LITERATURE CITED
( I ) Chance, B., Hughes, V., MacSichol E. F., Sayre, D., Killiams, F. C., eds., ‘VaveformS.” R.I.I.T. Radiation Lab-
oratory Series,’’ Vol. 19, 1st. ed., pp. 257-9, 278-88, and 664-5, LIcGraw Hill, Sew York, 1949. ( 2 ) IlkoviE, D., Semerano, G., Collection Czechoslov. Chem. Communs. 4, 176 (1932). (3) Kelley, M. T., Fisher, D. J., Cooke,
IT. D., Jones, H. C., “ControlledPotential and Derivative Polarography,” presented at, and to be published by Pergamon Press Ltd. in the Proceedings of the Second International Congress of Polarography, Cambridge, England, Aug. 24-29, 1959, pp. 158-
Inc., Boston, Mass., “GAP/R Electronic Analog Computers,” “-4pplications Manual for Philbrick Octal PlugIn Computing Amplifiers,” and catalog data sheets. RECEIVEDfor review January 2,5, 1960. Accepted July 1, 1960. Work performed under Contract No. W-7405-eng-26 at Oak Ridge National Laboratory, operated by Union Carbide Nuclear Co. for the U. S.Atomic Energy Commission.
182.
(4)Kelley, 11. T., Jones, H. C., Fisher, D. J., A N A L . CHEM.31, 1475 (1959). (5) Philbrick, George A., Researches
Automatic Coulometric Titrator Application to the Determination of Sulfur in Petroleum by High Frequency Combustion JOHN R. GLASS and EDWARD J. MOORE Research Department, Socony Mobil Oil Co., Inc., Paulsboro, N. 1.
In the determination of sulfur by combustion to sulfur dioxide, the sulfur dioxide must b e titrated a t the rate at which it is formed. An automatic coulometric titrator has been developed which relieves the analyst of titrating continually as the combustion proceeds. Titrant (iodine) is generated using from 0.2- to 5-second increments of constant current times to within 0.001 second. The generation of the titrant is controlled by amplified amperometric signals from a pair of detecting electrodes.
coulometric titrators have been described and the instruments have been reviewed b y DeFord (3-5). When sulfur is determined by combustion to sulfur dioxide (1, 6),the sulfur dioxide must be titrated at the rate a t which i t is formed. An automatic coulometric titrator for this purpose is shown schematically in Figure 1. ARIOUS
+haft of the stirrer. The c..amber at the middle of the cell has a larger diameter to provide space for the ends of the four electrodes and the four b1atic.s of the stirrer. The chamber a t the upper part is baffled to permit the gases to leave the cell without loss of electrolyte. The combustion gases enter through the small inlet a t the left and make thorough contact with the electrolyte in the loner part of the cell. At the p w k of t h r combustion the SO? may lxl evolved momentarily at a rate fa-ter than iodine can be generated. The gradual mixing of the electrolyte iii the lower section with that in the middle section enables the titrator to keep pace with the SO2 evolution. The
the constant current supply. This cycle of operation ib repeated many times during the combustion uiitil no more sulfur dioxide is liberated. The clock totals the time during whirh the constant current supply is connected and generating iodine. The product of the current and time (coulombs) is a measure of the amount of titrant added, hence the amount of sulfur in t h sample. TITRATION CELL
The titration cell is shown in Figure 2 . The chamber at the lower part of the cell is a long narrow annular space between the cell wall and the enlarged
si ELECTROLYTE
I
CLOCK
GENERAL OPERATION
The sample burns in an oxygen atmosphere. The combustion products, JThich include sulfur dioxide, are swept into the titration cell by a stream of nitrogen. The sulfur dioxide is absorbed and continuously titrated with coulometrically generated iodine. The detecting electrodes respond to the iodine concentration. When the iodine concentration is below a preset level, the detecting electrodes cause the amplifier and relay to connect the constant current supply to the pair of generating electrodes, thereby coulometrically generating free iodine. K h e n the iodine concentration rises above the preset level, the detecting electrodes cause the amplifier and relay to disconnect
L
CONSTANT CURRENT SUPPLY
AMPLIFIER
r-I I
1
TITRATION C E L L
TO DRAIN
I
::
ELECTRODES
I I
I I I I I L--i
O I L SAMPLE I N CRUCl0LE INSIDE
N2
I
.
