Automatic Coulometric Titrations with Externally Generated Reagents Applications to Acidimetry and Alkalimetry DOh-i-iLD D. DEFORD, C.4RLrON J . JOHSS, . m J.-IIIES ~ N. P I T I S .Verthwestpr n I -ni rersi t y , Ercrns ton. T l I .
I
P R I 3 I O C S pap~~i's ( 2 , 3 ) thv autliora outliiied thcb fundament,al principles of coulometi,ic titrations with externally gwerated reagents and presented the rrsults of typical manual titrations, One of thr niajor advantages of coulometric titrations is the ease of making the process conipletely automatic. Tlie rclagc.iit in all coulometric titrations is an elect,ric current. whirh can readily be regulated, measured, and controlled through the applicatioii of cmvriitional elert,rical eirruits. >[any automatic. instruments for use with stantlard solutions oi the titrant have been dwrribed ( 1 , 5 , 9 , 10, I S ) . At least I wn, one manufactured by the Prrrision Scientific Co., 3737 ('ol,tland St.. Chicago 47, Ill., and the other manufactured by Ilwknian Iristruments, Inc., South Pasadena 15, Calif., are now :t\-ailablv cwinmrrciall~-. .4utoniatic coulometric analysis bj- tlir c lirect method has heel1 rxplorrd by 1,ingaiie ( 7 ) and by Lingano and h a l l (8).I n both of thew studies the electrolysis electrodtis were placed in the solutioii cwitaining the sample, and the workiiip electrode was o p e r a t d at R suitable constant potential which permitted only the desired electrolysis reaction to occur. Thv completion of the electrolysis was indicated by a cessation of th(2 clr~rtrolysiscurrent, and the number of coulombs required was cic~terminedby means of a cnulonic~terin series TTith t h r rlwt rolysis cell. T h r only previoub \vvork in the fitild of automatic. roulomc>tric titrations is that of Shaffer, Ariglio, antl Brorkman (I?), who us~cl a c*ontinuousautomatic titrat,or for tlrtrrmining the roncentration of mustard gas in air. Thvy passed air through the titrat i o n asscmhly at a constant ratr and generated bromine cx1ec.tid,vtically at a rate that was just sufficient, to react with the niustard vapor. The generation rurrent thus provided an inytantaneous and vont,inuous ni('asure of the concentration of the niustard vapor in the air. ?;o previous studies on the automatic, cvulomctric titration of disiawtc. samplcs have been described. The present paper dewrilws xn instrument for the automatic performance of coulometric titrations with exteriially gcneratrd wagents. This instrummt (.inploys x roiistant and carefully regulated electrolysis cwrrrnt. The number of coulombs of electricity required for the titrxtioii is then determined from the product of the current and thr tiniv. T h e end point in the titration is detec.tc4 hy means of R I I nrdin:tr,y Jhlinian >Iode1 CT pF€ metcr, S
1uc.h actuates a relay antl thus terminate.%the titration when the equivalence point pH has been attained.
\I
.iPFARATUS
hutorriatic coulonictric titrations by nirans of the constant current technique require an instrunimt that is capabltl of maintaining a constant electrolysis current during the titration, tietecting the end point, and terminating the titration at this point. Current Regulator. Thv complete electrical circuit of the rurrent regulator is shown in the lower half of Figurch I . Its operation is best explained by referenre to t hri simplified wiiiiblock diagram shown in Figure 2.
Thc. regulator is placed in operation by adjustment of resistitnces R2,Rs, and Ri2,so that the desired electrolysis current, indicated either by the meter, Mi, or by the potential drop across the precision resistor, R,,, ia passing through the generator cell, and so that about one half the maximum plate current is flowing through the l l i L 7 control tube. Meter JT? should read about 40 ma. If now the resistance of the generator cell should increase, a momentary decrease in current through the cell, and hence also through resistors Rs, R,o, and R12,will result. The decrease in current through these resistors causes a decrease in the input potential t o the direct current amplifier. This small decrease in input potential is amplified, and results in a comparatively large decrease in the output potential, which is applied t o the grid of the control tube. The decrease in grid potential of this tube rauses a reduction in the plate current through the t.uhe. As thr plate current of the tuhe passrs through the resistor, R1, the climinution in plat,e current result,s in a diminution of the current passing through this resistor. The lower current. through the resistor decreases the potrntial drop across the resistor and hence makes point a more positive. The entire operation thus results in an increase in the potential across the electrolysis cell. ial applied to the cell tends to compensate t a m e antl hrnre maintains the current at a comtant value. The operation of the circuit in compenr:tting for chitngrs in the supplv vo1t:tge follows it similar pattern.
