Coulometric Titration of Dyes with Externally Generated Titanous Ion J. S. PARSONS and WILLIAM SEAMAN Research Division, American Cyanamid Co., Bound Brook, N.
J.
a
fritted disk was employed. Seepage mas made very slo~v(less than 1 ml. in several hours) by allowing some of the titanic soliition to hydrolyze so a s to clog the pores. The fritted-glass disk becomes clogged when titanium ion is generated for about 20 minutes a t 260 ma. with the titanic solution flowing through the cell at n slow flow rate (about 2 ml. per minute). A fritted-glass tube prepared in this way should be satisfactory for many weeks, provided the tube is not allowed to dry out. The tube may be unclogged by boiling in concentrated sulfuric acid. A zinc rod was used a s the anode with 5% sodium sulfate as the anolyte. The zinc rod should be fitted tight a t the upper end, so that the titanium tetrachloride solution cannot be forced into the anode compartment. The anolyte solution was renewed for each titration. It is recommended thaf, for best iesults a fresh mercury pool be used for each analysis. The mercury pool can be removed by releasing the rubber stopper a t the bottom of the cell, then replacing the stopper and allowing fresh mercury to flow from the reservoir. Titanium tetrachloride solution should not be allswed to stand in the cell overnight, a s it will hydrolyze. It is recommended that the electrolysis cell assembly be installed on the movable stand (8) (available from Scientific Glass Co., Bloomfield, S. J.), so that the delivery tube can be raised or loTyered into the titration vessel R ith ease.
Titanous ion has been generated electrolytically at a mercury cathode in an external electrolysis cell using constant current. The average current efficiency of 31 titrations was 99.8%, with a standard deviation for a single value of +O.l5%. The generated reagent has been used to titrate Orange 11, tartrazine, p-aminoazobenzene, amaranth, and methyl violet with results in satisfactory agreement with the conventional volumetric titanous chloride titration methods.
T
ITASOUS chloride is used extensively for titration of dyes. Coulometric titration with electrolytically generated titanous ion has a particular advantage, as titanous chloride reagent requires frequent standardization because of its instability. Arthur and Donahue ( 1 ) have reported the internal generation of titanous ion a t a gold electrode. However, the necessity for frequent reconditioning of the electrode so as to maintain a high current efficiency proved too bothersome in the authors’ hands. Titanous ion generation with 100% efficiency is difficult to achieve because of the competing reaction, H + e = HP,the redox potential of which is close t o that of the titanous-titanic system. In this work external generation of titanous ion at a mercury pool cathode was used. The use of externally generated reagents was first described by De Ford, Pitts, and Johns (3). Mercury, which has a much higher hydrogen overvoltage than gold, was found to be a better electrode material. The external electrolysis cell was preferable t o internal generation, since the near-boiling temperature at which some titrations of dyes are carried out caused certain difficulties in using internal generations a t the mercury cathode. Furthermore, external generation allows conditions for 100% efficient electrolysis to be standardized so that in setting up the coulometric method any necessary changes in the present dye titration procedures can be minimized.
+
RUBBER SLEEVE ED G L A S S D I S K
Hg CATHODE
112cmdtal RUBBER STOPPER/’
CAPILLARY TUBE 127cm i 0 6 m m ~ d m l
APPARATUS AND REAGEYTS T O TITRATlbN ASSEMBLY
Instrumentation. Precision coulometric titration equipment mhich was constructed in this laboratory will be described in another paper. The essential components consist of a Reilley, Adams, and Furman constant current supply ( 7 ) ,which was modified so that currents up to 450 ma. could be obtained and a clock (Standard Electric Time Co. RIodel S-1 with direct current clutch) which is operated by a Riverbank standard tuning fork (Cenco, 60 cycles, 5 p.p.m. precision). The current was measured from the potential drop across a standard resistor in the circuit by means of a Rubicon portable precision potentiometer (Catalog S o . 2710). Other equipment ( 3 ) would be satisfactory for the current levels used in this work. It was, however, necessary to use a frequency standard to operate the timer in the work reported here, because of frequency variations in the plant poa er. Electrolysis Cell. A diagram of the electrolysis cell used is shown in Figure 1. Approximately 0.4M titanium tetrachloride solution is allowed t o pass in a thin layer over a mercury pool cathode which is separated from the anode compartment by a fritted-glass disk. The titanous ion produced a t the cathode is carried to the titration beaker by a capillary tube. The flow rate is adjusted by keeping the electrolysis solution under slight nitrogen pressure with a mercury pressure regulator. A stopcock shell made a convenient cell (6). Rubber stoppers were used to connect the tube containing the fritted-glass disk and the mercury pool assembly to the stopcock a s illustrated in Figure 1. The capillary tubes mere fitted inside the stopcock tubes, so that the capillaries were flush with the inner wall of the stopcock shell. The mercury pool, which has an area of about I sq. cm., was adjusted so that the distance between the pool and fritted glass tube was about 1 mm. or less (mercury should not touch the fritted-glass disk). A tube with a coarse porosity
Figure 1.