:
~
~
~
] ~
oGENERATING E PAIR
INDICATING
_____
! J
HIGH FREQUENCY INDUCTION FURNACE
Figure 1 .
Schematic diagram of apparatus VOL. 32, NO. 10, SEPTEMBER 1960
1265
electrode controls the current flow through the detecting electlodes by the reaction: I2
+ 2e + 2 I-
The reference electrode is a silver n-ilc immersed in electrolyte but separated by a porous diaphragm from the main body of the electrolyte. A voltage IS applied to these electrodes by a batteiy; the resulting current through the cell controls the amplifier and relays. CENERliTING
ELECTRODES.'rh
anode is a 90% platinum-lO~c iridium 1.0-cm. disk immersed in the main body of the electrolyte. The generating current passing through the cell foi ins free iodine : 21-
+
I?
+ 2e
The iodine oxidizes the SOe: SO,
+ 1 2 + 2H2O + H,SO, + 21- + JH-
'The cathode is a platinum wire immersed in a catholyte buffer solutio11 (231 H3PO4, 2-11 NaH2P04) which is Teparated from the main body of the clectrolyte by a porous diaphragm. The generating current produces hydiogen a t this electrode: Figure 2.
qtirrer, revolving a t 3500 r.p.iii. just beneath the four electrodes, thoroughly mives the electrolyte in this zone and ensures a definite and rapid response of the detecting electrodes to a change in the iodine concentration. Electrodes. Four electiodes are in the clectrolyte sollition-oiir pair for
Titration cell
2H+
detectiiig the iodine coneenticition amPeron1ctricallJ' and t h e other Pail for generating free iodine CoulometricD E T W I I x G ELECTRODES. The indicating electrode is a 1.0-cm. disk of 90% platinum-lO% iridium immersed in the main hociy of the electrolytc. The conccntration of f w iodin? a t thib
+ 2e
+
HP
The porous diaphragms are made flom unglazed porcelain plate (Catalog ?;o. 13-752, Fisher Scientific eo.) sealed in the end of a borosilicate glass tube. Preparation of Catholyte. Add 135 1 ~ 1of , 85y0 &Po4 to 200 mi. of \\ater. Cool and add 100 ml. of 10.V K a O H slonly. Cool and dilute t o 500 ml. \\ith water.
t l P
Figure
3.
P
Wiring diagram of amplifier
Input transformer, UTC LSl 2 X Isolation transformer, Merit P3096 Power transformer, 275-0-275 Volts-50 ma., 6.3 Volts-2.6 amperes, 5 Volts-2 amperes, Merit P3154 Filament transformer, 6.3 Volts-1.2 amperes, SNC 4P 245 Power transformer, 340-0-340 Volts-330 ma., 5 Volts-6 amperes, 6.3 Volts-2.5 amperes, 6.3 Volts-5 Stancor 8 1 6 6 T& Filament transformer, 5 Volts-1 3 amperes, UTC 5 5 9 Filament transformer, 6.3 Volts-0.5 amperes, Merit P2964 T7. 1-1, 1-2. Chokes, 1 3 henries, Thordarson T' 13C .30 1-3. Choke, 1 2 henries-300 ma. Thordarson T 1 5 C 46 Relay 1. Relay BIXBX, Struthers Dunn Converter. 60-cycle, Brown Tuning fork. Vacuum tube tuning fork. General Radio Ca., Type 723-C, 34-volt output Electronic counter, Model 1 0 and decimal counting unit, Model 700A, Berkeley Scientific Corp. Galvanometer.* Sensitivity, 0.031 &a/mm; 3 1 3 ohms; period 3 seconds, Leeds & Northrup No. 2420D Standard celLa Eppley, student type, 1.0184 volts a For clarity, these a r e shown with meter range switch in several places. Only one i s required.
1266
ANALYTICAL CHEMISTRY
amperes, 6.3 Volts-5 amperes
Preparation of Electrolyte. To 14 liters of distilled water in a 5-gallon bottle, slowly add 15 pounds of S a R r (U.S.P. grade). Agitate t h e solution during t h e addition b y bubbling n i t h nitrogen. Add 330 ml. of 1 0 N NaOH, 450 ml. of 85% H3POa, and 6.9 granis of KI. Mix well. Paint t h e bottle opaque black t o exclude light.
