Automatic Differential Potentiometric Titrations Electrode Systems for Aqueous and Nonaqueous Titrations H. V. M A L M S T A D T
and E.
R. FETT
Noyes Chemistry Laboratory, University o f Illinois, Urbana,
‘The initial success and interest in the automatic differential potentiometric titrator have led to an investigation of its characteristics and applicability in all general types of titrations. Examples of precipitation, roniplex formation, aqueous and nonaqueous acidbase and oxidation-reduction titrations illustrate the important features of the automatic differential technique. The potential response characteristics of \ arious indicator electrodes for the different titration types are demonstrated with both recorded second derivative titration curves and automatically obtained titration data. Indicator and reference electrodes that shift or do not establish a reproducible potential are nevertheless applicable for automatic differential titrations. The effectiveness of an electrode systeni of platinum-rhodium allo? indicator electrode us. graphite reference electrode in nonaqueous titrations is demonstrated. The titrator yields automatic titration results of excellent precision, even with titrant delitery rates greater than 10 nil. per minute in some cases.
111.
The data presented for the examples of precipitation, soluble complex, aqueous acid-base, and nonaqueous acid-base titrations indicate that a wire of platinum-lO% rhodium alloy serves as an excellent indicator electrode. The platinum-rhodium wire might prove to be an extremely versatile indicator electrode for automatic differential potentiometric titrations; however, it will have to be tried in many more titration systems to establish its versatilitv.
110 v. 60 N
11 6
110 v. eo hr OUTPUT TO BURET MOTOR
YEl17 type of automtltic pot,entiometric tit,rator was recently described ( 2 4 ) whereby the inflection point of a potentiometric curve was automatically detected and used to Figure 1. Relay system for automatic differential titrator turn the buret off. This was accomplished by electronicall>producing a voltage which was proportional to the second derivaRI. ZOO-ohm wire-wound potentiometer CI. 8 d d . , 150-volt electrolytic condenser t i w of the ordinary potentiometric titration curve. This voltCz. 500 ilfd., 20 WVDC electrolytic condenser :igtx function was ideally suited to trigger a relay system which SI. Push button (spring return) SPST switch Relay 1. 110-volt a x . SPDT relay t,tii,iicd the Iiuret off a t the inflection point (end point) of t8he Relay 2. 6-volt d.c., SPST. normally closed relay Relay 3. 110-volt &.e., SPST, normally open relay titnttion. BI. 3-volt bias battery Lamp. 110-volt a.c. indicator lamp Sweral advantages (2.4) characterize the automatic differential VI. 2D21 thvratron tube titrator. The equipment is simple, compact, and inexpensive, Ti. TransfoFmer (isolation) ; primary 110-volt, 60 cycles: secondary 110volt, 60 cycles, 100 ma., and 6-volt, 60 cycles, 1 ampere anti there are no end-point potentials to set or other instrument SP. Selenium half-wave rectifier adjustments to make. Various types of reference and indicator electrodes can be used, even t,hough their absolute potentials shift from one titration to the nest, or the electrodes undergo a The relay system as originally designed prevents false end drift in potential during a titrat,ion. points from various types of electrode noise which sometimes The automatic differential titrator is not suit,ed for titrat’ions occur in certain titration procedures. This is illustrated Tvith where the solution or electrodes reach equilibria very slowly ( 2 4 ) . recorded curves for specific systems. Titrant flow rates up to ITowever, for most titrations the solution equilibria are attained 10 ml. per minute are possible in some cases without appreciable sufficiently rapidly, even for rat,her rapid titration rates. Elecovershoot of the equivalence point. Examples are also given trodes are available n-hich respond very rapidly t.0 potential where flow rates should not exceed about 2 ml. per minute so that rhanges a t the end point for precipitaDion, soluble complex, oxisolution equilibria are maintained. Many of the experimental tiation-reduction, aqueous acid-base, and nonaqueous acid-base data were obtained a t relatively fast flow rates, which are estitrations. Although certain electrodes have slow rates of pecially desirable for routine titrations. potential response, these rates are relatively constant, and t’he precision obtained with such electrodes is good. EQUIP3IENT AND GENERAL PROCEDURE The glass electrode cannot be used with the titrator as orig911 experimental results were obtained using the automatic in:tlly described because of its high resistance, but this can be titrator (2.4). A uniform flow buret (4)in conjunction with a wmedied by preceding the first amplifier stage Iyith a high input relay-operated pinch-clamp device was used for delivery of the impedance stage. However, a noble metal indicator elect’rode titrant. I t was found desirable to supplement the sinteredc u i he used in the titration cases where a glass electrode is usglass tubes for varied lengths of capillary tubing for use with ually used thus eliminating the necessity of a high input impedalkali titrants, and to replace daily the rubber tubing running ance stage, unless the solution resistance across the electrode through the pinch clamp when used with the benzene-sodium pair is estremely high, as in the case of certain nonaqueous titramethoxide solutions. tions. If it is necessary or desirable to use t.he glass electrode or I n most cases the relay system was satisfactory (24). This if the resistance of the electrode pair is very large, a commercial system, however, required that the electrode potential swing direct-reading p H meter can be used by connecting directly from negative in the end-point region, because the relay operated on the feed-back resistor of the pH meter to the input of the differenthe negative pulse of the second derivative curve. I n the case tiator. 1757