R,,,
This rttgulator differs from most current regulators which have heen tltwrihed prrvioris1~-in that the load current is not required to pass through the control tuhe. This feature is a significant advantage when generation currents in excess of about 100 ma. are being employed. This l)y-pasi. repulutor permits rrgulation
In the final analysis all coulometric titrations employ an electric current as the titrant. Because electric currents are readily susceptible to automatic regulation and control, this type of titration can be performed automatically with comparative ease. A n automatic instrument for the performance of coulometric titrations with externally generated reagents has been employed successfully in several acid-base titrations w-ith externally generated hydrogen and hydroxyl ions. The average error for individual determinations is slightly less than *0.3%. As the entire titration is automatic, and no standard solutions are required, only a few seconds of operator time are required for each titration.
941
ANALYTICAL CHEMISTRY
942
of gen~rationcurrents of a t least 500 ma. by means of a single tube which has a maximum plate current of about 80 ma. The values of the circuit components given in Figure 1 are suitable for an electrolysis current in the neighborhood of 250 ma. By appropriate changes in the values of resistors R1,Rz, Rg, Rlo, &I, and Rlz, any desired electrolvsis current between 10 and 500 ma. may be secured. I n order to achieve maximum stability and frkedom from drift, the direct current amplifier employed in the regulator is batterroperated. The 115-volt direct current was obtained from the laboratory supply, which originated from a bank of storage batteries. After an initial warm-up period of about 15 minutes, the current drift a t a generation rate of 250 ma. has never been observed to exceed 5 parts in 10,000 over a period of 1 hour. The variation in current for a change in supply voltage of *4 volts or for a change in cell resistance of *16 ohms is less than 5 parts in 10,000a t a generation rate of 250 ma. Terminator Circuit. The terminator circuit is shonn in thc upper half of Figure 1. The terminator consists essentially of a Beckman Model G pH meter, the output of which is connected to a lamp and scale galvanometer, and a photo relay unit which is actuated by the light beam from the galvanometer. Two binding postq, connected by internal leads to the terminals of the null point indicating meter, u-ere affixed to thtx Rrckman mpter in t h c manner deseribed by Lingane ( 6 ) . Thrw
Figure 1.
posts were then connected through the voltage-dividing network, Rl,-Rla-R,5, to the galvanometer terminals of a Leeds & Northrup No. 2420c lamp and scale galvanometer. The resistors in thc voltage-dividing network were chosen so that the galvanometer index would traverse its full scale as the null meter of the Beckman instrument traversed it’s full scale. The galvanometer is critically damped to prevent response to erratic fluctuations in the reading of the p H meter. The connection of the voltagcdividing network and galvanometer to the pH meter in no T-iay affects the operation of the pH meter. The zero of the galvanometer was adjusted so that the index was in the center of its scale when the Beckman meter was a t the null position. The ground-glass scale of the galvanometer was removed and replaced by a light shield covering half of the opening. The photo relay unit (G. M. Laboratories photo relay unit, Catalog No. 1222-E) was placed directly in front of the opening in the galvanometer case and was adjusted so that the relay was actuated when the galvanometer index passed the end of the light shield. The photo relay was then connected to a double-pole double-throw power relay which makes and breaks the necesEary circuits for starting and stopping t,he titration. The direction switch, 8 3 , is included in the circuit so that the titration may be terminated by movement of the pointer of the Beckman null meter from right t,o left or from left to right past the null position, whichever is required for the titrat,ion. The operntion switch, Si, makes possible preliminary adjustments and preparations for the titration with the generator cell disconnected from the circuit (OFF position). The titration is then h e w n by throwing this snitch to t,he O S position. The pilot light, I.’?, indicates that a titration is either in progress (operation switch on) or will begin as soon as the operation switch is turned on. The reversing switch, Sa,permits the electrodes of the grncrator rcll to be operated a t the desired polarity.
