An Experiment i& Colorimetry and Reaction Kinetics for Phvsical Chemistrv RICHARD WISTAR and LYDIA NELSON' Mills College, Oakland, California
EXPERIMENT described below offers a proTHE cedure for studying reaction kinetics which is exceedmgly simple yet suitable for an elementary course in physical chemistry. No expensive or special equipment is needed, the reagent-quality chemicals used are readily available, and the reaction is conveniently rapid. In the concentration ratios recommended, diazotized sulfanilic acid couples with 1-naphthylamine-4-sulfonic acid to form an amber-colored solution. The reaction can be followed colorimetrically to 77 per cent completion in approximately 20 minutes a t 20°C. No thermostat need be used for a reaction as rapid as this, and the colorimetric comparison with standards is made by looking through the solutions in regular 200 X 25mm. test tubes. In comparing the standards with the reacting mixture it is easier if two standards of nearly the same concentration are used, correspondingto 18per cent and 20 per cent complete reaction, for instance. When the color in the reaction tube is darker than the first and lighter than the second, it is recorded as 19 per cent reacted. This method has proved more successfnl than trying to match a certain standard. The reaction involved is summarized by the following equation:
As long as the two reactants are present in equal concentrations, or when the naphthylamine sulfouate is in excess, the reaction is kinetically of the second order and can be followed to 80 per cent completion with an accuracy of about *2 per cent. The reaction mixture is buffered a t pH 7 with 0.2 M ammonium acetate. It is not necessary to measure the pH because the rate of the coupling reaction is not affected by changing hydrogen ion concentration above pH 3.5.l In soluL P r e s e n t address: Shell Development Company, San Francisco, California. 1W I AND~ BARTLETT, "Kinetics and mechanism of the in hufIer coupling of diazonium salts with aromatic solutions." J. Am. Ckem. Soc., 63, 414 (1941).
tions more acid than pH 3.5, the following change
begins to take place to an appreciable extent and the coupling reaction is slowed down. The directions for carrying out the experiment follow. Object: The determination of the reaction velocity constant of a second order reaction. Apparatus: Three test. tube racks; iiiteen %inch test tubes; eight pipets: one I-ml., two 5-ml., two 10ml., one 25-ml., one 50-ml., and one 10-ml. graduated in 0.1 ml.; three volumetric flasks: one 1-liter, one 500-ml., and one 50-ml.; one 10-ml. buret; stop watch; thermometer; stirring rod; box of weights; analytical balance; weighing bottle; desiccator. To obtain &inch test tubes of the same internal diameter, pipet 50 ml. of water into several and choose those in which the level is the same. For introduction of the naphthionic acid solution, the 50-ml. pipet should be calibrated to deliver 49.0 ml., and a file mark should be made a t the proper place. Reagents: Snlfanilic acid, pnitraniline, sodium nitrite, ammonium acetate, hydrochloric acid, and lnaphthylamine-4-snlfonic acid. Reagent-grade chemicals should need no further purification. The naphthionic acid obtained from the Eastman Kodak Company is of satisfactory purity. Prefiaration of Standard Solutions: The sulfanilic acid is dried to constant weight a t 110". A 0.1 M solution is prepared by dissolving 8.65 g. (0.05 mols) in some water containing 5 ml. of concentrated ammonia. This solution is diluted to 500 ml. in a volumetric flask. To prepare the diazonium solution, 10 ml. of this 0.1 M sulfanilic acid is pipetted into a 50-ml. Erlenmeyer flask. To this is added from a pipet 5 ml. of 0.8 M hydrochloric acid. The mixture is cooled in an ice-water bath for 10 minutes to brine the temoerature below 5OC. Using a 10-ml. buret, 3 ml. of 6.2 M sodium nitrite solution is slowly dropped in while the flask is gently rotated. The diazotization is allowed to continue at the temperature of the ice bath with fre-