Electrolysis cell
Reagents. The stock titanic chloride solution was prepared according to the procedure described by Arthur and Donahue ( 1 ) . Water was added slowly to 200 ml. of C . P . titanic chloride (Amend Drug & Chemical Co., S e w Tork) v i t h stirring until vigorous hydrolysis ceased. The mixture goes through a doughy stage, making stirring difficult. However, by macerating with a large stirring rod and adding more uater, complete solution is obtained. The final solution %vasdiluted to 500 ml. This solution is approximately 3.6M in titanic ion and is stable t o hydrolvsis for a t least 6 months. A small amount of titanium dioxide formed by hydrolysis ma\- settle out, but this is of no consequence, a s the supernatant liquor is clear. The stock titanium tetrachloride solution gave a negative test for ferric ion when 2.5 ml. of stock solution was diluted to 50 ml. in a Sessler tube and treated with 2.5 grams ammonium thiocyanate. The pale yellow color of this solution mas definitely distinguishable from the red tint of a similar solution to m hich had been added 0.0007 mg. of ferric ion. The electrolysis solution 15 as prepared by diluting the concentrated stock ten times with distilled water. This solution, which had a pH of 0.6, began to hpdrolvae seriously in 12 to 2.2 hours. Hence, the solution must be used within a n hour or two after preparation. Nitrogen (prepurified grade) should be bubbled through the solution for a few minutes before use in order to remove dissolved oxygen. The standard potassium dichiomate solution used for testing the efficiency of titanous ion generation was prepared by weighing 210
V O L U M E 2 7 , N O . 2, F E B R U A R Y 1 9 5 5 out National Bureau of Standards potassium dichromate and making up to volume in carefully calibrated glassware. Based upon the weight of potassium dichromate this solution n a = 0.02504S; by coulometric titration with electrolytically gene] ated ferrous ion ( 2 ) five titrations gave 0.02500, 0.02502, 0.02.509. 0.02508, 0.02501 ; average, 0.02504. Aitriple-distilled grade of nirrcury was used. METHOD FOR TESTI\G EFFICIENCY OF TITANOUS IOh GENERATION A 200-ml. tall-form electrolvsis beaker was provided nith a magnetic stirrer and rubber stopper, so that an atmosphere of carbon dioxide could be maintained. Approximately 35 ml. oi G S sulfuric acid v e r e added and carbon dioxide (purified by passage through a chromous solution) was bubbled through the solution for a f e x minutes. Then after 3 grams of reagent grade Nohr’s salt crystals had bren added and dissolved, a 25-ml aliquot (calibrated pipet) of 0.02504N potassium dichromate was added. Carbon dioxide was bubbled through the solution for a few minutes and finally carbon dioxide was allowed to pass over the surface of the solution during the titration. The electrolysis delivei y tube was lowered into the titration henker so that the tip Tyas immersed in the solution. The flow of titanic chloride reagent (8 to 9 ml. per minute) was started and the electrolysis current was snitched on to begin the titration. S e a r the end point 10 ml. of 6.6M ammonium thiocyanate indicator were added 2nd the current was then interrupted a t 0.25second intervals until the reddish ferric thiocyanate color was just discharged. A blank vras run in exactly the same way, substituti,ig 25 ml. of distilled nater for the dichromate solution and adding about the same volume of titanic chloride solution as for the titration. The efficiency was calculated a s follows:
