The Rate of Oxidation of Hydriodic Acid by Sulphuric Acid Citation Tracy, Willard Harmon (1924) The Rate of Oxidation of Hydriodic Acid by Sulphuric Acid. Bachelor's thesis, California Institute of Technology. doi:10.7907/MSN0-6E90. https://resolver.caltech.edu/CaltechETD:etd-07012004-111626 Abstract No Abstract. Item Type: Thesis (Bachelor's thesis) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: None, None Defense Date: 1 January 1924 Additional Information: Authors's name in 1924 Commencement program is listed as William Harmon Tracy. Record Number: CaltechETD:etd-07012004-111626 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-07012004-111626 DOI: 10.7907/MSN0-6E90 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2798 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 02 Jul 2004 Last Modified: 03 Oct 2019 23:20 Thesis Files Preview PDF (Tracy_wh_1924.pdf) - Final Version See Usage Policy. 665kB Repository Staff Only: item control page

_ THE RATE OF OXIDATION OF HYDRIODIC ACID. BY SULPHURIC ACID Thesis by Willard H. Tracy In partial fulfillment of the requirements for the degree of Bachelor of Science in Chemistry CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 1924 The Rate of Oxidation of Hydriedie Acid by Sulphuric Acid. ‘At the suggestion of Dr. D.f. Smith a determination of the rate at which hydriodic acid is exidized by relatively concentrated solutions of sulphuric acid, with the forma- tion of free iodine and sulfur, was attempted. The pur- pose of the investigation was to ascertain whether er not the rate of this oxidation was substantially proportional _to the activity ef the sulphuric acid in the solution, at & constant concentration of hydriodic acid, since the ac- tivities of sulphuric ecid solutions of various concentra- tions at 4 temperature of 80°C. have recently been calenl- ated by Smith and Mayer. ( J. Am. Chem. Soc. 46, 79-80 (1924).) The course of the reaction was to be follewed by titration of the liberated iodine. ‘This investigation has hud to be discontinued: the work alresdy accomplished is reported in this paper. | Method and Apparatus The work of Smith and dayer above referred to had shown that the reaction oceurred with measurable velocity in ite initial stages at a temperature of 80°S. This temperature was accordingly fixed upen, and a water thermostat covered with a layer of “Transil O11" to reduce evaporation was adjusted to maintain this temperature to within .02°, as indicated on a thermometer graduated in .1° and calibratea by the Bureau of Standards. 48 hydriodic acid is rapidly oxidized by atmospheric oxygen at this temperature, an all-glass apparatus was de- signed which permitted of carrying out the reaction in an atmosphere of nitrogen. This apparatus was constructed of Pyrex glass according to the fello.ing diagran. 2 ~-—) Th Te UP (\ | t) a _ bath 4 — | level = ) ie : a The apparatus was supported in the thermostat with the bath level at the point indicated. Nitrogen, previously passed through the glass bead saturator I filled with a solution of sulphuric acid prepared to approximate the - -vaper pressure of the hydriedic acid solution used, was passed into the apparatus threugh the stopeecks A and B, and allowed to escape through tubes C and D and around £. After having swept out the air from the bulbs and connect~ ing tubes, mercury was poured into the mereury seal E of the stirrer *. The stream of nitrogen was reduced in a- "mount ana the golutions were weighed into the bulbe G and a by means of weighing pipettes provided with long, narrow tips, capable of extending into the bulbs. the hya@riodie acid pipette carried a glass steppered side tube and a | stopcock on the top tube. In filling it with hydriodic acid a stream of nitrogen was first passed through the bulb, after which acid was allowed to run inte the pipette from the stock receptacle through the unstoppered side tube from which a current of nitrogen was permitted to escape, thus preventing the influx of air. The hydriodic acid was placed in G and the sulphuric acid in H. The stopcocks A and B were then closed and the ground glass stoppers at © and D inserted, after which the solutions were allowed to come to the temperature of the bath. the glase propeller stirrer F, cennected direetly to the shaft of a small motor by means of a short length of rubber tubing, was then start- ed. The stepper C was removed, and, at an accurately noted time, a measured volume of nitrogen waa admitted through 3, thus fercing the contents