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Continuous coulometric titration of various oxidising substances by electrogenerated ironII
Trdanta.
1963, Vol. 10. pp. 971 to 979.
F’ergamon
Press Ltd.
Printed
in Northern
Ireland
CONTINUOUS COULOMETRIC TITRATION OF VARIOUS OXIDISING SUBSTANCES BY ELECTROGENERATED IRONI TAKEO TAKAHASHI and HIROSHI SAKURAI Institute of Industrial Science, University of Tokyo, Azabu-Shinryudocho Tokyo, Japan (Received 8 February 1963. Accepted 7 April 1963) Summa~-Ironrr reacts rapidly and quantitatively with various oxidising substances, such as potassium permanganate, potassium bichromate or free chlorine, in acidic solution. Continuous coulometric titrations of such solutions have been carried out with ironrr electrogenerated from ironrII at a platinum cathode. The progress of the reaction in the titration cell is followed potentiometrically with a platinum-calomel electrode pair. The potential difference between the detected potential and the end-point potential is amplified with a d.c. amplifier, whose output controls the electrolytic current so that the detected potential becomes equal to the end-point potential. Thus the electrolytic current which generates the irontr is directly proportional to the concentration of oxidising constituent in the sample solution. In order to obtain good results, it is desirable to make a dynamic titration curve empirically in order to locate the optimum for the end-point in the coulometric titration. INTRODUCTION POTASSIUM permanganate, potassium bichromate, ceriumIV sulphate and other oxidising substances can be determined by coulometric titration with electrogenerated ironrr. This method is free from the trouble of atmospheric oxidation of ironrr which occurs in a conventional titration with an iron11 solution. Oelsen and Gobbelsl proposed a determination of vanadate and permanganate by coulometric titration with ironI electrogenerated at constant current. This method was subsequently applied to the determination of vanadium and chromium in steel.2 Cooke, Reilley and Furman used the coulometric titration with electrogenerated ironrr for determination of ceriumIV and bichromate,3 vanadium in uranium4 and a minute amount of permanganate. 5 Futhermore, Meites6 determined ceriumrv, vanadiumv, chromiumvr and manganesevrr. Takahashi, Kimoto and Sakurai’ developed a back-titration coulometric method which consisted of electrogeneration of ceriumIV from ceriumIIr in the presence of ironrrr, reaction of cerium with the components to be determined in the sample and coulometric titration of the excess cerium Iv. This back-titration method was comfirmed by Fenton and Furman8 to be an excellent method because of the almost 100 % current efficiency during the coulometric titration. The present authors s~lohave constructed a continuous titration apparatus using coulometry and used it, for example, to carry out a continuous titration of arsenous acid with bromine electrogenerated from potassium bromide; the principle of such a continuous coulometric titration apparatus and its manipulating conditions have also been discussed. 4
971
972
TAKEOTAKAHAWI~II~Hmanr
SAKURAI
Furthermore, the authors have investigated the continuous determination of micro quantities of iron dissolved in water by coulometric titration with electrogenerated bromineli and confirmed the applicability of the continuous coulometric titration method. In the present paper the authors have studied the continuous coulometric titration of potassium permanganate, bichromate and chlorine in dilute solutions using ironrrr as the electrolyte solution. EXPERIMENTAL Reagents Electrolyte solution (about 0.1 N in ammonium iron111w&hate and2N in sulphuric acid): Prepared by dissolving 100 g of extra-purified sulphuric acid and 48 g of extra-purified ammonium iron111 sulphate in 1 litre of water. O-1N Potassium permanganate and bichromate solutions: Prepared by dissolution of extra-purified reagents in water and suitable dilution. Chlorine water: Prepared by absorption of chlorine gas evolved from 1: 1 hydrochloric acid and bleaching powder in 1N hydrochloric acid and subsequent dilution with water. The concentration of free chlorine was determined by iodometric titration. Apparatus The apparatus used in the present experiments (Fig. 2) is as in previous reportsgJO trolyte solution (ammonium ironrrr sulphate solution) is introduced into the titration
Electrolytic
FIO. l.--Dynamic
titration
current,
curves for determination iron”.
