where the third transition time, ro, is due to the reoxidation of species 2 (Zn metal). For the 23 determinations shown in Table 11, the standard deviation is less than 3.0%. CONCLUSION
Tn routine analytical applications, chronopotentiometry with step-functional changes in current offers no significant advantages over conventional chronopotentiometry, and in some cases, particularly current decrease during electrolysis, leads to unfavorable complications. However, in studies of
kinetics and mechanism of electrode processes a number of interesting and useful applications suggest themselves. Some uses of current reversal have been made previously; other8 are suggested by the present study. Current cessation in multicomponent systems appears to have value in the investigation of purely chemical reactions between reactants produced by electrochemical means. LITERATURE CITED
(1) Berzins, T.,Delahay, P., J. Am. C h . SOC.75,4205(1953). (2) Delahay, P., Mattax, C. C., Ibid., 76,874(1954).
(3) Delahay, P., Mamantov, G., ANAL. CHEM. 27,478 (1955). (4) Drdka, O., Collection Czechosloo. Chem. Communs. 25,338 (1960). (5)Furlani, C., Morpewigo, G., J . Electroanal. Chem. 1, 351 (1960). (6) Geske, D.H.,J. Am. Chem. SOC.81, 4145 (1959). (7)King, R. M.,Reilley, C. N., J.Electroanal. Chem. 1,434(1960). (8) Reilley, C. N., Everett, G. W., Johns, R. H., ANAL. CHEM.27,483 (1955). (9) Testa, A. C., Reinmuth, W. H., Ibid., 32, 1512 (1960).
RECEVEDfor review October 10, 1960 Acce ted June 12,1961. Presented at the Pitbiurgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Feb. 29, 1960.
Continuous Differential Potentiometric Titration M. M. NICHOLSON Humble Oil & Refining Co., Baytown, Tex.
b The principle of differential potentiometric titration with two indicating electrodes of the same type has been applied to flowing streams for use in continuous analysis and automatic plant process control. An advantage of the differential system is that it seeks the shoulder of the potentiometric titration curve automatically and is thus nearly independent of displacements of the curve on the voltage scale. Results are given for the continuous titration of ferrous iron with dichromate solution.
D
IFFERENTIAL POTENTIOMETRIC TI-
with two like indicating electrodes is one of the classical electroanalytical methods. The original technique, due to Cox, involved the simultaneous titration of identical samples in separate beakers connected by a salt bridge (1). MacInnes and coworkers introduced the more familiar modification with a single vessel containing a temporarily isolated electrode and applied it to several precise analyb ical determinations (3-6). This paper describes differential potentiometric titration in a flowing system, for use in continuous analysis and automatic plant process control. Apparatus for performing such a continuous titration is depicted schematically in Figure 1. The sample and reagent streams are divided and distributed to the two half cells by capillary inlet tubes, which are adjusted to give slightly different fractions titrated in the cell compartments. The potential difference AE betweer +.he two in1328
TRATION
ANALYTICAL CHEMISTRY
and the ratio of the fractions titrated is I SAMPLE
TITRANT
1
I
I
t ,
AE
XI
A_.
Xn
A.
(2)
In general, X = rcr/sc,, with c, and c. representing concentrations. For 8 s m d control interval, X is given
D
I ,
implicitly by XE'(X) =
2
1
E'(X) =
Figure 1. Continuous differential titration svstem
s1 82 and A ,
71
= rz -
(3)
(l)
AE -x2 - XI
(4)
For practical purposes, the total r e agent flow rate, T1 rz, is directly proportional to the sample concentration, c., if sl SZ, c, and AE are constant, and the function E ' ( X ) is not very dependent on c,. A linear calibration does not require that X approach unity -only that it be essentially constant ubder the conditions of intereat. The analyzer can be applied to automatic plant process control by causing a signal from the flowmeter to actuate a second control unit, which will adjust a suitable process variable in response to changes in the sample concentration. A laboratory evaluation of the differential titrator as a continuous analyzer waa made by the titration of ferrous iron solutions with potaaaium dichromate, using two platinum indicator electrodes. Results obtained with the difEerentia1 system are compared with those from an analogous Ringle cell
+
+
dicator electrodes is applied to a d e tector-controller system, D, which automatically regulates the flow of titrant by adjusting the valve, V . In thb way, a selected value of A,?? is maintained across the cell, and the titrant addition rate, indicated by the flowmeter, MI is a measure of the sample composition. The inlet capillaries UBually will serve also as a salt bridge for the potentiometric mettsurement. The ratio of the fractions titrated in the two half cells is fixed by the geometry of the capillaries. If SI and 82 are the sample flow rates, and rl and rs are the reagent rates, then capillary constants A , and A , may be defined by A, =
-1
where E ' ( X ) is the derivative of the titration curve when given in terms of the fraction titrated and is approxi-
1
1
AE (AaIAr)
.o 9 1
1
-
0.2 I
Figure 2. rnium(VI)
I
I
I
1
Potentiometric titration of iron(ll) with chro-
--
Figure 3. Continuous differential titration of iron(ll) with chrorniurn(V1) AE = 20 mv. --t 40 mv.
arrangement using an indicator and a reference electrode. An example of a commercial single cell titration unit is the Dow-Hart instrument supplied by the Milton-Roy Co. (2, 7 ) .