Xlliequivalents of potassium dichromate taken X 96.5 X 100 Time for titration in seconds X current in amperes % efficiency PROCEDURE FOR DYE TITRATIONS
+
Keigh 5 grams of sodium bitartrate (XaHC4H4O6 HzO) into the titration beaker and add 50 ml. of distilled water. Start the flow of carbon dioxide through the beaker and heat to boiling for a fea- minutes to remove dissolved oxygen. Lower the electrolysis cell delivery tube into the beaker with the flow of titanium tetrachloride solution adjusted to 8 to 9 ml. per minute. Switch on the current (adjusted to 260 ma.) and generate titanous ion for a few seconds so as to reduce any reducible substances in the reagents. Remove the electrolysis assembly and boil for 3 minutes. Back-titrate the excess titanous ion with 0.025Aamaranth to a faint pink n-hich persists for 30 seconds. To avoid the back-titration, the standard amaranth solution may be added with a Mohr pipet to give a permanent red, indicating the presence of excess amaranth; then the solution may be brought to a n exact end point by generating titanous ion just to decolorize the amaranth. Pipet into the beaker a 25-1111. aliquot of the dye solution-e.g., 2.000 grams of Orange I1 dissolved and made up to 1 liter in distilled n-ater. Bring to a boil with carbon dioxide bubbling through the s o l u t h . Commence the titration, keeping a; atmosphere of purified carbon dioxide above the titration solution a t all times and the temperature just under the boiling point. The end point is taken, for instance, in the case of Orange 11, when the orange color is just discharged or when a 0.25-second addition of titanous ion causes no further change in color in the solution. The titration required about 190 seconds. Other dyes require variations in this procedure. With aminoazobenzene, tartrazine, and methyl violet excess titanous ion is generated and the excess is back-titrated to a pink tinge a i t h 0.025-\- amaranth. The amaranth may be added u i t h a pipet and titanous ion generated just to decolorize the excess amaranth, as previously described. The aminoazobenzene hydrochloride is made up in 3.4 alcohol and the titration is carried out in approximately 35% 3.4 alcohol-water medium. I n the case of tartrazine a n d methyl violet, 15 grams of sodium bitartrate are used. RESULTS AIVD DISCUSSION
Values of the current efficiencies for external generation of titanous ion in the cell shown in Figure 1 a t several current levels are presented in Table I. The average current efficiency and the standard deviation of a single value calculated from all the data in Table I were 99.8 and *O.l5%. Factors which tend to affect the current efficiency are:
21 1
Table I.
Current Efficiency for External Generation of Titanous Ion at 3llercury Cathode Ciirient. Flow Rate, KO.of Efficiency. Standard hIl./Mn. Detns. 70 Deviation hla. 111.0
1.i8 1 260 4 207 0 297 0 420.0
8-9 8-9 8-9 8-9 15
2 17
15
2
4 2 4
99.95 99.94.100.02 99.76 99.97, 9 9 . 9 2 99 94 99 59, 99.89
0.2
o:i4 0: is
..