-0
0
.-+3.-Y U
Function of Electrolyte. T h e electrolyte is 4 N S a B r , 0.0025M XI, 0.2M H3P04, and 0.2M KaHJ'Oh. T h e high concentration of halide lon-ers the cell resistance and ensures generation of free halogen at t h e surface of the anode. A slight cwess of halogen must be present at all times during the titration. Because iodine is less likely t o be volatilized than bromine, it is preferable to use a n iodometric titration rather than a bromometric one. However, if iodide ) \ c w used exclusively as a source of halogw, it mould be partially oxidized by the o\ygen in the gas stream; this does not happen n i t h bromide. Therefore, to reconcile these opposing (+i+vts, a ..mall amount of iodide is used with a large amount of bromide. The amount of iodide is sufficient to rcact with any free broniine that is formed during the generation: SKI
--AT+-
+ Br2S 2KBr + 1 2
The acidity of the electrolyte represents a compromise which keeps the loss of SO2 to a minimum. .4t greater acidity, the SOz is not strongly held in solution and may be swept out more readily. At lrsser acidity the SO2 is oxidized more rapidly by the oxygen dissolved in the electrolyte. Traces of combustion products that s l o d y Consume iodine may accuniulste in the electrolyte. To stabilize the end point, fresh dectrolyte is used for each sulfur detcrrnination. -4 single threeway stopcock on the right side of the titration cell faci1it:itc.s rapid change of the electroljte. CIRCUITRY
The complete wiring diagram of thP instrument is shown in Figures 3-5. Detection System. T h e current output from the indicating electrodes is filtered t o remove noise due t o stirrer, bubbles, and switching of t h e generating electrodes. The voltage supply, the detecting electrodes, and the input to the Bro\\n converter arc connected in series. A visual indication of the input signal to the Brown converter is given by the galvanometer nhen the meter rang2 switch is set a t L. M. or €1. The electromotive force of the cell containing a trace of iodine is opposed, but not quite overcome, by the voltage suppl).. A bias current supply is also ronrwted VOL. 32, NO. 10, SEPTEMBER 1960
1267
Figure 5.
Wiring diagram of constant current supply See legend Figure 3
to the input of the amplifier so that its current opposes that froin the titration cell. The bias current is adjusted so that with a slight excess of iodine in the cell, the current to the amplifier is zero. The cell operates as an amperometric indicator, the current output being proportional to the iodine concentration of the solution. For titration of SO2 an excess of iodine equivalent to 0.6 coulomb is maintained. The Bias Adjust and Voltage Adjust permit adjustment for decline of voltage as the batteries age. The voltage and bias are standardized by comparison with the standard cell The voltage applied to the electrodes can be reversed by the Voltage sn-itch. The input current to the converter can be reversed by the Detect switch. The instrument can be used with other low-resistance electrodes for potentiometric titrations by reducing the bias current to zero and adjusting the voltage supply to the potential of the electrodes a t the end point. K i t h such flexibility it is possible to adapt this instrument to a variety of uses. Amplifier. T h e Brown converter changes t h e input current into a 60cycle square wave which is amplified b y the 6SF5 tube and both halves of t h e first 6SN7. The 6ilL5 limits t h e maximum signal size. T h e signal then passes through a 6SN7 phase splitter t o t h e 6J6 driver stage and finally to the 656 amplifier output tube. The plates of the final stage are supplied with 60-cycle a x . from the amplifier power transformer. When the signal voltage on the grid and the voltage on the corresponding plate of the output tube are in phase, the tube acts as a full wave rectifier. Current flows through the tube during the whole cycle, energizing relay 2, thereby sending current through the cell. When the 1268
ANALYTICAL CHEMISTRY
input current to the Brown converter reverses, the grid voltage of the 6J6 changes phase by l S O o , and neit'her half of the output tube conducts, thus deenergizing relay 2 . K h e n relay 2 is de-energized, the generating current is shorter around tho cell. Timer. As the end of a titration is approached, i t is desirable to have a standard waiting period during n-hicli to check for drifting of the end point. The instrument' contains a timing circuit which can he adjusted to give a suitable Raiting period for any particular titration. When this period ends, the Finished light and the Buzzer signal the end of the titration. The timing circuit is comprised of the 6J5 tube, the 2D21 t'hyratron, a 24-pf. condenser, and a variable 10-megohm resistor (Tinier Adjust) on the grid of the 6J5, Assuming the Titrant sritch to be closed and the 24-pf. condenser moment,arily shorted by the Timer, the 6J5 passes more current, making its cathode more positive. This makes the grid of the 2D21 more positive, causing it to fire, thereby energizing relay 1. This turns off the Finished warning light, silences the buzzer, and connects relay 2 to t'he output tube of the amplifier. Every time relay 2 is energized, closing cont'act 2c. the timer is reset by shorting the 24-pf. condenser. Aft'er a sufficient waiting period a t the end of the titration, determined by the time constant of the 24-pf. condenser and the setting on the Timer Adjust resistor, the grid of the 655 becomes charged more negatively. This decreases the current t'hrough the cathode resistor of the 6J5, making the grid of the 2D21 more negative. This stops the firing of the 2D21, de-energizing relay 1, which sounds the buzzer, turns on the Finished light, and disconnects relay 2 from the amplifier output tube.