1758
ANALYTICAL CHEMISTRY
of the glass electrode it was impossible to reverse the electrode connections on the p H meter input. When the glass electrode was used and the potential break was initially a positive voltage swing a t the end point, use of a redesigned relay circuit was necessary to operate on the positive pulse of the second derivative curve. This was accomplished by connecting the relay circuit as shown in Figure 1. The operation of the relay circuit is essentially the same as described (Zd), but the thyratron is biased negatively by a 3-volt bias cell so that the thyratron does not conduct until a positive voltage is applied to the grid. Relay 1 in the plate circuit of the thyratron must be connected so that the contacts are as shown in Figure 1 when not energized (no signal on grid of thyratron). An isolation transformer for the 60-cycle, 110-volt plate supply of the thyratron also is provided to eliminate the necessity of a polarized plug. The 6-volt winding on the isolation transformer is used in conjunction with a half-wave selenium rectifier to provide the char ing voltage for condenser CS. A variable time delay is provifed by the potentiometer, R1. -4Beckman Model H-2 p H meter was employed for use with the high resistance glass electrode. I n this case, the electrodes were connected to the pH meter in the usual manner, and the voltage across the feed-back resistor was fed to the input of the amplifier-differentiator circuit. No connections in the pH meter need to be changed, as the leads across the feed-back resistor can be attached with clips. I n a nitrogen atmosphere the titration vessel was closed mith a large rubber stopper equipped with holes for the electrodee, stirrer, buret tip, and nitrogen inlet and outlet. The buret was filled by nitrogen pressure by employing a v,-ide-top buret, which had a rubber stopper with holes for delivery and an air outlet equipped with an Ascarite drying tube. The recording of the second derivative titration curves served primarily as a permanent record for each titration, and, also, made it simple to observe electrode behavior while trying different electrode systems for various types of titrations. The two types of recorders employed were a full scale Brown recording potentiometer, 50 my., and an Esterline-Angus recording milliammeter with an amplifier to convert it to a recording millivoltmeter. The curves discussed in the following sections are faithful reproductions of the titration curves obtained with one of the above recorders in the circuit. The electrode tip must be below the surface of the solution and the stirring must be efficient. Motor-driven paddle stirrers provide good stirring, but magnetic stirrers do not generally stir effectively as they usually result in streaming of titrant from the buret tip. This often causes electrode noise. REAGENTS AND ELECTRODES
HYDRkZINE SULFATE, commercially pure. POT4SSIUM IODATE SOLUTIOS.-4 5.350-gram sample of potassium iodate was dissolved and diluted to 1.000 liter nith freshly boiled distilled water to make a 0.0250021f solution. ACETIC ACID SOLUTION.Prepared to approximately 0.1.v by diluting reagent grade glacial acetic acid. HYDROCHLORIC ACID SOLUTIOS. Prepared approximately to 0.lh’ by diluting concentrated reagent grade hydrochloric acid. SODIUM HYDROXIDE SOLUTIOS. Si.; milliliters of saturated sodium hydroxide was diluted with freshly boiled distilled Tyater to make 1 liter of approximately 0.lA- solution. SODIUM CHLORIDESOLUTIOS. Six grams of reagent grade sodium chloride was dissolved and diluted n i t h 1%-aterto make 1 liter of approximately 0.1S solution. SILVERN I T R ~ T E SOLUTIOS.A 17-gram Pample of reagent grade silver nitrate was dissolved and diluted to make 1 liter of approximatell 0.1.1- solution POTASSIUM CYASIDESOLUTIOS. Seven grams of reagent grade potassium cyanide was dissolved and diluted with distilled water to make 1 liter of approximately 0 . 1 S solution. MERCURICS I T R ~ SOLUTIO\. TE A 25-gram sample of reagent grade mercuric nitrate 11-as dissolved and diluted with distilled water to make 1 liter of approximately 0 1K solution. GLACI 4~ ACETIC ACID,reagent grade. ASILISE SOLCTIOS. iipproximately 9 ml. of redistilled aniline was dissolved and diluted with glacial acetic acid to make 1 liter of approximately 0.1-V solution. PERCHLORIC ACIDSOLUTIOS. Kith glacial acetic acid 9.8 ml. of 72% perchloric acid was diluted to 1 liter. Then 18.5 ml. of acetic anhydride was added and the solution allowed to stand for 24 hours. This makes an approximately 0.LV solution. BENZEXE, purified grade. BESZOICACID,primary standard grade. METHANOL, absolute.