The hydrogen ion concentration of thr titanium tetrachloride plectrolysis solution should not be increased. The approximately 0 . 4 s titanium tetrachloride solution as made up has a p H of 0.6. A titanium tetrachloride solution which was stabilized by adjusting the acidity to approximately IN in sulfuric acid gave a low current efficiency. A solution of 0.4N titanic sulfate (pH = 0.4) gave a rapid evolution of hydrogen a t the mercury pool. A fast flow rate through the cell is desirable in order to keep the depolarizer concentration a t the electrode surface high. Hydrogen &-as evolved a t the mercury pool when the flow rate was decreased to less than 2 to 3 ml. per minute. The hold-up volume in the electrolysis cell and delivery tube must be kept as small as possible for better flow and flushing characteristics. A bright mercury surface is important for best results. Repeated use of the mercury pool without renewing the surface may lead to l o a efficiencies. The titanium tetrachloride solution should not be allowed to hydrolyze in the cell. Caution should be taken to ensure that traces of metals or other impurities do not get into the titanium tetrachloride reagent. Internal Versus External Generation. One advantage of internal electrolysis is the large electrode surface that can be used, so that the current density can be kept low with resulting better current efficiency. Although nearly 1 0 0 ~current o efficiency was obtained in this laboratory for several titrations of ferric ion with titanous ion by the use of the gold electrode with internal generation, other titrations were ruined by hydrogen evolution. The frequent cleaning and flaming treatment of the gold electrode, recommended by Arthur and Donahue to avoid such erratic behavior, was not convenient. Preliminary experiments in generating titanous ion a t a large mercury cathode indicated that Orange I1 could be titrated by the internal generation of titanous ion if the temperature of the titration was considerably below the boiling point. However, the rate of reaction of titanous ion with the dye was slowed down. On raising the temperature to near the boiling point, h>drolysis of titanium chloride solution and bubble formation a t the mercury surface led to low current efficiencies. De Ford (3’) has pointed out that one of the advantages of external generation is that the electrolysis can be controlled xithout sample interference a t the electrodes. Hence, the external generation method was considered a better approach. since titanous ion could be generated a t room temperature and then delivered to the hot dye solution. Preliminary experiments were carried out with a small mercury pool as the cathode in one arm of the De Ford T-tube cell. Current efficiencies of 98 to 99% were obtained. illercury plated on a platinum wire coil and silver wire coil cathodes amalgamated with mercury, which were tried in order to obtain a larger electrode area, gave very low current efficiencies (54 to 75%). Pitts, De Ford, Martin, and Schmall (6) have used a single flow of electrolysis solution through a platinum gauze electrode contained in a stopcock for external generation of chlorine, bromine, and iodine. .4 somewhat similar cell design was used for generation of titanous ion, as this design was better suited for obtaining a larger mercury pool cathode surface and smaller hold-up volume. Furthermore, the single flow has further advantages over the T-tube arrangement in the saving of
ANALYTICAL CHEMISTRY
212
Table 11.
Dye Orange I1 Direct titration Indirect titration Tartrazine, indirect titration p-bminoazobenaene HC1 paste, indirect titration Methyl violet. indirect titration
Titration of Dyes Titanous Ion Coulometric TitrAtion % No. of Standard dye detns. deviation 88.70 88.96 80.60 44.4
77.7
4 4 5 1
7
Titration
wgt&::z Chloride, % Dye
0.17 0.20 0.23
0: 58
88.1 88.5 79.5 41.4
..
and Nagel ( 4 ) state that the function of the buffer (buffer catalyst) is not only to regulate the hydrogen ion concentration, but also to promote the reduction of the dye. The rate of reaction of titanous ion with the dye may be somewhat slower by the coulometric titration, as the titanic ion is about 20 times the titanous ion added-for example, the rate of reaction for the direct titration of tartrazine was slowed so that it was necessary to employ the indirect end point procedure. The titration of aminoazobenzene (oil-soluble) in alcohol-water solution was satisfactory; this dye is normally run by the indirect procedure. ACKNOWLEDGMENT