Constant Current Supply System. This system provides five current ranges: 6.02, 30.1, 60.2, 120.4, and 301 ma. These currents generate iodine equivalent t o 1, 5 , 10, 20, and 50 p g . of sulfur (as SO2) per second. Any small change in the current through the cathode resistor of the 6Y6 tubes changes the grid voltage of the 6V6 tube. The resulting change in plate voltage of the BY6 is then fed back to the control grids of the 6Y6 tubes to maintain the current constant at the selected value. The Range sn-itch connects the necessary number of BY6 tubes in parallel for the current range desired. The current is set to the correct amperage by varying the 30 K Range Adjust so that no galvanometer deflection is obtained when the voltage drop across the standardizing resistor is compared with the standard cell. Operation. Relays 3 and 4 are powered b y a separate filtered current from t h e selenium rectifier and are interconnected t o function as a makebefore-break sii-itch. Thus, t h e constant current is never interrupted, thereby eliminating arcing of t h e contacts. The sequence of operation is as follows : Starting with the constant current supply shorted around the cell, a loTTering of the iodine concentration in the cell causes the amplifier to energize relay 2, opening contact 2a and closing contact 2b. This de-energizes relay 3, closing contacts 3a and 3b. The closing of these contacts energizes relay 4 and connects the constant current supply to the cell. Energizing of relay 4 opens contacts 4a and 4b. Opening of contact 4b removes the short around the cell, forcing the current through the cell, therehy generating titrant. Accumulation of excess titrant causes the amplifier to de-energize relay 2 ,
opening contact 2b and closing contact 2a. Thir; de-energizes relay 4 , closing contact 4a (which energizes relay 3 ) arid contact 4b (which shorts the constant current, supply around the cell). iziiig of relay 3 opens contact's 3n and 3b. Opening of contact 3b discoiinccts the constant current supply fr(,Ii1 the cell, thus completing the seclii('IIc('. The momentary Manual snitch Can lie used t,o cause generation of titrant v-hen desired, as for the final adjustment of the end point. During the brief moment when the ccll is shorted, the polarization of the geiiei,ating anode and cathode tends t o sencl a titrant-consuming current in the opposite direction to the generating curimt' through t'he short and the generating electrodes. To prevent thip! tn-o 5U4 iwtifier tuhes are placed in se1,ies 11-ith the cell. Anticipator. T h e four electrodes are spaced so t h a t t h e p a t h of t'he coilstant generating current through t h e cell is a t right angles to t h e path of the detecting current (Figure 5). This minimizes the tendency of the genrrabing curient to disturb tlie detecting currrnt. Some interaction docs occur, h o ~ e v e r :when t h e generating electrodes arc operating; this affects the current' through the detecting electrodes so that the end point is overshot slight'ly. This is corrected by an anticip:itor circuit t h a t feeds back a small current froin the generating anode to the input of the amplifier. When this anticipator is properly set, the end pciint is not overshot. This control also can be used to givc a more precise end point by shortening the t>imeincrenwnts during which titrant' is being gmcrated. Electronic Clock. .in electronic clock, registering in t,housandths of a second! totals t h e t'iine that) t h e titrant is being generated. K h e n t h e cons t a n t current, flows through the selected clock resistor, t h e grid of t h e kft half of t h e 12ATi becomes more positive. This makes the plate of the left half and t h e connected cathode of the right half less positive. T h e removal of the positive bias from the cathode of the right half permits the positive half cycles of the 1000-cycle tuning fork to actuate the counter. When the constant current is shorted around the cell and the clock resistor, positive bias is applied to the cathode of t'he right half of the 12XT7, blocking the output of the tuning fork so that no counts are registered. 4 750-ppf.