of H ever inte G. The amount ef solution remaining in H and J was determined by calibra- tion of the apparatua with a constant amount, 500cc, of nitrogen passed through. This calibration was used rather than a determination of the residual solution in H after each experiment in order that the composition of the reac- tion mixture could be se adjusted as to maintain a cen- stant concentration of hydriedic acid in ail the experi- mente. Standara sulfuric acid was used for the calibration. After blowing over the charge, the tube J and bulb H were | | rinsed out with water and the rinsings titrated with stand - ard alkali. The residual volume was found te be quite con- stant, values obtained being 40, .38, and .43 co. At noted times after mixing the solutions, samples ef about 5ec were withdrawn through ©, with pipettes previously filled with nitrogen, and quickly discharged inte 150cs flssks contain- ing 100co water, previously weighed. The flaake were re- weighed and the solutions titrated for iodine at once, using starch as indicator. Preparation of Materials Sulphuric Acid: C. P. sulphuric acid ef commerce was dil- uted to the requisite concentration and the solution boil- ed to expel air, after which it was kept in full bettles until it was used. Hydriedic Acid: The se-called ¢C. P. hydricdic acid of commerce available contained large quantities of reducing substances, as was demonstrated by adding a little iodine, ‘warming, and noting that the solution was decolerized. Distillation of this product did not remove the impurities. & supply of hydriocdic acid was available which had been meade frem iodine by means of hydregen sulfide. This mat- erial contained @ large amount ef free iodine. “Repeated attempts to remove this, as by reduction with sulphur adi- oxide and precipitation of the sulphate formed with barium fedide, followed by distillation at a lew pressure in an atmosphere of hydrogen or carbon dioxide, proved unavail- ing. At the time when the work was discontinued, an appa- ratus for the production of pure hydriodic acid by cate- lytic combination of hydrogen and iodine vaper over plat- inized asbestos, according to Bodenstein, -( 2. physik. Chem. 13, 56(1894)) had been set up. Nitrogen: A quantity of nitregen wes generated by slowly drepping a strong solution ef sodium nitrite into a4 strong, hot solution of emmonium ehloride, washing the gas with strong sodium hydroxide solution, and storing over water in a gasometer. Standard Alkali: Crystallized C. P. barium hydroxide of commerce was used for the preparation of standard alkali solutions. Standard Acid: Censtant boiling hydrochloric acid prepared according to the method ef Poulk and Hollingsworth, (J. Am. Chem. Sec. 45, 1220(1923)) was used for standardizing the Standard Iodine Selution: On account of a shertage of. potassium iodide, the stendard iodine solution used was borrowed from Hr. Hod. Winegarden. It was later found to contain large quantities of iodate. A sample of potassipm fodide was afterwards ebtained which contained little 40- date; this was then used, together with thrice sublimea iodine, for the standard iodine solution. ‘Standara Thiosulphate Solution: Imperted sodium thicsulphate, "for analysis”, was used. _ Standard Arsenite Solution: This solution was prepared as. recommended by Washburn, (J. Am. Chem. Soc. 30, 16(1908)) and diluted as required. Preliminary Experiments e Some preliminary experiments were made to ascertain what concentrations of hydriodic acid and sulphuric acid would give measurable reaction velocities. The results obtained indicated that the range from 2 toe 6 mols ef sulphuric acid at 1 mol of hydriodic acid per kilogram of solution might be studied. It was afterward found that the hydriodic acid used for these experiments contained a large quantity of reducing substances. The results obtain- ed are therefore certainly unreliable -if not tetally mean- ingless. ‘The Titration of Small Amounts of Iedine in the Presence of Concentrated Acid A great deal of time was spent in attempting to dis- cover a method of accurately determining small quantities of iodine in the presence of large amounts of strong ecid. In the use of sodium arsenite as @& reducing agent the hydro- gen ion concentration must be very carefully controlled; this is also true to a less extent with sodium thiosulphate as the reducing agent. With sedium thiogulphate, in addi- tion, the acid concentration must at no time exceed a rather small value if the formation of sulphurous acid