Electrical Circuit of Titration Instrument
V O L U M E 23, N O . 7, J U L Y 1 9 5 1 The rheostat, RIG, is automatically switched irito the circuit i n place of the generator cell whenever a titration is not in prog1 ~ 8 s . This rheostat is adjusted so that its resistance is the same : i ~the effective resistance of the generator cell. (The effective iwistance of the generator cell is defined as the potential applied t o the cell divided by the current flowing through the cell.) l~ecausethe current regulator drifts badly until it has been 111 operation long enough for the resistors to come to temperature tquilibrium, the rheostat must be provided so that the regulator nil1 continue in operation at all times. GENERATOR CELL 0
D.C. AMPLIFIER
943 1)y .I!].
Allow 15 minutes for the apparatus to wariii u p , and then repeat the adjustment of K , and R12, if necessary. 6. Adjust Rg as necessary, Y O that the potentionietrr iiidicates a current of 250.0 ma. through the precision resistor. 7 . Turn on and calibrak the p H meter in the conveiitioiial manner as outlined in the manufacturer’s instructions. 8. Set the direction switch, S3,so that control in thr draiwd direction is obtained. 9. Set the reversing switch, S4,so that the geiiei,atoi, ele+ trodes are of the desired polarity. 10. Place the sample, dissolved in allout 150 nil. of‘ water, in the titration beaker. 11. Set the drum on the p H meter to read thr drsirrd titration exponent-the p H of the equivalence point. 12. Lock the push button on the pH meter donn. 13. Turn the stirring motor on. 14. Set the electric timer at zero. 15. Turn the operation switch to the OS positioii to stapt the titration. 16. Check the current by means of the poteiitiometei, and, if Iiecessary, repeat operation 6. (If rheostat Rls has beeii set at the proper value, no further adjustment of current will be newssary.)
D C
Figure 2. Semiblock Diagram of Current Regulator
The same power relay which controls the electrolysis current also controls the operation of an electric timer and a solenoid valve. The value of the resistance, E,,, in series with the solenoid was selected empirically to provide the maximum current through the solenoid which would not result in overheating.
Titration Assembly.
GENERATol
ELECTROLY
The titration assembly is shown in
Figure 3. C
The titration vessel was a 400-ml. beaker into which the electrodes of the p H meter, an electric stirrer, and one delivery arni of the generator cell were dipping. The generator cell has been described ( 2 , 3 ) . The generator electrolyte was supplied to the cell by hydrostatic pressure from an overhead stock bottle. .-i solenoid valve in the supply line permitted solution to flow to the cell during the titration and terminated the flow a t the end of the titration. The flush-out tube, which becomes filled with electrolyte when the solenoid valve is opened, was provided to supply a few milliliters of electrolyte to flush all products of electrolysis out of the generator cell a t the end of the titration. The rate of flow of electrolyte through the generator cell waB maintained a t about 0.2 ml. per second (0.1 ml. per second through each delivery arm). The flow rate was not carefully controlled, as it appeared to have no effect on the results. Very slow flow rates were avoided, because the products of electrolysis must be delivered to the titration beaker without significant delay. The glass electrode was placed approximately 5 mm. from the delivery tip of the generator cell and in such a position that the rotating liquid in the titration vessel passed from the delivery tip toward the glass electrode. Because an appreciable time is rexuired for the stirrer to accomplish complete mixing, it is essential t a t the glass electrode be placed very close to the delivery tip if overtitration is to be avoided. There is no danger of error due t o incomplete titration; the generator is automatically turned on again if, after mixing is complete, the p H meter indicates that the set equivalence point p H has not been passed.
d
Figure 3.