The eleccell at a
mA
of an oxidising substance with
definite rate, ironrr being generated electrolytically at the cathode surface; the solution then reaches the titration cell where it is mixed with the stream of sample solution. The titration cell is provided with a platinum indicating electrode and a saturated calomel electrode, between which is applied a d.c. constant voltage, slightly more negative than the potential corresponding to the end-point. The solution in the titration cell is always automatically controlled so as to contain a small excess of ironrr. The electrogenerated ironI reacts with the oxidising substance in the sample solution and the remaining iron rr changes the potential at the indicating electrode; the potential difference between the indicator and reference electrodes is fed into the d.c. amplifier and its output supplied as electrolytic current for the electrogeneration of iron”. Fig. 1 shows the characteristics obtained when using the continuous coulometric titrator for the determination of an oxidising substance with iron rr. The curves represent the titration of sample solutions containing increasing amounts of the reacting component. A is the applied voltage, E is the theoretical potential at the end-point, and the line AB represents the working characteristic of
Titration of various oxidising substances
973
the d.c. amplifier, i.e., the relationship between the input (the potential difference between the indicator and reference voltages) and the output current (the electrolytic current). In the present work the iron” was generated coulometrically, so that the titration curve had a sigmoid shape as shown in Fig. 1, and the end-point indication circuit as shown in Fig. 2 was inverse compared with that in the coulometric titration with electrogenerated bromine. The positive side of the applied voltage was connected to earth, while the negative side was connected to the platinum indicator electrode. The above applied voltage (A in Fig. 1) was less than that at the equivalence point. The potential difference between the applied voltage and the changing potential during the titration was fed into the d.c. amplifier, whose output was applied to the electrogenerating electrodes (platinum cathode and anode in electrolysis cell).
Recorder
d.c. omplifer
FIG.
2.-Schematic
diagram of automatic continuous coulometric
titrator.
Current eficiency of electrogeneration of iron!? To find the effect of the concentration of electrolyte solution and electrolytic current intensity on the current efficiency, three electrolytic solutions (O.lN, 0.03N and O.OlN ammonium ironrrr sulphate in 2N sulphuric acid) were supplied to the electrolysis cell, whose e&rent containing electrogenerated iron” was taken into the titration cell. The flow rate of the electrolyte solution was held at 0.1-045 ml/set. The effluent was gathered during the electrolysis for 600 set and analysed for potentiometric titration with 0405N potassium electrogenerated iron rr by a conventional permanganate standard solution. The results are shown in Table I. From Table I the concentration of the ammonium ironrrr sulphate must be at least O=lN, even if the generating current was less than 3 mA, and this concentration was employed for the electrolyte solution in the subsequent experiments. Continuous coulometric titration of potassium permanganate Potassium permanganate is conventionally titrated with ironrr in acidic solution. In the present experiment, however, a minute amount of potassium permanganate was determined by continuous coulometric titration with electrogenerated iron II. Dynamic curves of the titration of potassium permanganate with ammonium ironI sulphate are given in Fig. 3, which indicate that the electrolytic current balancing the applied voltage changes in accordance with the applied voltage when potassium permanganate solution is fed in at a constant flow-rate of 0.16 ml/set. The dynamic curve corresponds to the lower part of the titration curve. The titration proceeded slightly through the end-point and only a continuous coulometric titration could be carried out, because the equivalence potential frequently fluctuated with the large oscillation, although it was very sensitive at the equivalence. The relationship between the applied potential and amplitude of fluctuation (the vibrating distance) of the writing pen in the recorder is shown in Table II. The optimum applied voltage can be found from Fig. 3 and Table II, and is about 0.62 V.
TAKXJ TAKAHASHIand HIROSHISAKLJRAI
974
TABLE I.-CURRENT EFFICIENCY Electrolytic current,
IN ELECTROGENEU~~ON OF IRON”
IronIIr electrolyte solution,
OGO5N KMnO,,
Current efficiency,
No.
mA
N
ml
%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
0.95 1.00 213 2.16 2.97 2.95 0 0 1.09 1.09 2.16 2.23 3.34 3.33 0 1.03 1.03 2.12 2.10 3.19 3.18 0 0
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
1.250 1.333 2.650 2.657 3.624 3.610 0.080 0.080 1.344 1.348 2.134 2.255 2.523 2.568 0.049 0.794 0.885 1.064 1.063 1.095 1.132 0.070 0.064
98.9 100.1 96.8 95.7 95.9 978
95.4 95.7 77.5 79.4 59.5 60.7 56.7 63.8 37.6 38.1 25.8 26.9
0.60 -
I
0
I
I
Electrolytic
FIG.
I
I.0
0.5
current,
I.5 mA
3.-Dynamic curves for titration of potassium permanganate with (A) H,O, (B) 0.5 x lo-*N KMnO,, (C) 1 x 10-4N KMn04.
ironrr:
975
Titration of various oxidising substances
The relationship between the concentration of potassium permanganate and the electrolytic current intensity is shown in Table III. In these experiments the sample solutions (0.5 x lo-‘, 1.0 x 10-4, l-5 x lo-* and 2.0 x 10-4N potassium permanganate solutions) were fed successively, but alternately with water, into the titration cell for 10 min and the electrolytic currents recorded. The applied voltage was set at 0.61 V; the feed rate of sample solutions was 0.16 ml/set and that of the electrolyte solution 0.15 ml/set. This series of experiments with the 4 sample solutions was repeated seven times and the electrolytic currents recorded in Table III have the background current (the electrolytic current for water alone in the titration cell) deducted.
