Jan., 19513 BELCHER AND STEPHEN 45 The Determination of Small Amounts of Hydroquinone BY R. BELCHER AND W. I. STEPHEN A method is described for the determination of small amounts of hydroquinone in the presence of methacrylic acid. The solution con- taining hydroquinone is allowed to react with an excess of ferric ammonium sulphate and the ferrous iron produced is treated with o-phenanthroline and determined absorptiometrically. The interference of the ferric iron un- consumed in the reaction is avoided by conversion to the ferrifluoride ion. HYDROQUINONE is used as an inhibitor in the polymerisation of methacrylic acid, and it is often necessary to determine the amounts present at various stages of the reaction. None of the methods available for the determination of hydroquinone was sufficiently sensitive to determine accurately the small amounts present under our conditions (1 to 200 pg), and it became necessary to find an entirely new method.Several reactions are known whereby various substances are determined colorimetrically after treatment with hydroquinone ; hence it seemed possible that by suitable modification, the same reactions might be exploited for the determination of hydroquinone itself. Hydroquinone has been used to determine the following substances colorimetrically : silver,l copper2 and nitritea3 It has also been used to reduce ferric iron to the ferrous state before determining iron colorimetrically by its reaction with o-phenanthr~line.~ All these reactions were examined in turn, but only the last proved to be suitable for the determination of hydroquinone.Excess of a ferric salt is added to the solution containing hydroquinone. The ferric salt is reduced,with the production of an amount of ferrous iron equivalent to the amount of hydroquinone originally present. o-Phenanthroline is added, and the amount of ferrous iron is determined colorimetrically after suppressing the colour due to the excess ferric iron by addition of a suitable complexing agent. A visual comparison method based on this principle was first developed, and a brief note on its use has appeared elsewhere.6 More recently another method has been described for the determina- tion of similar amounts of hydroquinone based on the formation of molybdenum blue by reduction of a solution containing phosphotungstate.s The present method uses a Spekker absorptiometer for the measurement of the colour intensity and is, accordingly, much more accurate than the visual comparison method.THE DETERMINATION OF FERROUS IRON- As the first step in the development of the method, solutions containing ferrous iron were used to find the optimum conditions for maximum colour development. Graphs were prepared, by using 0.5-cmJ 1-cm and 2-cm cuvettes, relating the amount of ferrous iron present to the drum reading. An Ilford spectrum blue filter No. 602 was used, with the instrument set at 1-0 against water. It was found that a period of 30 minutes was required a t a pH between 3 and 4 for the maximum colour development. The correct pH range was obtained by adding suitable amounts of sodium acetate solution.A straight line relationship between the amount of ferrous iron and the drum reading was found under these conditions (Figs. 1, 2 and 3). THE EFFECT OF FERRIC IRON- Since an excess of ferric iron would necessarily be present in an actual determination, it would be expected to interfere unless its effect could be eliminated in some way. Suitable amounts were added to solutions containing known amounts of ferrous iron, and the latter was then determined. Although the interference was not marked, it was sufficient to affect the accuracy of the determination. However, it was found that the interference could be eliminated by complexing the ferric iron by the addition of ammonium fluoride. THE DETERMINATION OF HYDROQUINONE- Known amounts of hydroquinone were allowed to react with ferric iron for various times, the solution then being treated with o-phenanthroline and the intensity of colour The basis of the reaction is as follows.46 BELCHER AND STEPHEN : THE DETERMINATION OF [vol.76 50 30 HY DUOQUINONE.Pg Fig. 1. Graph obtained with 2-cm cuvette HYDROQUINONE,pg Fig. 2. Graph obtained with 1-cm cuvetteJan., 19511 SMALL AMOUNTS OF HYDROQUISONE 47 measured after complexing the excess ferric iron with ammonium fluoride. A reaction time of 30 minutes was found necessary for complete oxidation of the hydroquinone by the ferric iron. Under these conditions the reaction was found to proceed quantitatively, and hence it was possible to prepare the standard graphs by using a solution of ferrous