The following procedure was followed in the performance of all titrations. 1. Fill the generator electrolyte reservoir with a solution that is approximately 1.0 X in sodium sulfate. Adjust the p H to 7.0 * 0.5, if necessary, by addition of dilute sodium hydroxide or
dilute sulfuric acid. 2. Set the operation switch, 8 6 , in the OFF position. 3. Adjust the resistance of the rheostat, RIB, so that it is equal to the effective resistance of the generator cell. 4. Turn on the power switches, Si and S p . 5. Set the resistance of Rs at about one half of its maximum value, and adjust rheostats RPand R12 so that a current of 250 ma. is indicated by Af2 and a current of about 40 ma. is indicated
Titration Assembly
17. When the titration has been completed, as indicated by the pilot, Pg, return the operation switch to the OFF position and read the electric timer. 18. Calculate the number of milliequivalents in the sample from the relationship Me. =
PROCEDURE
SOLENOID VALVE
current (amperes) X time (seconds) 96.5
After the preliminary adjustments have been accomplished, the operations that must be carried out by the analyst for each titration are very few and simple. I t is necessary only to place the sample in the titration beaker, return the timer to zero, and start the titration by turning the operation switch on. At the end of the titration the operation switch is returned to the OFF position and the reading of the electric timer is recorded. RESULTS AND DISCUSSIOS
The results of several series of typical acid-base titrations are summarized in Table I. The hydrochloric acid and sodium hv-
A N A L Y T I C A L CHEMISTRY 'Table I. $ample
HCl
Tahrn .TI e . 0 ?444
Time Calcil. SlC.
94.3
.\\..
Tilnr Ohserred .Set.
94.3 94.4 95. I 94.2 134.3 93.6 04.3
Found Me. 0.2444
0,2446 0,2464 0,2440 0,2444 0.2425 0,2444
Del-iation from Mean
Hestilts of Typical 'I'itratioii. I-rmr
(7
c
0.0 0.1 0 . (1 0.2 0.0 0.8 0 3
0 0 -0.1 -0 9 -0 2 0.0 -0 8 0.0
Sample
Taken Me.
S-aOH
1 268
Tillie
Calcd Sec 489 4
1 222
47l.i
.4r.
H C1
Av.
2 444
$4.3 4
473 4 469,3 472.4 470. 2 471.6 472.1 471.5
1 226 1.216 1.224 1,218 1.222 1.223 1 221
0 4 0.4 0.2 0.2 0.1 0.2 0 2
-0
-0
I
938.0 041,l 943.5 939.4 941.0 945.1 941.4
2.430 2.438 2,444 2.434 2.438 2.448 2,439
0.4 0.0
-0 -0 0 -0 -0 -0 -0
6 2 0 4 2
0.2 0.2 0.0 0.4 0 "
3
RHP
1.347
.;in n
-0.2
-0.2 -0.3 0.0 -0 1
2
2
tlrositie samples werc aliquots of holutioris that had been carefully standardized b y c~onventionalprocedures. The potassium acid phthalate and the sodium carlionate samples n-rre prepared l)y direct weighing of the reaycwt grade salts. The average deviation of individual results from the mean is o f the order of 0.2 to 0.37, for all systems studied. T ~ results P of more recent work in this lahoratory indicate that prerisions much bet,ter than this can be obtained with rartxful \vork. Hooker ( 4 ) has rcwntly completeti a rwies of titrations of hydrochloik acid samples in Tvhich the avtlragr deviation of individurtl results from the mean is of the order of 0.04 to 0.06%. \Tith t,he exception of the det,erminations of sodium carbonate, positive and negative errors occur in approximately equal numbers, indicating the absence of an) teniatic error in the method. The analysis of the sodium carbonate samples was based upon the t,it,rationof carb0nat.e with hytlrogcri ions to form bicarbonate. The wrors in the rcwlts obtained in this series of determinations arc rrmarkahly small in view of the small slope of the tit,ration m"'e at the equivalence point. The consistent positive wror would seem to indicate that the titration was not tc~minatctlvxavtly a t the equivalcrice point. Preliminary titrations of -odium t*art)onate l)y convrntional methods indicated that the rquivnl i w w point oc:curred at a pH of 8.0; the pH meter was;.t1icwfoi.r. adjusted to terminate the titration at this pH. A sec~nidswit+ of titrations \vas performed in whirh thc. titration was terminatcti at pH 8.3, which is the theoretically pi,tvlictrd pH for thtl equiva1enc.e point if values of 4.3 X lo-' and 5.6 X lo-" are employed for the acid dissociation ronstants of carbonic acid. This series of titrations gave results which were consistently low by an average of 1.5O;. It would appear that a pH of about 8.1 should be selected as t,he proper point a t \vhich to terminate thi. pwticular titration. (:0\cLus101 s
4utomatic coulometric titration of ac~idi :inti hawk ('an be accomplished with a preciiion and accuracy 14 h1t.h are adequate for most determinations After initial adjustment of the instrum m t , only a fen- qcronds of the analyst's timtl ai(' iequired for cw,h titration. Work now in progresi: in this laboratory indicates that the tttrminator circuit may also he actuated by an amperometrie indirator system connectrd directly to the terminator galvanomtxter (11). Still other indicating deviw.: may he made to actu:itrJ th[i terminator.