-A-
compartment was then established a t potentials of 600, 550, and 580 mv. va.
S.C.E.
EXPERIMENTAL
The magnetically stirred titration cell was similar to that in Figure 1, with a capacity in each side of about 50 ml. a t the overflow level. The capillary ratio A , was 1.02,; A , was 0.91. The two platinum indicator electrodes were connected in series with sn opposing potentiometer on which the voltage eF was set, and the unbalance between the potentiometer and cell voltages was detected on a Kintel Model 202B microvoltmeter. The amplified output of the microvoltmeter was applied to a United Hydraulics proportional control unit, which operated an Ideal-Aerosmith No. 54-2-11 stainless steel needle valve in the titrant inlet line. The sample flow rate was controlled by the hydrostatic pressure in a large elevated sup ly bottle; the titrant was kept un&r a slight gas pressure to bring its rate of addition within a desirable range. The total flow rates of both solutions were measured on rotameters reading from 0 to 10 ml./min. The sample solutions were from ferrous ammonium sulate and ferric chloride, at concentrations recorded in Table I. They contained, in addition, 0.2M sulfuric acid. The reagent was 0.0167M Dotassium dichromate. Under steady conditions of automatic operation, the selected diflerential voltage'intervak of 20,40,and 100 mv. remained constant to about *lo% and the reagent flow rate seldom varied more than 10.1 ml./min. The measured flow rates were within the range of 2 to 9 ml./min. To compare continuous differential and continuous single cell potentiometric titrations, a saturated calomel electrode was inserted in one of the half celle., and the inlet t u b a to the other side were closed. Automatic control in the one
prepared
100 mv.
- - - - Equivalence line
Conventional potentiometric titration curves for the various solutions were recorded on a Precision-Dow Recordomtic titrator at a high sensitivity to the rate of voltage change. RESULTS AND DISCUSSION
In figure 2, titration curves are shown for the four ferrous iron solutions, and some of the the control conditions that were used in the automatic operation are indicated. The calibration plots for the dserential titration, Figure 3, are linear within the experimental accuracy and a p proach the equivalence line with increasing AE, as expected. The potentials of the platinum electrodes were stable in the presence of the ferricferrous couple but wandered erratically in excess dichromate, due to irreversibility of the potential-determining process. For this reason, automatic control was established differentially only on the chromatedeficient side of the equivalence point. A slow titration reaction would also impose a limitation. With chemical systems that
respond rapidly on both sides, however, the control interval may be located before or after the end point. This choice will be determined by the. orientation of the leads to the cell, since the two control regions require opposite directional responses of the titrant addition mechanism. In general, the most satisfactory continuous performance of the differential titrator will be obtained on a shoulder of the titration curve. The tendency to overshoot the equivalence point is thus minimized, and the system is not unduly sensitive tq minor fluctuations, as it might be when operating on a steeper slope. Although the effect of overshooting was observed occasionally with the ferrousdichromate reaction, it did not present serious difficulties. An automatic resetting device could be used to circumvent this problem in unattended installations. Data for continuous differential and single cell titrations are given in Table I. In the differential cell, the total fraction titrated is equal to (rl r2)cr/ [(sl s2)c8]. The results for solution 4 emphasize a n advantage offered by the differential method when the titration curve shifts on the voltage axis.
+
+
Table I, Fractions of Im(ll)Tltrated in Differential and Single Cells Total Fraction Titrated
soh.
in Differential Cell
E
'-
No.
Fe(I1)
1
0,0975M
OM
0.84
0 0 0.4
0.81
3 4
0.0682 0.0388 0.0388
a
Fe(II1)
Fraction Titrated in Sigle Cell
20 mv. 40 mv. 100 mv. 500 mv. 660 mv. 680 mv
0.76 0.88
0.92 0.90 0.91 0.95
1.01 0.94 0.97 1.03
0.96 0.97 0.89 0.00
0.99 1.00 0.93
0.37
VOL 33, NO. 10, SEPTEMBER 1961
1.01 1.02 0.98 0.78
1329
A shift of this kind, due to a change in the activity of one or more potentialcontrolling species, will be encountered frequently in solutions that are recycled in continuous plant processes. The differential system is essentially independent of these displacements because it automatically seeks the shoulder of the titration curve, Its use under such circumstances advantageous.
be
ACKNOWLEDGMENT
(3) MacInnes, D. A., 2. physik. Chem.
The writer thanks Marjorie IT. Eastwood for laboratory work in evaluation Of the COntinUOUS titrator and W. A. Morgan for recommending the detectorcontroller system which was used.
(4) 1.hIacInnes, A , , J . Am, Chem. D. A., Sot.Cowperthwaik, 5j, j55 (1931), (5) MacInnes, D. A., Dole, M., Zbid., 51,
LITERATURE CITED
(1) cox, D. c., J . Am. Chem. &c. 47, 2138 (1925). (2) Hart, Porter, ZSA Journal 4, 472 (1957).
l3OA! 217 (lg2V.
