of an analysis mixture is a potential interference. If the acidity of the components is sufficiently differente.g., the aluminum, h1,drogen ion caseboth materials may be determinable. If the acidities are similar, the polarographic waves will overlap and the method will show merely the sum of the acidic constituents. ACKNOWLEDGMENT
One of the authors (J. C. A.) gratefully acknowledges the assistance of a National Science Foiindation Suiniiier Fellowship during the course of this work.
LITERATURE CITED
(1) Abbott, J. C., M.Sc. thesis, Ohio State University, Columbus, Ohio, 1962. (2) Bates, R. G., “Electrometric pH Determination, Theory and Practice,” p. 105, Wiley, Sew York, 1954. ( 3 ) Ibid.,p. 171. (4) Collat, J. W., ANAL. CHEW30, 1726 (1955). (5) Conyay, B. E., “Electrochemical Data, p. 145, Elsevier, Amsterdam, 1952. (6) Erdtman, H., Granath, M., Schultz. G., Acta. Chem. Scand. 8 , 1442 (1954). ( 7 ) Kolthoff, I. RI., Lingane, J. J., “Polarography,” 2nd ed., p. 52, Interscience, New York, 1952. (8) Ibzd.,p. 203. (9) Ibid.,pp. 205 et seq.
(10) Kolthoff, I. M., Orlemann, E. F., J . Am. Chem. SOC.63,664(1941). (11) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitat,ive Inorganic Analysis,” 3rd ed., p. 321, Macmillan, Yew York, 1952. (12) Latimer, W. M., “Oxidation Potentials,” 2nd ed., Prentice-Hall, New York, 1952. (13) Mueller, 0. H., “Polarographic Method of Analysis,” pp. 106-13, Chemical Education Pub. Co., Easton, Pa., 1951. (14) Mueller, 0. H., Baumberger, J. P., Trans. Electrochem. SOC.71, 169 (1937). (15) Vetter, K. J., 2. Elektrochem. 56, 797 (1952). RECEIVEDfor review August 1, 1962. Accepted March 29, 1963.
Determination of Water below 10 p.p.m. in Benzene and Related Solvents, Using Coulometrically Generated Iodine R. F. SWENSEN and D. A. KEYWORTH Universal Oil Product!: Co., Des Plaines, 111. Iodine may be coiJlometrically generated from potassiurn iodide dissolved in a solvent containing pyridine, formamide, and sulfur dioxide, and may be used in a Karl Fisc:her type reaction for determination of water below 10 p.p.m. in benzene and related solvents. The formamide performs the usual chemical function of methanol in the Karl Fischer reaction but avoids production of water from reaction of methanol with any carboxylic acids or carbonyl compourids present. End point detection i s accomplished by dead-stop technique. A special cell and a system of sample handling have enabled sample transfers without contamination. Synthetic samples were prepared and analyzed to demonstrate the usefulness of the new technique. A glycol extraction procedure was examined b y using the new coulometric technique. This showed the weakness of the glycol procedure for the determination of water below 10 p.p.m.
D
the prcfusion of papers describing procmedures for determination of water ( 7 ) ,there has been a real difficulty in sample handling and direct determination of concentrations of water smaller than 10 p.p.m. with accuracy. Direct memurement of water concentrations by infrared spectroscopy at the level of 1 p.p.rn. is not workable because of background and sensitivity ESPITE
limitations (4). Ilorrever, indirect infrared ( 4 ) , nondispersive infrared analyzers ( I ) , and gas chromatographic (6) methods exist, all based on the reaction of water with calcium carbide with subsequent measurement of acetylene formed. Sample handling problems between plants and the laboratory are circumvented by working with continuous streams in the infrared methods
(I,4).