SOLUTION. Mix three volumes of BENZENE-METHAXOL benzene with one of methanol. SODIUM METHOXIDE SOLUTION.Six grams of commercially available sodium methylate was added to 150 ml. of methanol and then diluted with 900 ml of benzene. GLASSELECTRODE, Beckman 4990-80. CALOMELELECTRODES, Beckman fiber capillary type and Leeds & Northrup sleeve type. ANTIMONY ELECTRODE. Spectrographically pure antimony was sealed in glass tubing with mercury contact to lead-out wire. GRAPHITEELECTRODE, six-inch lengths of I/r-inch spectrographic graphite electrodes. WIRE ELECTRODES. Platinum, platinum-10% rhodium! platitinum-40% rhodium, rhodium, palladium, and silver wires of about 1-inch lengths and 0.5 mm. in diameter were sealed into 6-ml. glass tubing with about 0.5 inch of wire protruding. Mercury was used inside the glass tube for contact between the electrode wire and lead-out wire. QUIKHYDRONE ELECTRODE, Leeds & riorthrup quinhydrone with platinum wire. AQUEOUS ACID-BASE TITRATIONS
There are more than 100 literature references to various indicator electrodes which have been used for pH measurements and acid-base titrations (I8-17). However, the glass electrode has become the favorite electrode in most laboratories for both pH measurements and acid-base titrations. The other pH-sensitive electrodes have been primarily of academic interest in recent gears. It, Kould seem logical, then, also to employ the glass electrode as the indicator electrode for acid-base titrations when using the automatic differential titrator, but there are two reaaons why other electrodes are usually preferable with this instrument, The primary reason is that the glass electrode does not respond instantaneously to changes of pH in the solution; therefore, the titrant flow rate must be relatively slow to prevent overshooting the equivalence point, or a blank correction must be applied to correct for the delay in electrode response. Another reason is that its very high resistance necessitates a high input impedance stage to precede the differentiator circuit, and this unnecessarily complicates the equipment if a suitable low resistance electrode is available. The antimony, platinum, platinum-10% rhodium alloy, platinum-40% rhodium alloy, rhodium, palladium, and graphite electrodes were used to compare the response of the glass electrode to other pH-sensitive electrodes for the titration of 10 ml. of approximately 0.1S acetic acid with 0.1-Y sodium hydroxide. The important point of the comparison is the relationship between the inflection point of the electrode potential curve to the equivalence point, as illustrated in Figure 2. Indicator and Reference Electrodes. The curves in Figure 2 are the ordinary and second derivative potentiometric titration curves for each electrode system as recorded on a Brown recording potentiometer for the titration of acetic acid with sodium hydroxide. -411 curves were obtained with a titrant flow rate of 2 ml. per minute. The broken lines represent the phenolphthalein end point. The displacement of the inflection point of the potential curves from the broken line roughly illustrates the delay of electrode response to changes of pH in the solution. Curve A in Figure 2 shows the ordinary and second derivative curves for a glass-calomel electrode system. The inflection point of the titration curves lags the phenolphthalein end point by a small amount when the titrant flow rate is 2 ml. per minute. -4greater lag of inflection point is illustrated in curve B for the antimony indicator electrode with all other conditions the same. The lag in the antimony electrode is not too surprising in view of its complicated electrode reaction (6). Quantitative data are presented to illustrate that the lag is reproducible. Curve C illustrates that an untreated platinum wire electrode also responds to changes of pH, but its response is not reproducible and the inflection point lags the phenolphthalein point by a rather large amount. Attempts to use a platinum electrode for acidbase titrations have been numerous (IS, 17, do), but it was found
V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5
1
1 I
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CURVE GLASS
v i
A
CALOMEL
I
1759 CURVE
1
1,
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ANTIMONY
I
YL
0
I
,
CALOKEL
-
CURVE
13
PLATINUM vs CALOMEL
CURdE
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CLRbE
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I I I
Figure
1
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2. Recorded ordinary and second derivatiwe potentiometric titration curves For
I
n 1 1 3 uelectrode ~ systems in aqueous