1119 (1929). (6) MacInnes, D. A., Jones, J. T., Zbid. 48, 2831 (1926). (7) Milton Roy Co., data sheet A-58-2.
RECEIVEDfor review March 6, 1961. Accepted May 15, 1961. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1961.
A Spectrophotometric Method for the Determination of Moisture and Active Hydrogen T. G. MUNGALL and J. H. MITCHEN Research and Development Departmenf, Ethyl Corp., Baton Rouge, l a . ,A new colorimetric procedure permits the rapid determination of moisture in many liquids, such as metal alkyls, where the Karl Fischer method is inapplicable. Microgram quantities of moisture can be accurately determined, and normally the entire procedure takes about 10 minutes. This method is based on the fact that water or other forms of active hydrogen destroy the intense red color of the diethylaluminum hydride-2-isoquinoline complex. Accordingly, it can also be used to determine the total reactivity based on water and/or active hydrogen.
0
i t is necessary to determine the inertness of solvents to be used in systems containing highly reactive materials, such as aluminum alkyls or Grignard reagents. Existing techniques for measuring moisture or other forms of active hydrogen in these solvents are cumbersome and frequently are inapplicable. For example, the Karl Fischer method requires special techniques and great care to obtain accuracy in the few parts per million range; furthermore, it is not sensitive to hydroxyl groups or acids, and cannot be used with unsaturates without modification. Gas evolution techniques based on Grignard reagents likewise are not easily adapted to determining low moisture concentrations; corrections for vapor pressure and solubility of the evolved gas in the solvent are particularly troublesome. This paper describes a new procedure for determining moisture and active hydrogen. It is based upon the reaction used by Mitchen (3) for the spectrophotometric determination of diethylaluminum hydride. The reaction and color measurement are carried out in FTEN
1330
ANALYTICAL CHEMISTRY
the same vessel, which eliminates most of the manipulative and sample handling difficulties of conventional techniques, enabling completion of the analysis in 10 minutes. Because the method is sensitive to microgram quantities of moisture or active hydrogen, a suitable choice of sample volumes will permit determinations over a wide range. Dissolved oxygen in the sample is a source of positive error, but this can be measured and corrected for. Organic functional groups that react with diethylaluminum hydride, such as aldehydes and ketones, constitute an interference. APPARATUS AND REAGENTS
The method employs a Beckman DU spectrophotometer with a modified cell compartment and a 200-ml. comparator tube fitted with a 1-cm. cell ( 2 ) . A 2.0-cc. hypodermic syringe is used to deliver measured amounts of standard or sample into the comparator tube. Benzene. Especially dry benzene is prepared by distilling benzene from a mixture of 1% triethylaluminum and reagent grade benzene. Standard Solutions of Water in Benzene. Oxygen-free benzene is prepared by passing nitrogen over hot reduced copper turnings and then through dry benzene. Several hours’ bubbling time a t a flow rate of approximately 50 cc. per min. is required to decrease the oxygen concentration in 500 ml. of benzene from 40 to 2 p.p.m. For calibration, water is added to the benzene by bubbling the nitrogen through a water scrubber located upstream from the oxygen removal system. Bubbling time with the water-saturated nitrogen may be varied to obtain several different water concentrations which are then determined by the Karl Fischer method. To avoid contamination by oxygen or water prior to use, these standards are covered with Sani-Tab
caps and stored in a water- and oxygenfree dry box; even with these precautions they should be used within 2 hours after preparation. 10% Isoquinoline Solution. Ten milliliters of freshly distilled isoquinoline is diluted to 100 ml. with dry benzene. Diethylaluminum Hydride. It is not necessary to have pure diethylaluminum hydride for development of the red isoquinoline complex. However, for best results the purity of this reagent should be a t least 50%. Reduced Copper Solution. A solution of 5 grams of cupric sulfate pentahydrate and 75 grams of ammonium chloride in 500 ml. of 501, ammonium hydroxide is reduced by storing it over copper wire in a container fitted with a serum-type rubber stopper. Reduction is evidenced when the solution in contact with the copper loses its dark blue color. It is not necessary for the solution to become completely colorless before it is used PROCEDURE
To prepare a suitable blank, place a sleeve-type serum stopper over the mouth of a dry, nitrogen-flushed, 200ml. comparator tube and secure it tightly with rubber bands. Introduce 20 ml. of dry benzene followed by 5 ml. of 10% isoquinoline solution via a hypodermic syringe. Add diethylaluminum hydride until a red isoquimline-Et2A1H complex is formed with an absorbance reading between 1.00 and 1.50 a t 460-mp wave length. Shake the comparator tube with its contents for 3 minutes prior to reading the spectrophotometer to ensure that all oxygen and water in the reagents have been reacted. It may become necessary a t this point to readjust the absorbance of the mixture to the above-mentioned limits by adding a few more drops of EtzAlH. Xext, prepare a calibration curve by injecting through the serum stopper a
