Coulometric Titration of p-Methylaminophenol Sulfate and Hydroquinone by Cerium(IV) N. HOWELL FURMAN
AND RALPH
N. ADAMS
Princeton University, Princeton, N . J .
HE
primary objective of this study was to develop microgram
Ttitration procedures for several organic compounds based on
coulometric cerate oxidimetry. Organic oxidations with electrolytically generated Ce( IV) have not been previously reported. The development of these titrations for hydroquinone and pmethylaminophenol sulfate (metol) clearly defined some important limitations of the sensitive end-point procedure of Cooke, Reilley, and Furman (6). Practically all of the common electrometric end-point methods h a w been employed in coulometric titrations ( 2 , 4, 6, 8, 12). Carson's recent work on uranium ( 3 )has proved the adaptability of the derivative polarographic technique developed by Reilley el al. (13). While experience in this laboratory has shown that both the potentiometric and the derivative polarographic methods may be used in fairly low microgram ranges, it is believed that these methods have more limitations in the range from 107 down than the sensitive end-point procedure, The latter involves restoration of a predetermined e.m.f. of the indicator cell through amperometric observation; this can be done with high precision. Ordinary amperometric techniques require precise control of the voltage impressed on the indicator electrode (6). Of all the methods available for end-point detection, the sensitive end-point procedure appears to be the most useful and accurate for microgram coulometric titrations (6, 7 ) . The sensitive end point employs a high sensitivity amperometric circuit to indicate potential differences between the solution and a platinum indicator electrode. The indicator electrode is fixed a t a preset potential. This potential is determined by the potentiometric end-point characteristics of the titration that is being studied, In ovidimetric titrations with Ce(IV), this preset potential is necessarily higher than the solution potential of the Ce(II1) generating medium. Current flows in the indirator circuit due to anodic osidation of Ce(II1) at the indicator electrode. To make the system ready for titration, the solution potential is raised to the indicator potential by generating Ce(1V). When the solution and indicator potentials are equal, no current flows in the indicator circuit. The sample of reductant is then added. The solution potential sharply decreases and is then brought back up to the preset potential by generating Ce(1V) a t constant current. When zero current is reached again in the indicating circuit, the time of generation is a measure of the reductant added. In order that this latter time of generation be a true indication of the amount of reductant added, there must be no potential changes in the solution other than those occasioned by the addition of the reductant. Two such interfering potential changes were encountered in this study. The first was due to a slight instability of very dilute solutions of Ce(1V) in sulfuric acid medium. After generating to the preset potential, a very slight drift toward lower solution potentials was observed. The magnitude of the error introduced by this drift was small. It can be completely eliminated by the addition of a small quantity of phosphoric acid to the Ce(II1)-sulfuric acid generating solution. The other interfering potential change was a far more serious one and limited the accuracy with which hydroquinone and p methylaminophenol sulfate can be titrated coulometrically. This is a drift toward lower solution potentials brought about by the oxidation products of the organic compounds. After the completion of a hydroquinone titration, the solution potential drifts considerably toward more negative values. This process occurp throughout the course of the titration. Thus, more Ce(1V)
than is equivalent to the hydroquinone must be generated to bring the solution back to the preset potential. While positive errors are usually obtained, microgram quantities of hydroquinone can be titrated quite accurately. Several precautions are indicated in later sections. p-Methylaminophenol sulfate is a typical photographic developing agent, While hydroquinone can be titrated volumetrically in the 0.01 iV region with no evidence of fleeting end points, p methylaminophenol sulfate titrated under these conditions shows falling potentials and very fleeting visual end points with ferroin and Erioglaucine. It is not surprising that similar behavior is found in the microgram coulometric titrations, In this case, the overtitration can be definitely traced to consumption of Ce(1V) by the primary oxidation products of the aminophenol. As originally described, the sensitive end-point procedure allowed successive titrations in the same solution, provided the previous sample had been titrated exactly to the end point (6). This procedure cannot be used with hydroquinone and p-methylaminophenol sulfate. The drifting potentials developed by products of the previous titration cause large positive errors in a subsequent titration. This necessary modification of the original method is not a serious disadvantage. A further precaution is necessary in adjusting the solution to the preset indicator potential. In cases where the preset potential was overrun by generating Ce(IV), it became common practice to adjust back with a very dilute solution of the reductant. Use of dilute hydroquinone or p-methylaminophenol sulfate is not permissible for this purpose; the solution becomes unstable and a drift develops before the titration can be started. All adjustments prior to the actual titration should be made with a very dilute solution of ferrous sulfate. APPARATUS