( 7 , $9) to give a poor change of potential a t the equivalence point, and this waa substantiated by curve C. The potential curves for the same acid-base titration with identical conditions, but with platinum-rhodium alloys and pure rhodium indicator electrodes, are given in Figure 2, curves D,E, and F . The inflection point of these curves nearly coincides with the phenolphthalein point, and the magnitude of the potential increases. This phenomenon is not considered in the present discussion, but should prove interesting for future investigations. All three of these electrodes appear ideal for acid-base automatic differential potentiometric titrations] although they n-ould not be too suitable for conventional potentiometric titration methods, because their absolute potentials often shift from one titration to the next and drift during the titration. Most of the quantitative data presented Fere obtained with the platinum-10% rhodium indicator electrode, because the rhodium and platinum-40~o rhodium electrodes were not available. The rapid response of the platinum-10% rhodium electrode to pH changes precludes much improvement mith the use of other electrodes. The use of platinum-tantalum (6),indium, rhodium, osmium, and ruthenium (30)as electrodes for acid-base systems has been reported. The response of a graphite rod electrode to pH changes in the solution is shown in curve G, resembling somewhat the response of a platinum wire electrode. There is a rather large lag in elec-
acid-base titrations
trode response, which makes it undesirable as an indicator electrode for automatic differential titrations, but it does suggest its use as a reference electrode (7, 19, sa). Curve H shows the ordinary and second derivative curves with a platinum-10% rhodium indicator electrode and a graphite reference electrode. The second derivative curve is obtained before the graphite starts to change potential] and the inflection point nearly coincides with the equivalence point. About 0.5 minute after the equivalence point is passed the potential difference across the electrode pair is again very small, and such a pair is useless for this titration by the conventional titration procedure. Because of the slow response of the platinum wire electrode as ehown in curve C, it also serves as a reference electrode for acid-base automatic differential titrations. Another possible reference electrode is an isolated indicator electrode. The electrode can be isolated by several procedures (18, 27, 34). Titration curves for the electrode system of a platinum-10% rhodium indicator electrode and a platinuni10% rhodium reference electrode isolated in the tip of the buret are shown in curve I , Figure 2. The titration curves with palladium as indicator electrode are not given in Figure 2. They are similar in shape to those for pIatinum-40% rhodium, curve E, but the magnitude of the potentisl change is less
1760
ANALYTICAL CHEMISTRY
Quantitative Results at Various Titrant Flow Rates. Table I summarizes the quantitative results obtained a t various flow rates for the titration of acetic acid with sodium hydroxide and with electrode systems of platinum-10% rhodium us. caloniel or graphite] glass vs. calomel, and antimony us. graphite. After the titration was terminated by the automatic differential titrator, the p H of the solution was measured on a calibrated p H meter and the values were recorded. The reproducibility of successive titrations is given as standard deviation expressed both in terms of p H and milliliters of 0.1X sodium hydroxide. At fast flow rates the over-shoot of the equivalence point becomes significant especially with the glass and antimony electrodes. Even with these electrodes the precision is good, and it is possible to make a blank correction for a given flow rate, but it would generally be
Table I.
best t o select an indicator electrode and flow rate where the blank is negligible or very small. Figure 3 shows a manual t'it'ration curve using a p H meter arid glass-calomel electrode system for the titration of 10 nil. of approximately 0.1S acetic acid in 50 ml. of solution mit,h 0.1S sodium hydroxide. Tit,rant. was delivered dropwise from a buret., and the solution was allowed to reach equilibrium before the p H values were measured. The average end point pH values for this same titration with the different indicator electrodes anti various flow rates are indicated on the titration curve in Figure 3.
I ANTIMONY - 6
ML./M'N
G L A S S - 6 ML /MIN
Automatic Differential Titrations of Acetic dcid with Sodium Hydroxide
95 PT(IOXRH)-I5 ML /MIN
(!O-ml. aliquots of approximately 0.1,V acetic acid in 50 ml. of solutlon were titrated with approximately 0 . 1 s sodium hydroxide a t various flow rates) Conditions P t (10% Rh) u s . calomel, 2 ml./minute
Pt. (10% Rh) us. calomel, 15 ml./minute
Pt (10% Rh) us. graphite, 6 ml./minute
Pt. (10% Rh) us. calomel, 6 ml./minute
Glass E S . calomel, 2 nil./minute
Glass u s . oalomel 6 ml./minute
Glass us. calomel, 12 ml./rninute
Antimony us. graphite, 2 ml./minute
Antimony u s . graphite, 6 ml./minute
Antimony us. graphite, 10 ml./minute
E n d Point p H 8.50 8.42 8.51 8.49 8.59 Av. 8 . 4 8 9.30 9.38 9.43 .4v. 9.37 8.93 8.88 9.02 8.97 A\'. 8.95 8.91 8.95 9.08 8.88 8.80 9.02 9.00 8.90 8.95 9.02 -4r. 8 . 9 6 9.13 9.08 9.11 9.01 9.07 9.08 9.12 9.08 9.14 9.06 AV. 9.09 9.64 9.57 9.68 9.51 9.60 Av. 9 . 6 0 10.20 10.18 10.10 10.10 10.14 .4v. 10.14 9.60 9.68 9.64 9.61 9.69 -41,. 9 , 64 10.30 10.20 10.36 10.27 10.31 .4v. 10.29 10.52 10,60 10.50 10.57 10 51 AV. 10. 54
PH 0.07
- 06
GLASS- 2 ML /MIN
Std. Dev.