Applied voltage, V KMn04, N
I-W 5 x 10-s 1 x lo-’
0.60
0.62
0.64
0.66
0.68
0.70
0.90
1X@
1.05
0 (mA) 0 0.02
0 0.01 0.04
0 0.05 0.45
0 0.45 0.75
0 0.70 0.95
0 0.95 -
0 0.75 1.20
0 0.45
0 0.03
TABLEIII.-RELATION~~-~BETWEENCONCENTRATIONOF~~TA~~ILJMPERMANGANATE AND ELECTROLYTVZ CURRENT KMnO,,N
No. 1 2 3 4 5 6 7 Av. Std. dev. Theoretical current
5 x 10-s
1 x 10-a
1.5 x 10-d
2 x 10-d
0.80 (mA) 0.78 0.88 0.93 0.86 0.88 0.82 0.85 0.049
1.53 (mA) 1.54 1.63 1.63 1.66 1.62 1.58 1.60 0,046
2.30 (mA) 2.30 2.35 2.48 244 2.37 2.38 2.37 0,062
3.00 (mA) 2.98 3.12 3.12 3.12 3.08 3.09 3.07 0,060
0.772
1.544
2.316
3.088
The results show that the theoretical and measured values coincide very well and the relationship between the concentration and electrolytic current is almost linearly proportional. It may be said more exactly that the ratio electrolytic current/concentration diminishes slightly with increase of concentration. This can be explained by the decrease of the excess concentration of iron’1 at the equivalence point with increase of the concentration of potassium permanganate. The stability of the recording was examined by the following procedure. Distilled water and 5 x 10-5N potassium permanganate were used as sample solutions flowing for 2 hr and the electrolytic current was measured every 4 min. The experimental results showed that it was almost constant (the average current for 30 measurements was 0.345 mA with a standard deviation of 0,005 mA in the case of distilled water, and 1.194 mA with a standard deviation of 0.039 mA in the case of 5 x 10-6N potassium permanganate). Continuous coulometric titration ofpotassium bichromate Potassium bichromate can be easily titrated by the conventional potentiometric method with iron” in acidic solution. The dynamic titration curves are shown in Fig. 4 and they were taken in the same manner as for potassium permanganate. These curves differ from those of potassium permanganate, however, in that they run smoothly, this resulting from the indistinctness of the equivalence potential. This difference is considered to be caused not only by the smaller potential change for potassium bichromate-iron I1 than that for potassium permanganate-ironrr, but also to a slow electrode reaction rate or to a slow following of the indicator electrodes near the equivalence point. The oscillation could not be seen when the applied voltage increased to 0.7 V.
976
TAKEOT AKAHASHI and
I
HIROSHI SAKURAI
I
I
PO
0.5 Electrolytic
current,
I.5 mA
FIG. 4.-Dynamic curves for titration of potassium bichromate with ironI1: (A) H,O, (B) 0.5 X 10-4NK,Cr,0,, (C) 1 X 10-‘NK,Cr,O,.
‘-70
_-.-d t .-
65
-'
---
--
l _--,.--___I._-_---__-\\ -.--___--.-_-_-‘.___-’ .
0.60
I __.---
\_
__---
I
I
0.5
0
Elecfrolytic FIG.
1
l-0 current,
I.5 mA
5.-Dynamic curves for titration of free chlorine in water: (A) HIO, (B) 4.5 x 10-6NCI~, (C) 9 x lo-‘NClp.
2
Titration of various oxidising substances TABLE IV.-RELATIONSHIP
BETWEEN CONCENTRATION OF WTASSIUM BICHROMATE AND EX,EcTROLvTK CURRENT
K&r&N
No. 1 2 3 4 5 Av. Std. dev. Theoretical current
977
5 x 10-s
1 x 10-d
1.5 x 10-a
2 x 10-a
0.63 (mA) 0.65 0.65 064 0.63 0.64 0009
1.31 (mA) 1.33 1.32 1.31 1.29 1.31 0.013
1.97 (mA) 1.98 1.98 1.96 1.96 1.97 oGO9
2.62 (mA) 2.59 2.61 2.59 2.58 2.58 0.023
0.772
1.544
2.316
3.088
The relationship between the concentration of potassium bichromate and electrolytic current intensity is shown in Table IV (the current intensity has the background current deducted). The applied voltage was set at 0.64 V. The results show a linear proportionality between the concentration of potassium bichromate and the electrolytic current, but the electrolytic current is lower than the theoretical value by about 16%. This may be caused by the large deviation between the dynamic and the static titration curves at the equivalence point. The deviation increases with increase of applied voltage. Continuous coulometric titration of free chlorine in water Free chlorine dissolved in water has been determined by the titration of liberated iodine or bromine from potassium iodide or potassium bromide. A very low concentration of free chlorine can be determined calorimetrically or spectrophotometrically using o-tolidine, benzidine hydrochloride or dimethyl-pphenylendiamine as reagents. In the present case the free chlorine content in water was determined potentiometrically by titration with electrogenerated iron” using the continuous coulometric titrator.
0
I I
I
I 3
2 C12.
ml
FIG. 6.-Relationship between concentration of chlorine in sample and electrolytic current.