The titration cell used for all work was a 25-ml. beaker yith the lip cut off at the top. No temperature control was provided other than a thin sheet of asbestos board between the cell and the magnetic stirrer top. The electrical circuit was identical with that described by Cooke et al. ( 7 ) . The timer was an inexpensive synchronous motor clock (Dimco-Gray Co., Model 201); The indicator electrode was a 1-sq.-cm. platinum-iridium foil. The reference electrode was a saturated lead amalgam-lead SUIfate-2 A f sulfuric acid electrode (1). The capillary-tipped side arm of this electrode was plugged with filter paper wet with the sulfuric acid solution. If the filter paper is properly tamped in place, these side arms provide a low resistance, nonleaking junction and appear easier to prepare than the more complicated agar or sintered glass connections. Electrodes of this type have been in continuous use for over a year without renewal of the paper plug. The generator anode was of platinum-iridium foil, about 2 sq. cm. The shielded cathode compartment was a ground glass construction made from soft glass tubing. This was filled with 2 M sulfuric acid, the same acid concentration present in the titration cell. to minimize any solution potential changes due to leakage. The cathode was small p l a h u m wire.
a
REAGENTS
The generating medium was a saturated solution of cerous sulfate in 2 M sulfuric acid, prepared from reagent grade cerous sulfate from the G. F. Smith Chemical Co. This generating medium was usually prepared in batches of 100 ml., made by adding 10 ml. of 85% phosphoric acid to 90 ml. of the saturated ceroua sulfate solution. The stock hydroquinone solution was 0.01 A'. It was repared by dissolving the appropriate weight of reagent crystays in airfree distilled water. A few drops of 2 M sulfuric acjd were added per liter of solution to stabilize this preparation against air ouida1564
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3
1565
tion. I t was standar3i:ed each morning by titration with standard ceric sulfate solution, using Alphazurine G as visual indicator (14) or Erioglaucine (9). Such a solution decreased in ceric titer only slightly-about 0.1% per day. Tenfold dilution of this sto;k gave a solution used for the very small samples of hydroquinone. Stock metol solution, 0.01 N , was prepared in a similar fashion from the crystalline p-methylaminophenol sulfate salt. The addition of a few drops of 2 M sulfuric acid stabilized this solution consilerably. Without this added acid, the solution turned red overnight, even with thoroughly deaerated water. This solution was standardized each morning by ceric sulfate titration, with the viwal end-point difficulties previously mentioned. PROCEDURE
About 20 ml. of the cerous solution were placed in the titration cell. Sitrogen was bubbled through the sdution for 2 t o 3 minutes. The gas inlet tube was then removed from the titration cell. The sdution potential was then raised t o the preset indicator potential by generating Ce(1V). If overrun, the required adjustment w a g made with very dilute ferrous sulfate solution. After adjustment, the unknown sample of hydroquinone or pmethylaminophenol sulfate was added by means of a 1-ml. hjpodermic syringe with an attached capillary tip. This was used as a small weight buret.
Table I.
Determination of Hydroquinone
Hydroquinone, y Taken Found ~~~~
203.8
289.5 249.0 228.5 202.6
133.9 125.4 123.7 101.5
133.8 125.6 125.4 100.9
Error, +1.4 -1.2 +0.4 -1.2 -0.1 f0.2 +1.7 -0.6
87.9 63.8 12.5
88.6 62.9 12.1
+O. 7 -0.9 -0.4
288.1 250.2 228 1
2.33 2.17 1.28
2.38 2.87 1.31
-,
Error, % +O. 5
App. Gen. Current, (ma.)
galvanometer, this is the only elaborate piece of electrical equipment needed. Since the galvanometer is essentially used only as a null-point instrument, the actual diffusion currents need not be measured. The shunt may be an inexpensive resistance box or a breadboard model constructed of available resistors. All the generating current required for microgram titrations can be drawn from a voltage supply of five 45-volt radio B batteries. The generating current is measured, as usual, as an I R drop across a standard resistance. While automatic coulometric titration equipment such as described hy Carson (3) and Deford e t al. (8) are very useful and may be essential for work with radioactive samples, ordinary coulometric microtitrations can be performed with relatively simple equipment.