90
MI. 0,005
P T ( I O I R H 1 - 6 ML /MIN
1
- 2o - 10 - 08 - o4
PHENOLPHTHALEIN
85 0.07
-I
PH
0.013
0.06
0,006
0.08
0.009
0.04
0.003
END p o ' N T ~ ~ P T ~ 1 0 % R HML./MIN. l - 2
8.0
INFLECTION POINT
I-
fl
i
I L
i r
I
--* -00
END
/
POINT
ERROR ( M L S I
i1 1
--I
MILLILITER I
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Figure 3. Potentiometric curve of end-point region for titration of acetic acid with sodium hydroxide
0.07
0 08
0.05
0.05
0.04
0.05
0.06
0.04
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The end-point error-Le., t,he difl'erence in milliliters betn.c.en equilibrium inflect'ion point and automatically terminated end point-is given in the vertical column t o the right of the curve. Strong Acid-Strong Base Titrations. Ten-milliliter aliquots of 0.1N hydrochloric acid in 50 ml. of solution were titrated with carbonate-free 0.1N sodium hydroxide. The electrode system was platinum-lo yo rhodium us. either calomel or graphite and the flow rate was 2 ml. per minute. Five aliquots of hydrochloric acid were automatically titrated in the presence of bromocresol green indicator, and five in the presence of phenolpht,halein indicator. The titrations were all automatically terminated on the basic color of bromocresol green (blue) and the acid side of phenolphthalein (colorless). S o attempts were made to measure the p H accurately after the tit,rations were automatically terniinated, but the above color test,s indicate that they were all terminated between about p H 6 and 8. For a strong acid-strong base titration this represents a very small milliliter error. -4t flow rates greater than 5 ml. per niinut,e the pink color of phenolphthalein was just percept,ible after automatic termination of the titration. The hydrochloric acid-sodium hydroxide titrations were also performed with a quinhydrone indicator electrode with very good results. Ten samplrs were automatically titrated and t8he
1761
V O L U M E 27, N O . 1 1 , N O V E M B E R 1 9 5 5 average of these results was 10.59 nil. with a standard deviation of 0.03 ml. (about reading error of the buret), which compares N i t 1 1 the average value of 10.58 nil. obtained manually using hromocresol green indicator. NONAQUEOUS ACID-BASE TITRATIONS
tkiti-base titrations in nonaqueous solvents have been rapidly gaining popularity in recent years. h serious drawback in the perfo~~mance of these tit,rations has been the difficult'y in the detection of the end point. Internal indicators, because of lack of a firin theoretical basis, have been chosen largely on a hit and misp kmsis. Likewise, lack of fundamental information on electrode systems has hindered tlie use of potentiometric methods. Many of t'he electrodes now used give an indication of the proper end point, but the potentials obt,ained are not reproducible (10). \\-it11 the automatic differential titrator it is not necessary t,o rc1l)roduce the elect,rode potrnt,inls from one titration t.o the nest,, n i i d tlwrefore the differential procedure should be used to advantxge i n nonaqueous titrations if elect,rodes respond rapidly at the equiv;ilence point. One nonaqueous system investigated was the titration of anilint, \r-it,h perchloric acid using glacial acetic acid as solvent. .~niongthe electrode systems suggested for this solvent have been tht, rliloranil-calomel (a), glass-calomel (66),glass-silver, silver chloricle (11). Even n.ith the glass-calomel system the absolute poteiitial curve was not reproducible from day to day, but varied :is much as 0.1 volt, The other nonaqueous system was the titi.:it#ionof benzoic acid n-it,h sodium methoside using benzenenwtlianol as solvent. The glass electrode cannot be used in this solvent ( 1 2 ) , but such systrms as antimony-calomel, hydrogenc3lonir1, and antimoiiy-isolakd antimony, where the reference :intinion>.electrode is placed in the hwet tip ( 2 7 ) have bern used.
CURVE A
CURVE B MILLILITERS
Figure 4.