4
TAKEO TAKAHASHI and Hraosrrr SAKURU
978
The dynamic titration curves are shown in Fig. 5. The equivalence potential is about 0.82 V (us. S.C.E.), where the oscillation occurs when the applied voltage is set near the equivalence point, because the potential sensibility is fairly large at the equivalence point. The relationship between applied voltage and amplitude of fluctuation is shown in Table V.
TAJSLEV.-RELATIONSHIPB~~EENAPPLIEDVOLTAGEANDAMPL~EOF~UCT~ATION
Applied voltage, V CL N 4.5 x 10-b 9 x 1o-6
0,60
0.62
0.64
0.66
0.68
0.70
0.80
090
1.00
1.10
O(nN O*Ol
0 0.01
0.005 0.020
0.01 0.05
0.25 0.35
0.35 0.45
0.50 0.80
0.45 0.75
0.25 0.60
0.05 0.20
Electrolyticcurrent, FIG. 7.-A
mA
typical chart recording in the continuous titration of chlorine.
Titration of various oxidising substances
979
The relationship between the concentration of chlorine in the sample solution and the electrolytic current is given in Fig. 6. The sample solutions were prepared from the original chlorine solution (O+lSN) by dilution of 1, 2, 3, or 4 ml to 1 litre with water. The abscissa in Fig. 6 shows the volume of the original solution taken. The ordinate is the average measured value of six electrolytic currents (standard deviations are 0~033,0~020,0~014 and 0.020 mA in order of increasing chlorine concentration). Fig. 7 is a typical chart recording for the continuous coulometric analysis of chlorine, in which the applied voltage was set at 0.64 V and the flow rate of the electrolytic solution was 0.15 ml/set. Zusamme&asung-Eisen(I1) reagiert in saurer LSstmg rasch und quantitativ mit verschiedenen Oxydationsmittehr wie Permanganat, Dichromat oder freiem Chlor. Kontinuierliche coulometrische Titrationen solcher Liisungen wurden mit an einer Platinkathode aus Eisen(II1) erzeugtem Eisen(I1) durchgefuhrt; der Reaktionsverlauf in der Titrationszelle wurde mit Hilfe eines Platin-Kalomel-ElektrodenDie PotentialditIerenz zwischen paares potentiometrisch verfolgt. gemessenem und Endptmktspotential wurde mit einem Gleichstromverstarker verstarkt; dessen Ausgang steuerte den Elektrolysestrom, soda0 das gemessene Potential gleich dem Endpunktspotential wurde. So warder das Eisen(I1) liefernde Elektrolysestrom der Konzentration des Oxydationsmittels direkt proportional. Urn gute Ergebnisse zu erzielen, ist vorher die Aufnahme einer dynamischen Titrationskurve empfehlenswert, urn das Optimum fur den Endpunkt der coulometrischen Titration festzulegen. R&sum&L’ion ferreux &git rapidement et quantitativement avec des oxydants vari6s comme le permanganate de potassium, le bichromate de potassium et le chlore libre en solution acide. Les dosages coulometriques de telles solutions ont Bte effectues par l’ion ferreux forme par galvanoplastie a partir de l’ion ferrique sur electrode de platine. La marche de la reaction dans la cellule de dosage est recueillie potentiometriquement par un couple d’electrodes platine/calomel. La difference de potentiel entre le potentiel detect6 et le potentiel du point equivalent est amplifie par un amplificateur d.c. dont le courant de sortie est controle de telle maniere que le potentiel detect6 devienne egal a celui du point equivalent. Ainsi, le courant electrolytique qui fournit l’ion ferreux est alors directement proportionnel a la concentration de l’agent oxydant dam la solution &udi&e. Pour obtenir de bons resultats, il est necessaire d’effectuer une courbe de dosage empirique en vue de localiser au mieux le point equivalent du dosage coulometrique. REFERENCES 1 W. Oelsen and P. Giibbels, Stahl II. Eisen, 1949,69, 33. 2 W. Oelsen, H. Haase and G. Graue, Angew. Chem., 1952,64,76. s W. D. Cooke and N. H. Furman, Analyt. Chem., 1950,22,896. 4 N. H. Furman, C. N. Reilley and W. D. Cooke, ibid., 1951, 23, 1665. 6 W. D. Cooke, C. N. Reilley and N. H. Furman, ibid., 1952,24,205. s L. Meites, ibid., 1952, 24, 1057. ’ T. Takahashi, K. Kimoto and H. Sakurai. Rep. Inst. Znd. Sci.. Univ. of Tokvo, 1955. 5, No. 6. ’ ’ . . * A. J. Fenton,.Jr. and N. H. Furman, Anaiyt. ‘Chem., 1957,29,. 221. s oT. Takahashi. E. Niki and H. Sakurai. J. Electroanalvt. Chem.. 1962. 3. 373. lo T. Takahashiand H. Sakurai, ibid., 1562,3, 381. ’ ’ ’ ’ I1 Idem, Talanta, 1962,9, 195.