quent agitation for 30 minutes. Then 20 ml. of distilled water is added. The solution is now 0.025 M in diazonium ion. A pipet is used to transfer 10 ml. of this solution to a 50-ml. volumetric flask. When diluted to the mark, this gives a solution which is 0.005 M in diazonium ion. One ml. of this solution, when added to the reaction mixture, is diluted to 50 ml., and the concentration of diazonium ion is then 0.0001 M. The 0.2 M sodium nitrite must he standardized, and this is conveniently done by diazotizing weighed quantities of pure p-nitraniline. An approximately 0.2 M solution is made up by dissolving 14 g. of c. p. sodium nitrite in water and diluting to 1 liter. Several 0.3-g. portions of p-nitraniline are weighed into 50-ml. Erlenmeyer flasks. To each, 2 ml. of 6 N hydrochloric acid and 10 ml. of water are added. The flasks are warmed until everything is dissolved, then cooled by the addition of small pieces of ice. The solutions are titrated a t 15°C. by running in the sodium nitrite solution from a buret, using potassium-iodide starch paper, and waiting about 20 seconds before testing near the end point. The instantaneous appearance of a blue color on the test paper indicates an excess of nitrite. It is convenient to dilute the nitrite solution to exactly 0.2 M. The ammonium acetate buffer is prepared by dissolving 15.4 g. of ammonium acetate in water and diluting to 1 liter, making a 0.2 Msolution. The naphthionic acid is dried to constant weight over sulfuric acid in a desiccator. A 0.057-g. sample of the dried product is dissolved and diluted to a volume of 500 ml. using the b&er solution. This solution is 0.00051 M in naphthionic acid and when 1 ml. of the 0.005 M diazoninm solution is added to 49 ml. of the naphthionic acid solution the resulting mixture is 0.0005 M iq naphthionic acid and 0.0001 M in diazonium ion. Procedure: This experiment covers two laboratory periods. During the first one, prepare the various solutions as directed above, and then prepare a standard reaction mixture as follows: Place five 49-ml. portions of the 0.00051 M naphthionic acid solution in a clean, dry 250.~1.&lenmeyer flask and to i t add from a pipet 5 ml. of the 0.005 M diazotized sulfanilic acid solution. This will reach substantially 100 per cent reaction overnight and will keep for several days. The following laboratory period (no more than a week later), standards of various percentages are pre~ a r e din the chosen test tubes. P i ~ e t s are used ihroughout to measure the desired amoints of both the completely coupled reaction solution and the buffer which is used for dilution. In 10 clean dry test tubes make up standards as in Table 1. Thoroughly mix each solution with a stirring rod and arrange consecutively in test tube racks. Timing the Coupling Reaction: Pipet 49 ml. of the 0.00051 M naphthionic acid solution into a clean, dry
P I ~ C I ~ I ~ ~ CMi
of Couplad Solwion 9
18 20 38 40 46 50 56 60
41 40 31 30 27 25 22 20 13 10
10 1-3 20 23 25 28 30 37 40
74
80
MI.of Buffir
TBLE 1
8-inch test tube. Using a dry 1-ml. pipet, withdraw 1 ml. of freshly prepared 0.005 M diazotized sulfanilic acid solution. Blow this into the naphthionic acid solution and start the stop watch at the instant the diazonium ion is introduced. Mix the reactants thoroughly by pouring the solution into another clean, dry test tube of the same size. Record the temperature a t the beginning, and again a t the end of the experiment. I t should not vary by more than 0.5'C. Place the reaction tube between the first pair of standards and record the time to the nearest second when the color is darker than the 18 per cent reacted standard and lighter than the 20 per cent reacted one. This is recorded as the time for 19 per cent reaction. I t helps to elevate the rack and back it up with a white or light colored wall. The comparison is made by looking across the diameter of the tube, not down its perpendicular axis. In a similar fashion, record the time for 39 per cent, 48 per cent, 58 per cent, and 77 per cent reaction by comparison with the proper pairs of standards. The timing run should be carried out two or three times. Treatment of Results: The determination of the velocity constant, K,is done graphically. For a semndorder reaction the momentary concentrations of the reacting substances A and B are expressed by the equation
Z
= b(a-x)(b-2)
in which a = initial concentration of A , b = initial concentration of B, x = amount reacted, t = time in minutes. This equation is used when a > b. Upon integrating and evaluating the integration constant by setting x = 0 when 1 = 0, this becomes
K=-
2.3 t(e - b)
b(a - z) m
log a
2.3 b(a - x ) . The expression w ~ t hthe concena - blogaib - x) trations expressed in mols per liter is plotted against t in minutes. The best straight line is determined by inspection and is drawn from the point of origin through the points on the graph. Stretching a piece of thread across the graph is helpful in determining the best straight line. K is calculated by solving for the slope of the line. The value of K is approximately 150 at 25°C.