Recorded second deri\ati\ e curves showing
eff'rct of lithium chloride in benzene-methanol solvent
The :iut,oinatic different,ial titrat.ions of aniline with perchloric. acid a n d benzoic acid with sodium mebhoxide were performed ivitli platinum-lO~o rhodium alloy, glass and antimony indicatoi rlertrodes, and a graphite refrrencr rlect'rode. The graphite rrfrienw electrode eliminates the use of salt bridges or special taquipnient, and is applicable to both types of solvent's. In :wetic acid and benzene-methanol, as well as in most, otlirr ~ioiinqueoussolvents, there is a very high resistance b e b e e n the iiidicat,or and reference electrodes, which generally requires a high input impedance circuit to the titrator. This high input inipedance stage can be readily provided in most laboratories by :I Beckman Model H-2 p H meter or other suitable pH meter or vlectrometer amplifier. For t,itrations in benzene-met,hanol
solvent about 0.1 gram of lithium chloride reduced the resistance a t the start of the titration (Figure 4 ) . The curves were recorded on the Esterline-Angus recorder and curve A was for the titration of benzoic acid with sodium methoxide in benzene-methanol solvent without addition and curve B with addition of lithium chloride. The titration curves were obtained with the relay system in the circuit and one half of the second derivative curve a t the end point accordingly is cut off. Figure 4, A , illustrates that the rapid oscillatory electrode noise does not turn the buret off. The rapidly oscillating voltage fed to the relay circuit causes the contacts of relay 1, Figure 1, to open and close rapidly, but the capacitance, C?, does not obtain a sufficient charge to open relay 2 until the end point is reached. The "noise" level of the derivative curve decreases as the titration proceedq, because of the salt formation, but the initial chatter of relay, K,, Figure 1, is annoying, and it is best to prevent this by adding some salt at the start of the titration. ~
~
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Tahle 11. Titration of Benzoic Acid with Sodiuni 3Iethoxide in Benzene-Methanol Solvent (Weighed aamples of benzoic acid were dissolved in about 50 nil. of 3 t o 1 benzene-nietlianol and titrated w i t h approximately 0.15N sodium methoxide in 6 t o 1 benzene-methanol solution) Equivalents per Rt. of hll. of Liter of Sodiuni 1Ietliod Sample Sodium Nethoxide Methoxide Manual 0.1246 6.98 0.1463 6.07 0.1484 Using thymol blue as 0 1081 6.32 0.1466 indicator 0.1130 Automatic 0,1152 6.46 0.1466 12.28 0.1467 P t (10% Rti) indicator 0.2198 electrode, 0 0648 3.63 0.1463 2 inl./niinute 0.2346 13.13 0.1464 Automatic 0 1094 ( 6.26-0.14) G.12 0.1465 Pt (10% R h ) electrode, 1835 (10.41-0.14) 1 0 . 2 7 0.1463 6 ml./minute 0.0583 ( 3.40-0.14) 3 . 2 6 0.1466 0.1913 (10.86-0.14)10.72 0.1463 Automatic 0.1360 ( 7.96-0.34) 7 . 6 2 0.1463 Sb electrode, 0.1806 (10.41-0.34)10.07 0.1462 2.4 nil./riiinute 0 . 2 2 2 8 (12.83-0.34) 12.49 0.1462 0 1200 ( 7 13-0.34) (i 71 0 1468
Titration of Benzoic Acid with Sodium Methoxide in Benzene-Methanol Solvent. Samples of reagent grade benzoic acid \ye re weighed and dissolved in about 50 ml. of benzene-methanol solvent and titrated with approximately 0.15N sodium methoside using the automat,ic differenhl t.it,rator to terminate the titrations and elect,rode syst,ems of platinuni-lO~o rhodium cs. graphite or ant'imony us. graphite. Data were obtained also by the manual titration proredure with thymol blue as indicator (Table 11). There is no apparent overshoot of the equivalence point at, a titrant flow rate of 2 nil. per minute with the platinum10% rhodium indicat'or elect'rode, but a t 6 nil. per minute there is a significant time lag of electrode response. The lag is reproducible, hon-ever, and can be correct,ed by a blank correction. The