is to be avoided. Some of the schemes tried will be mentioned. ein the first place, the addition of sodium bicarbonate to neutralize the acid seemed inadmissable on account of the probable lesa of iodine vapor with the escaping garben dioxide. In order to test this point, a stream of carbon -G@ioxide was passed inte a .01 WH solution of iodine in £% potassium iodide solution for several minutes. The iodine solution was then titrated with thiosulphate, when it was found to have lost some 7% of its strength. The use of sodium bicarbonate was therefore abandoned. Direct neutralization with sodium hydroxide was tried, but extremely low values always resulted. Apparently the fairly high concentration of hydroxyl ion present lecally even with good stirring resulted in the production ef hypo- f. dodite ion, even when the solution was only partially neu- tralized. The equilibrium constant ef the reaction repre- sented by the equation Ip +2 OH” = I0”™+ I” +He0 indicates that in solutions of greater pH than 9, only a negligible proportion of hypoiodite ion can exist et equilibrium. It would seem that the attainment of equilibrium must be slow, ag the iodine value did not appear to return on standing, ‘after partial neutralization with sedium hydroxide. The uddition of sodium monohydrogen phosphate ss &@ neu- tralizing agent was sttempted. In order, however, to attain the ratio of sodium monohydrogen phosphate to sedium dihydro- gen phosphate negeasary for proper control of the hydrogen ion concentration, a very large volume of solution, of the order gf 1 liter, would be required, owing to the relative- ly small solubility of sodium menehydrogen phosphate. Be- side this disadvantege, the end-point in such concentrated phosphate solution was very insensitive. The following precedure was tried in an attempt to em- “ploy sodium arsenite as the reducing ey-nt.. The sample was weighed into water, after which an excess of standard sod- | ium arsenite solution was added from a weight pipette. A drop of methyl red indicator was then added, and the selu- tion titrated with a solution of ammonium hydroxide and ammonium chloride so prepared that the pH was not greater than 9, thus presumably avoiding the preduction of hypoiodite ion. In this titration the strong iodine color faded com- pletely before the end-point for methyl red was reached, £0 that no trouble was experienced in observing this point. A phosphate buffer solution containing 2 mols of sodium monohydregen phosphate per mol of sedium dihydrogen phosphate was then added to bring the solution te pH 7. After the addition of starch end potassium iodide, the ex- cess of. standard sodium arsenite was titrated with stand- ard iodine solution. This complicated procedure also gave ‘low results, presumably owing to the eccurrence of the re- action represented by the equation NpHgIs = Np + SHI. Wo base of suitablé ionization constant except ammonium hydrox- ide could be discovered, so that this method had to be - abandoned. | 7 | Exeraction of the jodine from the strongly acid solu- tion with carbontetrachloride and titration of the extract with thiosulphate wes tried, with low results. Care was | taken to prevent evaporation of iodine: perhaps the iodine reacted with some unknown impurity in the carbon tetra- chloride. The use of sodium thiosulphate in elightly acid solu- tion ( see W. C. Vesburgh, J. Am. Chem. Soc. 44, 2125(192¢ )) was at first tried, with erratic results, ana given up. The poor results obtained were later found to be due to the presence of a large amount of iodate in the iodine solution with which the experiments were made. The method finally adepted, as the only feasible one, consisted in the simple 10 dilution ef the acid solution to an acidity of .2 N and titration with thiosulphate, using color standards for de- termining the end-point. These stendurds were made up with cobalt nitrate, copper nitrate, and picric acid. The pic- rico seid was found greatly superior to ferric chleride, some- times recommended for this purpose. The accuracy obtain- ea was of the order of 1% on 2500 ef .01 N iodine solution. Summary 1. An apparatus and method for determining the rate of ox- idation of hyariodic acid by ‘sulphuric acid are described. 2. Difficulties with the titration of small quantities of iodine in strongly acid solution are described. The only practicable method tried is the dilution of the acid sol- ution to a concentration of ~2 N in acid, and titration with thiosulphate, using celor standards.