Coulometric Titrations Automatic Titrator for Mercaptans FRANK A. LEISEY Research Department, Standard
Oil Co.
(Indiana), Whiting, Ind.
The determination of mercaptans in petroleum stocks is important in the control of refinery treating operations. Present titration methods require considerable timeand operator skill. For this reason, an instrument has heen developed for the automatio titration of mercaptans. The titrator uses a coulometric system for generating silver ions that react with mercaptans in a I-ml. sample. At the end point, excess silver ions cause an increase in current flow between arnperometrio eleotrodes and stop the titration. The time of titration gives a measure of the mercaptan content. Better than 2% agreement with potentiometric results is obtained on samples containing over 100 Y oC mercaptan sulhtr. The automatic titrator requires no standard solutions, titrations are rapid, and unusually low concentrations of mercaptan can he measured.
Standard Oil Co. (Indians) for commcrcisl manusactwe and sale by the Central Scientific Co., Chicago, Ill. Two electrical systems are used: one eoulomctrie Sor generating silver ions; the other ampemmetric ior det,ccting the end point. Silver mercaptide is precipit,ated from the sample in an alcoholic electrolyte by silver ions, coulomctricdly gcncrated in the titration cell. At the end point, the smpcrometrie system stops the titration. The generation timo is a ~ Z R S U I ' P of the mercaptan cantent of the sample.
T
HE efficient operation of refinery treating plants that remove mercaptan sulfur from light petroleum stooks requires a constant check on the mercaptan content of the product. Mast methods of determining memaptans (thiol) are based upon a titration with aqueous or alcoholic silver nitrate. The titration end point can he detected oolorimetrioally ( l ) ,potentiometrically (lS),or amperometricdly (7, 0). With improved electrodes and electronic measuring equipment (10)the potentiometric method has largely supplanted the colorimetric method. Potentiometric titration involves careful manipulation by an experienced operator and is not too satisfactory for plant control. For this reason, Pompeo and oowarkers ( 1 1 ) developed B semiautomatic titrat,ion instrument to meamre the mercaptan content of gesalines. The instrument titrates s simple to a fixed end-point voltage, and reoords the quantity of reagent required as per cent of mercaptan. This technique was also applied in 8 continuous recording instrument (8) in which %. sample stream is mtomatieally and continuously titrated. The amperometric method can be used to detect the mercaptan end point much more. rapidly than the potentiometric method, especially in very dilute solutions. Kolthoff and Harris (9) developed a titration system in which rotating platinum-wire indicator and mercury-mercuric iodide reference electrodes are connected to a galvanometer for detecting the end point. This amperometrio method offers several distinot advantages. It is simple, accurate, and rapid; however, it has the disadvantage of using a volumetric reagent. The coulometric method of eleetrolytioitlly generating reagents a t electrodes placed in the titration cell has become increasingly important in recent years. This method eliminates the prepamtion, standardization, storrage, controlled introduction, and measurement of volumetric reagents. Furthermore, the method readily lends itself to automatic control because the generated reagent e m be electrically regulated. Previous articles point out the many advantages and applications of this technique (8,4,6, I d ) . DeFord, Johns, and Pitts ( 5 )describe an automatic titrator using externally generated coulometrio reagents. The advantages of coulometric generation of reagent and of ampemmetric detection of the end point have been combined in a new instrument for titrating mercaptans. The new titrator, shown in Figure 1, is automat,ic and well suited for routine use. This instrument has been licensed under the patent rights OS the
Figure 1. Automatic Mercaptan Titrator
I n the ion-generating system, a pair of metal electrodes is suspended in the titration cell. A constant direct current passed through the cell generates silver ions by oxidieing the anode. The silver ions generated under these conditions are directly proportional to the generation time. I n the end-point detecting system, mother pair of metal electrodes in the titration mixture is connected to a galvanometer. Initially, the galvanometer deflect,ion is small, but a rapid inoreese occurs when the end point has been exceeded, because of the diffusion current of excess silver ions. In the titrator, the ion-generating and detecting circuits work together with a relay circuit to make the titration automatic. At the end. point, the galvanometer deflect.iou directs a narrow beam of light into B phototube, which activates a relay. The relay then turns off the generation current and a n electrical timer. API'ARATUS
A schcmittic diagram of the automatic mercaptan titrator is
50 ml. of benzene, ap