blank a t a given titrant flon- rat,e appears to be independent of concentration and temperature, and also to remain constant, from day to day. The antimony indicator electrode was found to have a large lag in potential response, similar to its action in aqueous acidbase t,it&ions. With the antimony indicator electrode and a flow rate of 0.040 ml. per second, it was repeatedly observed t,hat there was an 8-second time lag between the thymol blue color change and the automatic t,ermination of the titration. This represent,s about a 0.32-ml. blank correction, which is in good agreement with t,he calculated value of 0.34 ml. The data in Table I1 illustrate the excellent results that can be automatically obtained, even with the antimony electrode when the blank correct,ion is applied. Titration of Aniline with Perchloric Acid in Glacial Acetic Acid. Each aliquot of approximately 0.1N aniline was diluted i n the titration beaker to about 50 ml. with glacial acetic acid
1762
ANALYTICAL CHEMISTRY
Table 111. Titration of Aniline with Perchloric Acid in Glacial Acetic Acid (Aliquots of approximately 0.1s aniline were added t o about 50 ml. of glacial acetic acid and titrated with 0.1.V perchloric acid in glacial acetic acid) MI. Conditions of Aniline HClOa-Blank Pt (10% Rh) u s . graphite, 10.00 9 . 0 3 - 0.11 = 8 . 9 2 4 ml./minute I O . 00 9 . 0 9 - 0.11 = 8.98 10.00 9 . 0 5 - 0.11 = 8 . 9 4 .Iv. 8.95 20.00 17.97 0.11 = 17.86 20.00 18.01 - 0 . 1 1 = 17.90 20.00 17.96 - 0.11 = 17.85 Av. 17.87 Pt. (10% Rh) u s . graphite, 10.00 9 . 2 4 - 0 . 2 4 = 9.00 10 ml./minute 10.00 9 . 2 0 - 0 . 2 4 = 8.96 10.00 9.20 - 0.24 = 8.96 Av. 8.97 20.00 1 8 . 2 1 0 . 2 4 = 17.97 20.00 18.17 - 0 . 2 4 = 17.93 20.00 18.20 - 0 . 2 4 = 1 7 . 9 6 Av. 17.94 2 i . 1 7 - 0.24 = 26.93 30.00 30.00 2 7 . 1 3 - 0.24 = 26.89 30.00 2 7 . 1 5 - 0 . 2 4 = 26.91 AV. 26.91 Glass us. graphite, 10.00 9.10 0.16 = 8.94 4 ml./minute 10.00 9 . 1 4 - 0 . 1 6 = 8.98 10.00 9 . 1 1 - 0.16 = 8.95 Av. 8.96 20.00 18.09 0.16 17.93 20.00 18.06 - 0 . 1 6 = 17.90 20.00 18.10 - 0 . 1 6 = 17.94 Av. 17 92 Glass v s . graphite, 10 00 9 21 - 0 29 = 8 . 9 2 10 ml./minute 10.00 9.24 0 29 = 8 95 IO.00 9 29-0 299 00 Ar. 8 96 20.00 18.20 - 0 . 2 9 = 1 7 . 9 1 20.00 18.20 - 0.29 = 17.91 20.00 18.22 - 0 . 2 9 = 17.93 Av. 17.92
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point, and McBride and others (as), found it reproducible within 0.2y011hen done manually. An electrode system of platinum u s . calomel, and a titrant flow rate of 2 ml. per minute were used for the titrations. Titrant delivery rates faster than 3 ml. per minute resulted in the formation of free iodine during the course of the titration. The potential change a t the end point is large and the magnitude of the second derivative curve is also large as shown in Figure 5. Eight samples of hydrazine sulfate of varying size were dissolved in about 15 nil. of water and 45 ml. of concentrated hydrochloric acid. These solutions were then titrated with a 0.02500.M solution of potassium iodate. Four of these samples were titrated automatically a t a f l o ~ rate of 2 ml. per minute and four manually using brilliant scarlet 3R as internalindicator (TableIV). There is no apparent lag in electrode response at a flow rate of 2 ml. per minute, because of the excellent precision obtained on the automatic titrations for the various sample sizes of hydrazine. PRECIPITATION TITRATIOXS
Chloride was accurately titrated with standard silver nitrate automatically a t titrant flow rates up to 7 ml. per minute and with indicator electrodes of silver, platinum, or platinum-lO~o rhodium. A silver electrode with an area of approximately 7 sq. cm. was used, but this electrode had a slow potential response and gave results about 0.9% high. Upon substituting a No. 24 silver wire, this error was eliminated.