1963, Vol. 10. pp. 971 to 979.
F’ergamon
Press Ltd.
Printed
in Northern
Ireland
CONTINUOUS COULOMETRIC TITRATION OF VARIOUS OXIDISING SUBSTANCES BY ELECTROGENERATED IRONI TAKEO TAKAHASHI and HIROSHI SAKURAI Institute of Industrial Science, University of Tokyo, Azabu-Shinryudocho Tokyo, Japan (Received 8 February 1963. Accepted 7 April 1963) Summa~-Ironrr reacts rapidly and quantitatively with various oxidising substances, such as potassium permanganate, potassium bichromate or free chlorine, in acidic solution. Continuous coulometric titrations of such solutions have been carried out with ironrr electrogenerated from ironrII at a platinum cathode. The progress of the reaction in the titration cell is followed potentiometrically with a platinum-calomel electrode pair. The potential difference between the detected potential and the end-point potential is amplified with a d.c. amplifier, whose output controls the electrolytic current so that the detected potential becomes equal to the end-point potential. Thus the electrolytic current which generates the irontr is directly proportional to the concentration of oxidising constituent in the sample solution. In order to obtain good results, it is desirable to make a dynamic titration curve empirically in order to locate the optimum for the end-point in the coulometric titration. INTRODUCTION POTASSIUM permanganate, potassium bichromate, ceriumIV sulphate and other oxidising substances can be determined by coulometric titration with electrogenerated ironrr. This method is free from the trouble of atmospheric oxidation of ironrr which occurs in a conventional titration with an iron11 solution. Oelsen and Gobbelsl proposed a determination of vanadate and permanganate by coulometric titration with ironI electrogenerated at constant current. This method was subsequently applied to the determination of vanadium and chromium in steel.2 Cooke, Reilley and Furman used the coulometric titration with electrogenerated ironrr for determination of ceriumIV and bichromate,3 vanadium in uranium4 and a minute amount of permanganate. 5 Futhermore, Meites6 determined ceriumrv, vanadiumv, chromiumvr and manganesevrr. Takahashi, Kimoto and Sakurai’ developed a back-titration coulometric method which consisted of electrogeneration of ceriumIV from ceriumIIr in the presence of ironrrr, reaction of cerium with the components to be determined in the sample and coulometric titration of the excess cerium Iv. This back-titration method was comfirmed by Fenton and Furman8 to be an excellent method because of the almost 100 % current efficiency during the coulometric titration. The present authors s~lohave constructed a continuous titration apparatus using coulometry and used it, for example, to carry out a continuous titration of arsenous acid with bromine electrogenerated from potassium bromide; the principle of such a continuous coulometric titration apparatus and its manipulating conditions have also been discussed. 4
971
972
TAKEOTAKAHAWI~II~Hmanr
SAKURAI
Furthermore, the authors have investigated the continuous determination of micro quantities of iron dissolved in water by coulometric titration with electrogenerated bromineli and confirmed the applicability of the continuous coulometric titration method. In the present paper the authors have studied the continuous coulometric titration of potassium permanganate, bichromate and chlorine in dilute solutions using ironrrr as the electrolyte solution. EXPERIMENTAL Reagents Electrolyte solution (about 0.1 N in ammonium iron111w&hate and2N in sulphuric acid): Prepared by dissolving 100 g of extra-purified sulphuric acid and 48 g of extra-purified ammonium iron111 sulphate in 1 litre of water. O-1N Potassium permanganate and bichromate solutions: Prepared by dissolution of extra-purified reagents in water and suitable dilution. Chlorine water: Prepared by absorption of chlorine gas evolved from 1: 1 hydrochloric acid and bleaching powder in 1N hydrochloric acid and subsequent dilution with water. The concentration of free chlorine was determined by iodometric titration. Apparatus The apparatus used in the present experiments (Fig. 2) is as in previous reportsgJO trolyte solution (ammonium ironrrr sulphate solution) is introduced into the titration
Electrolytic
FIO. l.--Dynamic
titration
current,
curves for determination iron”.
The eleccell at a
mA
of an oxidising substance with
definite rate, ironrr being generated electrolytically at the cathode surface; the solution then reaches the titration cell where it is mixed with the stream of sample solution. The titration cell is provided with a platinum indicating electrode and a saturated calomel electrode, between which is applied a d.c. constant voltage, slightly more negative than the potential corresponding to the end-point. The solution in the titration cell is always automatically controlled so as to contain a small excess of ironrr. The electrogenerated ironI reacts with the oxidising substance in the sample solution and the remaining iron rr changes the potential at the indicating electrode; the potential difference between the indicator and reference electrodes is fed into the d.c. amplifier and its output supplied as electrolytic current for the electrogeneration of iron”. Fig. 1 shows the characteristics obtained when using the continuous coulometric titrator for the determination of an oxidising substance with iron rr. The curves represent the titration of sample solutions containing increasing amounts of the reacting component. A is the applied voltage, E is the theoretical potential at the end-point, and the line AB represents the working characteristic of
Titration of various oxidising substances
973
the d.c. amplifier, i.e., the relationship between the input (the potential difference between the indicator and reference voltages) and the output current (the electrolytic current). In the present work the iron” was generated coulometrically, so that the titration curve had a sigmoid shape as shown in Fig. 1, and the end-point indication circuit as shown in Fig. 2 was inverse compared with that in the coulometric titration with electrogenerated bromine. The positive side of the applied voltage was connected to earth, while the negative side was connected to the platinum indicator electrode. The above applied voltage (A in Fig. 1) was less than that at the equivalence point. The potential difference between the applied voltage and the changing potential during the titration was fed into the d.c. amplifier, whose output was applied to the electrogenerating electrodes (platinum cathode and anode in electrolysis cell).