condenser across the input of the electronic counter prevents false counts. CALIBRATION AND TESTING
The accuracy of the constant current supply, with the current flowing continuously, was checked by a n accurate series resistor and a potentiometer. The results are shown in Table I. The accuracy of the current supply and the clock, 11-itli the current flowing intermittently, was checked with a silver coulometer. An interrupter (Figure 4), which consisted of a motorized cam actuating a single-pole doublethron snitch 8 times per second, i n s inserted in the direct current power supply circuit of relays 3 and 4. This exaggerated the sv-itching of the constant current supply and the clock to approximately three times the number of interruptions occurring during a typical sulfur determination. The titrator agreed with the silver coulometer within 0.13%. IT-e believe that this method of obtaining and integrating a variable current by electronically timing pulses of constant current is potentially the most accurate method available. The accuracy could be improvpd further, if necessary, by using n higher frequency and a current supply of better constancy. PERFORMANCE
Table II. Determination of Sulfur in Oil Samples ASTU p,omb Induction Furnace Gravimetric Method, yo H Method, Manual Automatic titration" titration" COS 2 . 0 0 , 2.00 1.95, 1.91 1 . 7 7 , 1.76 0.85, 0 . 8 1 16.4, 16.4 1 2 . 7 , 12.6 0.0;. 0.05 i . 9 i : i.9i 2 . 4 5 , 2.46 0.44, 0.44 2 . 0 2 , 2.03 i & G , 4.81
1.9T, 1 . 9 6 1.87, 1.93 1.81, 1 . 7 4 0.81, 0.80 16.2, 16.1 12.5, 12.4 0 00. 0.01 1.831 1 . 8 8 2 . 3 4 , 2.36 0 . 4 1 , 0.47 1.93, 1.96 4 . 7 7 , 4.78
1.90, 1.96 1.89, 1.95 1 . 7 7 . 1.78 0 . 8 3 , 0.80 16.5, 16.8 13.0, 12.4 0 . 0 5 . 0 05 1.93: 1.84 2.43, 2.41 0.42, 0.41 2.01, 1.90 4.85, 4.81 Results calculated without use of empirical factor recommended in ( 2 ) . Q
than adequate since most of the error is associated with the combustion of the sample. The instrument has been used over a 3-year period for about 4000 sulfur determinations. Mercaptan titrations have also been carried out. It can be adjusted to perform coulometrically any amperometric or potentiometric titration employing lorn impedance electrodes. This includes acid-base, precipitation, and oxidation-reduction titrationq. ACKNOWLEDGMENT
The sulfur content of a variety of oil samples was determined by an induction furnace method similar to that described in ( I ) . The data obtained by both automatic and manual titration are shown in Table 11, which also s h o w results obtained by the ASTN bomb-gravimetric method ( 2 ) . An empirical factor was not used in calculating any of the results. I n general, the results obtained by the automatic instrument are comparable to those obtained by manual titration. For the determination of sulfur tlie accuracy of the instrument is more
The authors thank E. T. Scafe, who first suggested the possibility of using an automatic coulometric titrator for this purpose, and Perry Swanson and 0. I. llilner, who guided the development of the method; also John Wescott and S. L. Duncan for helpful suggestions in designing and constructing the instrument.
Accuracy of Constant Current Supply Current Current Range Measured, Error, Betting, Ma. Ma. 70
( 3 ) DeFord. D. D., AKAL.CHEU.28, 662 (1956). (4) DeFord, D. D., Record Cheiri. Progr. (Kresye-Hooker S a . L i b . ) 16, 165-74 (1955). ( 5 ) DeFord, D. D Bowers, R. C., AKAL.CHEW30,Sik (1958). (6) Rice-Jones, \T. G., Zbid., 2 5 , 1383 (1953).
Table 1.
301 . O 120.4 60.2 30.1 6.02
301.4 120.3 60.12 30.05 6.02
+o. 13 -0.08 -0.13 -0.1; 0.0
LITERATURE CITED
(I) Am. Soc. Testing AIaterials, Philadeluhia. Pa.. ASTRI Standards on Petroleum Products, Appendix 11, p.
944, 1956. (2) BSTM D 129, Test for Sulfur in Petroleum Products and Lubricants by
the Bomb Method.
RECEIVED for review September 23, 1959. Accepted June 1, 1960.
VOL. 32, NO. 10, SEPTEMBER 1960
1269