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and titrated with 0.1.V perchloric acid. An electrode pair of either platinum-lO% rhodium us. graphite or glass us. graphite was used in conjunction with the automatic differential titrator. The results of these titrations are summarized in Table 111. The potential break in this titration is not very sharp and the magnitude of the second derivative curve is accordingly small. Therefore, the titrant flow rate n a s maintained at about 4 ml. per minute or greater to ensure a derivative curve of sufficient magnitude to operate the relar system. Slower flow rates would be possible by modifications of either the amplifier-differentiator or relay circuit. At the relatively fast f l o ~rates used for this titration a blank correction is necessary with either thr platinum10% rhodium or glass indicator electrodes, although the blank correction for the glass electrode is somen-hat larger. The blank is constant for a given titrant flow rate and indicator electrode, remains constant from day to day, and is unaffected by concentration of reactant. OXIDATION-REDUCTION TITRATIONS
Sutomatic differential potentiometric titrations of ferrous iron n ith standard dichromate and ceric solutions are very precise ( 2 4 ) . Automatic titration results for another redov system are presented in this section. The titration of hydrazine sulfate with standard potassium iodate by the rlndrews method ( 2 ) is a rather complex oxidationreduction reaction, which was considered worthy of investigation to determine whether the solution equilibrium was established sufficiently rapidly that the automatic differential titrator could be successfully employed. The potentiometric study of this reaction in a t least 4F hydrochloric acid was found by Singh ( 3 1 ) to give accurate and reproducible results n i t h a sharp inflection
"OLTS
I
1
Figure 5. Recorded second derivative curve for titration of hydrazine sulfate with potassium iodate
The use of the platinum wire as an indicator electrode for silver ion is not new (13-17). It is generally agreed that a micro amount of silver on the surface of the platinum accounts for this phenomenon, which Allen and Hickling ( 1 ) recently demonstrated analytically. The mechanism for the appearance of this coating, however, is not clear. It has been suggested that it might be the result of a redox process involving a higher oxide of silver (14),or a trace of silver subnitrate rearranging to give the deposit of silver (29). Allen and Hickling ( 1 ) have shown the effects of pretreatment of the platinum and suggest that it is simply a reduction of the univalent silver ion. I n this laboratory the potential from a used platinum-calomel electrode pair was found to agree with that from a silver-calomel couple within 10 mv. a t several points along the titration curve, especially in the end-point region and beyond. Kolthoff ( 2 2 ) has shown, however, that in dilute solutions the platinum wire may not assume a reproducible potential, and Druet (9) has claimed a variation in the end point using a silver electrode which depends on the age and nature of the silver electrode. These factors are imignificant Then using the differential titrator, because the
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V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5 titration is automatically terminated a t the inflection point and not a t an absolute preset potential. Figure 6 shows a recorded second derivative t,itration curve for t,his titration, and is present,ed to illust.rate considerable voltage fluctuation on the upswing of the second derivative curve. The broken line represents the thyratron firing level. The first two sharp voltage pulses to go above and thsn below this level do not, homver, operate t,lie buret solenoid or niot.or because condenser C p ,Figure 1, does not, have sufficient time to charge to a value that will open relay 2. The titration curve in Figure 6 is typical for the titration of chloride Tvith silver nitrate a t a flow rate of 7 ml. per minute, but no false end points were experienced. In all the titrations 10-nil. aliquots of approximately 0.065.V sodium chloride were diluted with about 75 ml. of distilled water and titrated with 0.13- silver nitrate solut’ion. The results tabulated in Table V shoTv a precision n-ithin the reading error of the buret. KO significant differences in the second derivative titrat,ioii curves were noticed between the platinum-10% rhodium, platinum, and silver wire indicator electrodes, which were paired with a calomel reference electrode isolated from the solution with a salt bridge. Better precision might have been obtained if a buret wit,h less reading error had been used. COMPLEX FORM\ZATION TITRATIONS
The automatic differential tI., and Wang, C., J . P h y s . Cheni., 41, 539 (1937). Lux, H., Z . Elektrochem., 4 5 , 303 (1939). JIalmstadt, H. V., and Fett, E. R., ASAL. CHEM.,26, 1348 (1954). hlarkunas, P. C., and Riddick, J. A, Ibid., 23, 337 (1951). IIcBride. W.R., Henry, R. A , . and Skolnik, S., Ibid., 2 3 , 890 (1951). l l o s s , AI. L., Elliott, J. H.. and Hall, R. T., I b i d . , 20, 784 (1948). Muller, E., and Aarflot, H., Rec. trav. chim., 4 3 , 874 (1924). Obruchera, A., J . P h y s . Chem. (U.S.S.R.), 11, 473 (1938). Perley, G. A., and Godshalk, J. B., (to George Kent, Ltd.) Brit. Patent 567,722 (1945). Singh. B., and Ilani, I., J . I n d i a n Chem. SOC.,14, 376 (1937). Tikhonov, A. S., Zavodskaya Lab., 8, 17 (1939). Vyakhirev, D. A., and Guglina, S. A., Ibid., 15, 1426 (1949). Willard, H . H., and Boldyreff. A. W., J . Linz. Chenz. SOC.,51, 471 (1929).
RECEIVED for review Ma)- 12. 1955. Accppted July 21.
19,55.