Recorder
d.c. omplifer
FIG.
2.-Schematic
diagram of automatic continuous coulometric
titrator.
Current eficiency of electrogeneration of iron!? To find the effect of the concentration of electrolyte solution and electrolytic current intensity on the current efficiency, three electrolytic solutions (O.lN, 0.03N and O.OlN ammonium ironrrr sulphate in 2N sulphuric acid) were supplied to the electrolysis cell, whose e&rent containing electrogenerated iron” was taken into the titration cell. The flow rate of the electrolyte solution was held at 0.1-045 ml/set. The effluent was gathered during the electrolysis for 600 set and analysed for potentiometric titration with 0405N potassium electrogenerated iron rr by a conventional permanganate standard solution. The results are shown in Table I. From Table I the concentration of the ammonium ironrrr sulphate must be at least O=lN, even if the generating current was less than 3 mA, and this concentration was employed for the electrolyte solution in the subsequent experiments. Continuous coulometric titration of potassium permanganate Potassium permanganate is conventionally titrated with ironrr in acidic solution. In the present experiment, however, a minute amount of potassium permanganate was determined by continuous coulometric titration with electrogenerated iron II. Dynamic curves of the titration of potassium permanganate with ammonium ironI sulphate are given in Fig. 3, which indicate that the electrolytic current balancing the applied voltage changes in accordance with the applied voltage when potassium permanganate solution is fed in at a constant flow-rate of 0.16 ml/set. The dynamic curve corresponds to the lower part of the titration curve. The titration proceeded slightly through the end-point and only a continuous coulometric titration could be carried out, because the equivalence potential frequently fluctuated with the large oscillation, although it was very sensitive at the equivalence. The relationship between the applied potential and amplitude of fluctuation (the vibrating distance) of the writing pen in the recorder is shown in Table II. The optimum applied voltage can be found from Fig. 3 and Table II, and is about 0.62 V.
TAKXJ TAKAHASHIand HIROSHISAKLJRAI
974
TABLE I.-CURRENT EFFICIENCY Electrolytic current,
IN ELECTROGENEU~~ON OF IRON”
IronIIr electrolyte solution,
OGO5N KMnO,,
Current efficiency,
No.
mA
N
ml
%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
0.95 1.00 213 2.16 2.97 2.95 0 0 1.09 1.09 2.16 2.23 3.34 3.33 0 1.03 1.03 2.12 2.10 3.19 3.18 0 0
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
1.250 1.333 2.650 2.657 3.624 3.610 0.080 0.080 1.344 1.348 2.134 2.255 2.523 2.568 0.049 0.794 0.885 1.064 1.063 1.095 1.132 0.070 0.064
98.9 100.1 96.8 95.7 95.9 978
95.4 95.7 77.5 79.4 59.5 60.7 56.7 63.8 37.6 38.1 25.8 26.9
0.60 -
I
0
I
I
Electrolytic
FIG.
I
I.0
0.5
current,
I.5 mA
3.-Dynamic curves for titration of potassium permanganate with (A) H,O, (B) 0.5 x lo-*N KMnO,, (C) 1 x 10-4N KMn04.
ironrr:
975
Titration of various oxidising substances
The relationship between the concentration of potassium permanganate and the electrolytic current intensity is shown in Table III. In these experiments the sample solutions (0.5 x lo-‘, 1.0 x 10-4, l-5 x lo-* and 2.0 x 10-4N potassium permanganate solutions) were fed successively, but alternately with water, into the titration cell for 10 min and the electrolytic currents recorded. The applied voltage was set at 0.61 V; the feed rate of sample solutions was 0.16 ml/set and that of the electrolyte solution 0.15 ml/set. This series of experiments with the 4 sample solutions was repeated seven times and the electrolytic currents recorded in Table III have the background current (the electrolytic current for water alone in the titration cell) deducted.