reagent, and the electric field can be evenly distributed directly above the mercury pool. Titration of Dyes. Results of the coulometric titration of several (technical grade) dyes are shown in Table I1 along with the corresponding results obtained by ordinary standard titanous chloride titration (6) for some of these. A current of 260 ma, was used in order to obtain a 0.25-second visual sensitivity for the Orange I1 direct end point. Sample sizes were taken such that the titration required about 200 seconds. It was important to limit the time of titration, as the addition of too much titanium tetrachloride solution caused the titanium dioxide to precipitate, thus obscuring the end point. The values in Table 11indicate the kind of precision which can be obtained by the coulometric titration. In addition, there are given some values for these samples which were obtained in the authors’ Control Laboratories under normal control laboratory conditions by titration with standard titanous chloride. I n view of the generally accepted estimate of the accuracy and reproducibility of such determinations as given, for example, in Siggia ( 9 ) , it may be concluded that there is no serious disagreement between the two methods. The large excess of titanium chloride added by the coulometric method may have some effect on the buffer action. Evenson
The authors wish to express their appreciation to S . Howell Furman, Princeton University, and to G. L. Royer, for their interest and many helpful suggestions; to D. D. De Ford, Northwestern University, for prepublication information on the single arm cell design ( 6 ) ; and to G. E. Gerhardt and H. C. Lawrence, who constructed and assembled the electrical equipment. LITERATURE CITED
(1) Arthur, P., and Donahue, J. F., ANAL.CHEM., 24, 1612 (1952). (2) Cooke, W. D., and Furman, N. H., Ibid., 22, 896 (1950). (3) De Ford, D. D., Pitts, J. N., and Johns, C. J., Ibid., 23, 938, 941 (1951). (4) Evenson, 0. L., and Nagel, R. H., IND. EXG.CHEM., A s 4 r . . ED., 3, 167 (1931). (5) Knecht, E., and Hibbert, E., “New Reduction Methods in Volumetric Analysis,” Longmans, Green & Co., S e w York, 1925. (6) Pitts, J. N., De Ford, D. D., Martin, T. ’A‘., and Schmall, E. A , , ANAL.CHEM., 26, 628 (1954). (7) Reilley, C. N., Adanis, R. N.. and Furman. S . H., Ihid., 24, 1044 (1952). (8) Seaman, W., and .411en, W., Sewage and Ind. Wastes, 22, 912 (1950). (9) Siggia, S., ‘Quantitative Organic Analysis via Functional Groups,” p. 84, John Wiley & Sons, Sew York, 1949.
RECEIVED for review July 30, 1954. Accepted October 14, 1954.
Titration of Acids in Dimethylformamide Using High Frequency JOHN A. DEAN and CARL CAIN, JR. Department of Chemistry, University of Tennessee, Knoxville, Tenn.
The purpose of the investigation was to determine the applicability of a high frequency oscillator to the titration of acids in dimethylformamide. Sharp V-shaped titration curves are obtained for strong acids and the ammonium ion over the complete range of the sensitivity of the instrument which includes concentrations as small as 0.0001M. Less acute angles are obtained in the titration of acids of intermediate strength, although generally adequate end point discrimination is possible for acids whose pK, values do not exceed 7 in water. The high frequency method should be a useful adjunct for titrations in dimethylformamide, particularly as only a very limited number of color indicators are available at present.
I
T W-kS the purpose of the present investigation to determine the applicability of a high frequency oscillometer to the titration of acids in dimethylformamide. Wagner and Kauffman (6) have reported a successful application of high frequency titrations to organic bases in glacial acetic acid, and Ishidate and 1Zasui (3) have titrated a number of organic acids in a benzenemethanol mixture by high frequency. Dimethylformamide has been used as a titration medium for
a number of weak organic acids. Fritz ( 2 ) has recommended the use of the solvent for the titration of enols and imides and negatively substituted phenols. Vespe and Fritz ( 5 ) have determined many of the sulfa drugs as acids by titration in dimethylformamide. Fritz (1) has also suggested a procedure for the determination of salts of strong bases-i.e., their conjugate acids-in dimethylformamide. The lack of a variety of suitable visual indicators, or adequate electrode systems for potentiometric titrations, is a major limitation to the use of dimethylformamide. The range of acid strengths which may be successfully titrated in the solvent has only been qualitatively estimated. . APPARATUS 4ND RE4GEYT3
The high frequency measurements were made R ith the Sargent Chemical oscillometer, Model V. The frequency of the instrument was approximately 4.89 megacycles. hfter the addition of each titration increment the instrument was brought back into resonance by adjusting appropriate capacitances in parallel with the titration cell ( 4 ) . All subsequent references to capacitance readings or measurements in this paper refer t o the high frequency capacitances as measured by this instrument. The titrations were carried out in a standard cell supplied with the instrument. The annular space hetween the condenser