.SW.
485.7 489.5 494.2 490.0
AV.
H CI
riiiie
O!)wrred
... 0.821 0.892 1.129 0.668 0.433
...
...
,.
3lti!r 344.3
435.8 257.8 167 9
...
491.2 490.3 &SO. 8 400.2 321.5 335.0 376.2 280.9 1032.0 80.5.2 865.0 971.9 317.8
348.1 436.9 2 5 8 , $4 I67 4
...
I'ound .Me,
1,258 1.268 1.280 1.269 1.273 1.270 1.271 1.270 1,351 0.868 0.975 0.728 1.673 2 086 2.241 2.318
...
0 823 0,902 1.132 0.671 0 434
...
Deviation trom Mean
El 1'111.
O&
0 0 0 0 0 0 0 0
9 2 8 1 2 0 1 3
-0
s
(I ( I !J
-0 -0 -0 -0 -0 -(I
1
4 2 2 2
- (I
:i .i
-0 -0 -0 0
''
-0 -0
1
I
"
0
-0 1 -0 I -0 2 -1
-0 -0 -0 -0
1 3 4 2 4
By ubing the current r~gulatordescribed above in conjuiic,tion with a recording pH meter, it should be possible to recon! titration curves of acids anti hasps automatically. This mat t('r is now being investigated. The coulometric titration nivthod appears t,o be well adaptwl to continuous automatic titrations, and thus provides a means u!' obtaining a continuous rwor(l of instantaneous concentrations in flow processes. .4n instrunic~it for the cont,inuous autoniatir titrat,ion of acids in the efflurnt solution from a chromatographic, rolumn. now being tested. grrwrates hydroxyl ion at' a rate just sufficient t,o neutralize the acids as they emerge from thr cshromatographir column. .4recording ammet'er in series with thr generator cell provides a continuous record of acid coricentra tion in the effluent as a function of time. The necessity of having a supply ot' 115-volt direct c u r ~ t ~ i i t for the current regulator described is a distinct disadvantage. Instrument,a operating direcatly from 115-volt alternating current are now bring tested. Iniproved terminator circuits which arr less bulky arid more 1~11ggrtlthan th(1 nne rlescrihed are also hriiig twtcd. ACKNOWLEDGMENT
The authors are indebted to Donald T. Hooker f o r p c ~ r f n i ~ n i i i ~ g the carbonate titrations. LITER4TURE CITE11
Barredo, J. AI. G., anti Tag-lor. J. K , . T m r r s . E l e c t r o c l i r m
.hr,,
92, 437 (1947).
DeFord, D. D.. Pitts. J. X . , and Johns. C. J., ANAL.CHEM..23. 938 (1951). DeFord, D. D., Pitts. J. S . . and .Johns C..I Proc S u f i dcuci. S c i . I'. S., 36, 612 11950). Hooker, D. T.. unpublished results. . 20, 285 i1948). Iingane. J. J..A N ~ LCHEX.. l h i d . . 21, 497 (1949). Iingane, J . J.. J . .im.Chem. Soc., 67, 1916 (1945). Lingaiie, J. J..a n d Small. I.. -\., . \ s i r . . CHEM.,21, 1119 (1949). Ponipeo, D . .J.. Penther. C . .J.. anti Hsllikainen. K. E., I n s f w mrnts, 16, 402 (19431. Robinson, H. -4..T m m . Elsctroclrern. .k., 92, 445 (1947).
Schmali, E. -4.. unpublished results. .TI... Shaffer, P. A , . J r . , Briglio. A., .II,.. and Brockman. J. -i., ANAL.CHEM..20, 1008 11948). Widing, R. -4,, and Farquharson, .J.. L-.5. Atomic Energy ('ommission. AECD-2836 (Dec. 23, 1949). RLCEIVED February 7 . 1950. Presented before the Division of Analstival Chemistry a t t h e 119th Meeting of the AvEmr.tx CHEMICAL SOCIETY,Hrist o n , Mass.