Applied voltage, V KMn04, N
I-W 5 x 10-s 1 x lo-’
0.60
0.62
0.64
0.66
0.68
0.70
0.90
1X@
1.05
0 (mA) 0 0.02
0 0.01 0.04
0 0.05 0.45
0 0.45 0.75
0 0.70 0.95
0 0.95 -
0 0.75 1.20
0 0.45
0 0.03
TABLEIII.-RELATION~~-~BETWEENCONCENTRATIONOF~~TA~~ILJMPERMANGANATE AND ELECTROLYTVZ CURRENT KMnO,,N
No. 1 2 3 4 5 6 7 Av. Std. dev. Theoretical current
5 x 10-s
1 x 10-a
1.5 x 10-d
2 x 10-d
0.80 (mA) 0.78 0.88 0.93 0.86 0.88 0.82 0.85 0.049
1.53 (mA) 1.54 1.63 1.63 1.66 1.62 1.58 1.60 0,046
2.30 (mA) 2.30 2.35 2.48 244 2.37 2.38 2.37 0,062
3.00 (mA) 2.98 3.12 3.12 3.12 3.08 3.09 3.07 0,060
0.772
1.544
2.316
3.088
The results show that the theoretical and measured values coincide very well and the relationship between the concentration and electrolytic current is almost linearly proportional. It may be said more exactly that the ratio electrolytic current/concentration diminishes slightly with increase of concentration. This can be explained by the decrease of the excess concentration of iron’1 at the equivalence point with increase of the concentration of potassium permanganate. The stability of the recording was examined by the following procedure. Distilled water and 5 x 10-5N potassium permanganate were used as sample solutions flowing for 2 hr and the electrolytic current was measured every 4 min. The experimental results showed that it was almost constant (the average current for 30 measurements was 0.345 mA with a standard deviation of 0,005 mA in the case of distilled water, and 1.194 mA with a standard deviation of 0.039 mA in the case of 5 x 10-6N potassium permanganate). Continuous coulometric titration ofpotassium bichromate Potassium bichromate can be easily titrated by the conventional potentiometric method with iron” in acidic solution. The dynamic titration curves are shown in Fig. 4 and they were taken in the same manner as for potassium permanganate. These curves differ from those of potassium permanganate, however, in that they run smoothly, this resulting from the indistinctness of the equivalence potential. This difference is considered to be caused not only by the smaller potential change for potassium bichromate-iron I1 than that for potassium permanganate-ironrr, but also to a slow electrode reaction rate or to a slow following of the indicator electrodes near the equivalence point. The oscillation could not be seen when the applied voltage increased to 0.7 V.
976
TAKEOT AKAHASHI and
I
HIROSHI SAKURAI
I
I
PO
0.5 Electrolytic
current,
I.5 mA
FIG. 4.-Dynamic curves for titration of potassium bichromate with ironI1: (A) H,O, (B) 0.5 X 10-4NK,Cr,0,, (C) 1 X 10-‘NK,Cr,O,.
‘-70
_-.-d t .-
65
-'
---
--
l _--,.--___I._-_---__-\\ -.--___--.-_-_-‘.___-’ .
0.60
I __.---
\_
__---
I
I
0.5
0
Elecfrolytic FIG.
1
l-0 current,
I.5 mA
5.-Dynamic curves for titration of free chlorine in water: (A) HIO, (B) 4.5 x 10-6NCI~, (C) 9 x lo-‘NClp.
2
Titration of various oxidising substances TABLE IV.-RELATIONSHIP
BETWEEN CONCENTRATION OF WTASSIUM BICHROMATE AND EX,EcTROLvTK CURRENT
K&r&N
No. 1 2 3 4 5 Av. Std. dev. Theoretical current
977
5 x 10-s
1 x 10-d
1.5 x 10-a
2 x 10-a
0.63 (mA) 0.65 0.65 064 0.63 0.64 0009
1.31 (mA) 1.33 1.32 1.31 1.29 1.31 0.013
1.97 (mA) 1.98 1.98 1.96 1.96 1.97 oGO9
2.62 (mA) 2.59 2.61 2.59 2.58 2.58 0.023
0.772
1.544
2.316
3.088
The relationship between the concentration of potassium bichromate and electrolytic current intensity is shown in Table IV (the current intensity has the background current deducted). The applied voltage was set at 0.64 V. The results show a linear proportionality between the concentration of potassium bichromate and the electrolytic current, but the electrolytic current is lower than the theoretical value by about 16%. This may be caused by the large deviation between the dynamic and the static titration curves at the equivalence point. The deviation increases with increase of applied voltage. Continuous coulometric titration of free chlorine in water Free chlorine dissolved in water has been determined by the titration of liberated iodine or bromine from potassium iodide or potassium bromide. A very low concentration of free chlorine can be determined calorimetrically or spectrophotometrically using o-tolidine, benzidine hydrochloride or dimethyl-pphenylendiamine as reagents. In the present case the free chlorine content in water was determined potentiometrically by titration with electrogenerated iron” using the continuous coulometric titrator.
0
I I
I
I 3
2 C12.
ml
FIG. 6.-Relationship between concentration of chlorine in sample and electrolytic current.