ammonium HY DROQUlNONE~g Fig.3. Graph obtained with 0-5-cm cuvette sulphate instead of a solution of hydroquinone. The equivalent amount of hydroquinone is calculated on the basis that 2 molecules of ferrous ammonium sulphate are equivalent to 1 molecule of hydroquinone. Some results are shown in Table I and indicate that the method is sufficiently accurate for a determination of this type. TABLE I THE DETERMINATION OF HYDROQUINONE IN THE ABSENCE OF METHACRYLIC ACID Hydroquinone present, CLg 275 2 20 165 110 55 44 33 22 11 6 3 2 1 Hydroquinone found, Pg 272, 273 222, 221 164 112, 110 56 45 33 23 11 5 3 2 1 THE DETERMINATION OF HYDROQUINONE IN THE PRESENCE OF METHACRYLIC ACID- The amount of methacrylic acid likely to be present in the solutions to be analysed was of the order of 2 to 3 ml of a 1 per cent. solution.Three-millilitre quantities of a 1 per cent. solution were added to various known amounts of hydroquinone and the latter was then determined. In every case a control determination was carried out in which the same48 BELCHER AND STEPHEN : ‘THE DETERMINATION OF [Vol. 76 amount of hydroquinone was determined withlout the addition of methacrylic acid. From the results included in Table I1 it can be seen that the methacrylic acid has no effect on the determination. TABLE I1 THE DETERMINATION OF HYDROQUINONE IN THE PRESENCE OF 3 M L OF 1 PER CENT. METHACRYLIC ACID SOLUTION Hydroquinone found Hydroquinone present, CLg 220 165 110 55 33 17 6 3 1 r Methacrylic acid absent, Pg 220 164 110 545 33 16 (i 3 1 3 Methacrylic acid present, Pg 221 167 110 55 32 17 7 4 2 Some further tests were also carried out .with much stronger solutions of methacrylic acid, 3 ml of an 8-6 per cent.solution being added. The results are included in Table I11 and show that the accuracy of the determination is not impaired by the presence of fairly large amounts of methacrylic acid. TABLE I11 THE DETERMINATION OF HYDROQUINONE I N THE PRESENCE OF 3 M L OF 8.6 PER CENT. METHACRYLIC ACID SOLUTION Hydroquinone found A 7 \ Hydroquinone Methacrylic acid Methacrylic acid present, absent, present, Pg Pg tLg 220 220 220 55 5 8 57 6 6 7 4 4 5 PROCEDURE FOR THE DETERMINATION OF HYDROQUINONE REAGENTS- sulphuric acid per litre. Ferric ammonium sulphate-A 0.001 M solution containing 4 ml of 50 per cent. v/v Ammonium JEuoride-A 0-1 M solution.Sodium acetate-A 1 M solution. o-Phenanthroline-A 0.1 per cent. aqueous, solution. PROCEDURE- Transfer 5 ml of the solution to be analysed to a 25-ml graduated flask and add 5 ml of 0.001 M ferric ammonium sulphate. Add sufficient of the 1 M sodium acetate solution to bring the pH to 3.5 to 4. The appropriate amount should be determined beforehand by adding bromophenol blue indicator to separate quantities of ferric ammonium sulphate and hydroquinone solution, and noting the minimum amount required to give a blue colour to the solution. Shake the flask to mix the solutions thoroughly and allow to stand for 30 minutes. Add 5 ml of 0.1 per cent. o-phenanthroline and after 5 minutes add 1 ml of 0.1 M ammonium fluoride solution and dilute the solution with water to the mark.It is essential to add the reagents in this order. Stand the solutions for 30 minutes and read the absorption on the Spekker absorptiometer with Ilford spectrum No. 602 filters and a setting of 1.0 to water. PREPARATION OF THE GRAPH- Prepare solutions of ferrous ammonium sulphate of the concentration required by appropriate dilution of a 0.2 M solution containing 4 ml of 50 per cent. v/v sulphuric acidJan., 19511 SMALL AMOUNTS OF HYDROQUINONE 49 per litre. Maintain the same acid concentration in the diluted solutions. Transfer suitable amounts to a 25-ml graduated flask. Adjust the pH as described above, add 5 ml of 0.1 per cent. o-phenanthroline and dilute the solution to the mark. Allow the solutions to stand for 30 minutes and then take the absorptiometer reading as before. Convert the amounts of ferrous iron present to the equivalent amounts of hydroquinone and prepare graphs relating concentration to drum reading. Prepare graphs for the 0.5-cm, the l*O-cm and the 2.0-cm cuvet tes. Thanks are due to Mr. T. S. West for independently carrying out a large number of check determinations. REFERENCES 1. 2. 3. 4. 5. 6. Costeanu, R. N., Mikrochemze, 1939, 26, 170. Muller, R. H., and Burtsell, A. T., Ibid, 1940, 28, 209. Vagi, S., 2. anal. Chem., 1925, 66, 101. Willard, H. H., and Hummel, F. C . , I n d . Eng. Chem., Anal. Ed., 1938, 10, 13. Stephen, T?'. I., Metallurgia, 1948, 37, 333. Whettem, S. M. A., Analyst, 1949, 74, 485. DEPARTMENT OF CHEMISTRY THE UNIVERSITY BIRMINGHAM May, 1950