the mixed crystal formation 11-hich is knon-n to occur in silver halide s p t e i n r . or, according to Clark (S), to an adsorption phenomenon. Flood and coworkers (5, 6, 7 ) have studied the titration of bromide in the 1
Ill.
EXPERIMENTAL
Reagents. Stock solutions or' sodium chloride, potassium bromide,
ANALYTICAL CHEMISTRY
and silver nitrate were prepared from reagent grade chemicals. T h e relative concentrations of these solutions were checked b y titration of each of the halide solutions with silver nitrate. Solutions of potassium nitrate, calcium nitrate, and aluminum nitrate were prepared from reagent grade chemicals. Apparatus and Procedure. All of the titrations were carried o u t with a n automatic titrator. T h e functional component of the automatic titrator was a hypodermic syringe, t h e plunger of which was driven b y a machined screw. T h e rate of titrant delivery could be varied over a considerable range b y changing t h e gear arrangement, which coupled the motor to the machined screw. Unless otherwise noted, all titrations were carried out a t a speed of 19.24 pl. per second. Calibration of the titrator was accomplished by carrying out titrations of various amounts of a standard potassium bromide solution with standard silver nitrate solution. The potential of the silver m Furmari, S. H., "Potentiometric Titrat ion. '' in>. 154-8, \Tilev. S e n York, 1931. 11) Iiolthoff, I. 11.. O'Rrien. .I. S., .I. .ivi. Chern. SOC.61,3409 (1939). 12) Leden, I., SLerisk Kern. Tzi16jl.i. 64, 249 (1952). (13) Thirl, -I.,2. anorg. Chein. 24, 1 (1 ROO). I I
RECEIVEDfor rwiev- June 20, 1960. Accepted Sovember 8, 1960. Division of .Inal>-tical Chemistrr, 137th JIeeting, ACS, Cleveland, Ohio, April 1960.
Amperometric Titration of Sulfhydryl and Disulfide Groups with Organic Mercury Compounds at the Rotated Dropping Mercury Indicator Electrode WALTER STRICKS and S. K. CHAKRAVARTI Department o f Chemistry, Marquetfe University, Milwaukee 3, Wis.
b The rotated dropping mercury electrode (R.D.M.E.) has the same sensitivity as the rotated platinum electrode. Polarographic studies at the R.D.M.E. of the reaction of organic mercury compounds with cysteine, reduced glutathione, cystine (in the presence of sulfite), and albumin in the undenatured and denatured states show that this reaction can be made the basis of a titration of sulfhydryl in these ma194
ANALYTICAL CHEMISTRY
terials, using the R.D.M.E. as indicator electrode. Procedures are given for the accurate and rapid determination of traces of sulfhydryl in amino acids, pepticks, and proteins with ethylmercury chloride as titrant. The method allows the determination of 6 to 120 pg. of sulfhydryl in biological materials in aqueous solutions with an accuracy of &O.s%. The reaction of phenylmercury hydroxide with sulf-
hydryl and disulfide i s slow. Titrations with phenylmercury hydroxide are therefore inferior to those with ethylmercury chloride.
A
( 6 ) and mercurimetric (7') amperometric titrations of sulfhydryl and disulfide groups in amino acids, peptides, and proteins at the rotated platinum wire electrode as RGENTIMETRIC