4
TAKEO TAKAHASHI and Hraosrrr SAKURU
978
The dynamic titration curves are shown in Fig. 5. The equivalence potential is about 0.82 V (us. S.C.E.), where the oscillation occurs when the applied voltage is set near the equivalence point, because the potential sensibility is fairly large at the equivalence point. The relationship between applied voltage and amplitude of fluctuation is shown in Table V.
TAJSLEV.-RELATIONSHIPB~~EENAPPLIEDVOLTAGEANDAMPL~EOF~UCT~ATION
Applied voltage, V CL N 4.5 x 10-b 9 x 1o-6
0,60
0.62
0.64
0.66
0.68
0.70
0.80
090
1.00
1.10
O(nN O*Ol
0 0.01
0.005 0.020
0.01 0.05
0.25 0.35
0.35 0.45
0.50 0.80
0.45 0.75
0.25 0.60
0.05 0.20
Electrolyticcurrent, FIG. 7.-A
mA
typical chart recording in the continuous titration of chlorine.
Titration of various oxidising substances
979
The relationship between the concentration of chlorine in the sample solution and the electrolytic current is given in Fig. 6. The sample solutions were prepared from the original chlorine solution (O+lSN) by dilution of 1, 2, 3, or 4 ml to 1 litre with water. The abscissa in Fig. 6 shows the volume of the original solution taken. The ordinate is the average measured value of six electrolytic currents (standard deviations are 0~033,0~020,0~014 and 0.020 mA in order of increasing chlorine concentration). Fig. 7 is a typical chart recording for the continuous coulometric analysis of chlorine, in which the applied voltage was set at 0.64 V and the flow rate of the electrolytic solution was 0.15 ml/set. Zusamme&asung-Eisen(I1) reagiert in saurer LSstmg rasch und quantitativ mit verschiedenen Oxydationsmittehr wie Permanganat, Dichromat oder freiem Chlor. Kontinuierliche coulometrische Titrationen solcher Liisungen wurden mit an einer Platinkathode aus Eisen(II1) erzeugtem Eisen(I1) durchgefuhrt; der Reaktionsverlauf in der Titrationszelle wurde mit Hilfe eines Platin-Kalomel-ElektrodenDie PotentialditIerenz zwischen paares potentiometrisch verfolgt. gemessenem und Endptmktspotential wurde mit einem Gleichstromverstarker verstarkt; dessen Ausgang steuerte den Elektrolysestrom, soda0 das gemessene Potential gleich dem Endpunktspotential wurde. So warder das Eisen(I1) liefernde Elektrolysestrom der Konzentration des Oxydationsmittels direkt proportional. Urn gute Ergebnisse zu erzielen, ist vorher die Aufnahme einer dynamischen Titrationskurve empfehlenswert, urn das Optimum fur den Endpunkt der coulometrischen Titration festzulegen. R&sum&L’ion ferreux &git rapidement et quantitativement avec des oxydants vari6s comme le permanganate de potassium, le bichromate de potassium et le chlore libre en solution acide. Les dosages coulometriques de telles solutions ont Bte effectues par l’ion ferreux forme par galvanoplastie a partir de l’ion ferrique sur electrode de platine. La marche de la reaction dans la cellule de dosage est recueillie potentiometriquement par un couple d’electrodes platine/calomel. La difference de potentiel entre le potentiel detect6 et le potentiel du point equivalent est amplifie par un amplificateur d.c. dont le courant de sortie est controle de telle maniere que le potentiel detect6 devienne egal a celui du point equivalent. Ainsi, le courant electrolytique qui fournit l’ion ferreux est alors directement proportionnel a la concentration de l’agent oxydant dam la solution &udi&e. Pour obtenir de bons resultats, il est necessaire d’effectuer une courbe de dosage empirique en vue de localiser au mieux le point equivalent du dosage coulometrique. REFERENCES 1 W. Oelsen and P. Giibbels, Stahl II. Eisen, 1949,69, 33. 2 W. Oelsen, H. Haase and G. Graue, Angew. Chem., 1952,64,76. s W. D. Cooke and N. H. Furman, Analyt. Chem., 1950,22,896. 4 N. H. Furman, C. N. Reilley and W. D. Cooke, ibid., 1951, 23, 1665. 6 W. D. Cooke, C. N. Reilley and N. H. Furman, ibid., 1952,24,205. s L. Meites, ibid., 1952, 24, 1057. ’ T. Takahashi, K. Kimoto and H. Sakurai. Rep. Inst. Znd. Sci.. Univ. of Tokvo, 1955. 5, No. 6. ’ ’ . . * A. J. Fenton,.Jr. and N. H. Furman, Anaiyt. ‘Chem., 1957,29,. 221. s oT. Takahashi. E. Niki and H. Sakurai. J. Electroanalvt. Chem.. 1962. 3. 373. lo T. Takahashiand H. Sakurai, ibid., 1562,3, 381. ’ ’ ’ ’ I1 Idem, Talanta, 1962,9, 195.