Table 11. Determination of Metol (p-Methglaminophenol Sulfate) hfetol, y 110.4
112.5
67.8 38.4 27.1 10.7
68.4 39.2 27.7 11.2
8.97 4.46
Error. (7c
.4pp. Gen. Current. (ma.)
+2.1
+1.4
0.5
+O. 6 fO. 8
+O. 4 +2.1 f2.2 +4.7
0 . or,
-0.19 +0.47
-2.1
~~~
Found
Error, y
fO. 6 +O. 5
8.78 4.93
+10
1.0
-0.5
+o. 2 -0.6
-0.1 CO.2 +1.4
0.5
-0.6
+0.8 .
+0.0.5 +o. 10 + O . 03
-1.4 -3.2
+2.?
0.05
+3. D +2.3
The clock was reset to zero and Ce(1V) was generated again until zero current was indicated in the galvanometer circuit. The galvanometer was kept a t a fixed sensitivity a t all times. The full sensitivity was never used. It was switched into the indicator circuit only when required t o take readings. The indicator voltage chosen for both titrations was 1.00 volt vs. hydrogen. Titrations were run in the case of hydroquinone at indicator vdtages of 0.05 to 1.05 volts with the same results.
The results of this study indicate that organic compounds which can be directly titrated with Ce(1V) on the macro scale can be titrated coulometrically in microgram quantities. Considering the lack of temperature control and the probable 1% weighing error involved, the results for 1 to 27 of hydroquinone are quite sztisfactory. Attempts to titrate resorc~inoland catechol met with no success. The drifts with these compounds were too rapid and large t o permit reproducible resultfi. With p-methylaminophenol sulfate, the results were considerably less accurate than for hydroquinone. I t would be expected that other similar photographic developers could be determined. The accuracy of each individual determination would depend to a large eytent on the labile nature of the primary oxidation products. ACKNOWLEDGMENT
The authors gratefully acknowledge the aid of a fellowship from the A411iedChemical and Dye Corp.
RESULTS
The results for hydroquinone are indicated in Table 1. The determinations of p-methylaminophenol sulfate are listed in Ta1)le 11. Only samples of 1007 or less were determined for the lattcr compound. DISCUSSION
Ceric sulfate is considered the most satisfactory oxidizing agent for microgram osidimetric titrations ( I O ) . However, dilute solutions prepared for microgram titrations are difficult to prepare and sometimes give spurious results ( I f ). Coulometric titrations with electrolytically generated Ce(1V) should provide an ideal answer to these difficulties. In order to be attractive t o the analyst, coulometric procedures must be relatively simple in operation and require a minimum of elaborate electrical equipment. The sensitive end-point procedure is not a complex technique. The determinations in this study were purposely carried out without temperature control and the general design of the titration apparatus was made as simple as possible. While it is true that the sensitive end-point method requires a high sensitivity
LITER4TURE CITED
.tdams, R. N., Reilley, C. N . , and Furman, K. H., ASAL. CHEM.,25, 1160 (1953). Carson, W. N., Ibid., 22, 1565 (1950). Carson, W. N., Ibid., 25, 226 (1953). Carson, W.N., and KO,R., Ibid., 23, 1019 (1951). Cooke, W.D., and Furman, K.H., Ibid., 22, 896 (1950). Cooke, W. D., Reilley, C. N., and Furman, K. H., Ibid., 23, 1662 (1951). Cooke, W. D., Reilley, C. S . ,and Furman, N. H., Ihid., 24, 205 (1 952). Deford, D. D., Johns, C. J., and Pitts, J . K.,Ihid., 23, 941 (1951). Furman, &’. H., and Wallace, J. H., J . .4m. Chem. Soc., 52, 1443 (1930). Kirk, P. L., “Quantitative Ultramicroannlysis,” p . 127, New York, John Wiley & Sons, 1950. Ibid., P. 129-30. lleyers, R. J., and Swift, E. H.. J . A m . Chem. Soc., 70, 1047 (1 948). Reillev, C. K.,Cooke, W. D., and Furman, N. H., ANAL. CHEM.,23, 1223 (1951). Whitmoyer, R. R., IND.ESG.CHEM.,ANAL.ED.,6 , 268 (1934). RECEIVEDfor review April 1 5 , 1953.
Accepted July 8, 1953,
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