Commercial moisture monitors are available employing absorption of the mater on P205and subsequent electrolysis ( 3 ) . X procedure employing diethylaluminum hydride as a reagent has also been described ( 9 ) , but t,his reagent is usually difficult to obtain and use. A procedure which concentrates water by ethylene glycol extraction ( 5 ) , and then measures the increase in water concentration in the glycol by Karl Fischer titration, does not work well for water eoncentrations below 10 p.p.m. Water measurements below concentrations of 10 p.p.m. hare been hampered by two major difficulties. It is difficult to obtain and work with solvents containing less than 10 p.p.m. of mat,er because samples are so easily cuntaminated by water during the transfer and sample-handling operations. It is also estremelj- difficult to prepare and maintain the exact water concentration of samples used to evalnate a proposed analytical method. Because of these considerations, an
approach was examined employing the coulometric generation of iodine for a Karl Fischer type reaction. This approach has several advantages. The coulometric generation of small amounts of iodine for the Karl Fischer reaction has been studied and the stoichiometry is known (8). Coulometric generators are available which produce precise small currents for timed interrals, thereby avoiding the problem of standardizing and protecting the very dilute and highly moisture-sensitive Karl Fischer reagents. Although coulometric generation of iodine for the Karl Fischer reaction has been previously described (8),the unique advantages of this approach have not been reported as having been applied to water concentrations below 10 p.p.m. The coulometric approach must be coupled with highly sensitive end point detection such as the dead-stop technique ( I O ) . The first part of this investigation dealt with the feasibility of this approach when applied to low water concentrations. Many systems were examined, and equipment and solvent .!-:terns were selected. As might be espected from the earlier literature, several combinations were satisfactory. The second and major portion of the study dealt with sampling, transfer of samples, protection of the sample from water contaniinat,ion during the measuring process, and a system of crosschecks to establish confidence in the accuracy of the measurements. VOL. 35, NO. 7, JUNE 1963
863
electrodes with large surface areas. The rectangular platinum electrodes (No. 117142) give a satisfactory response. The generation electrode pair (KO. 117137) should consist of an anode of large surface area and a cathode, which may be the same as the rlcctrodes nsed in the dctector system. The shield tube (KO. 167514) is a satisfactory container for the salt bridge. The cell (No. 7961) consists of a glass container (No, 122043) and a polyethylene cap (Xo. 067513) designed for the electrode assembly described, where the above numbers reier to the Leeds and Northrup Co. catalog. This was tried, but the ~iolyethylenecap soon developed leaks which admitted sufficient air to constitute a source of significant error. Therefore, an allglass cell was made from a 55/50 T joint. The female portion of the joint was used for the cell cap and the electrodes 17-ere fused into the top (Figure 1). Openings for a rubber septum and a capillary tube aere added. The capillary tnbe extended t.o the bottom of the cell and was used to drain the cell without exposing it to atmospheric moisture. Titration Solvent. The Karl Fischer reaction occursin two steps (11). Figure 1.
Electrode assembly for cell
1s
+ SOB+ HzO + 3 C
N -
EXPERIMENTAL
The eouinment reauired consists of
a coulometric source, Eel1 assembly, and end point detector (Figure 1). Coulometric Source. A satisfactory coulometric source is the Sargent Model I V coulometric generator. This unit has various multiplier settings which allow accurate generation of small currents at several possible levels for the 0.05 t o 2 microequivalent-per-second range. A timer on the instrument was calibrated so t h a t the product of the tinier reading and t h e multiplier setting equaled microeqnivalents. The instrument is capable of ~ O . l ~relative , error as long as external resistances do not exceed 1700 ohms. First investigations used a chart recorder t o determine the end point and the number of microeyuivs, lents, but later the recorder was replaced by the simpler dead-stop end point detection, and the timer and multiplier settings were used to determine the number of microequivalents. End Point Detection. 0riein:illv t h e - polarized platinum ele%rod"c arrangement described by Pyrah and Roberton (IO) was used. Although thls system worked well, the commercially available Beckman Model 76 expanded scale pH meter was employed in later work. This unit has an expanded millivolt scale, a polarizing system for the detector electrodes, and a very rapid and nearly full-ecalc needle deflrction at the end point with the use of the expanded scale function. Electrode System. A rapid responsc of the detector system reyuires 864
ANALYTICAL CHEMISTRY
Methanol is added to the system to prcvent water from adding to the pyridinesulfur trioxide addition compound, thereby avoiding a change in stoichiometry of the reaction and a decrease in the sensitivity of the reagent. Since the presence of methanol in the system is a source of water in some samples, because of esterification reactions mhen acids are present, or because of acetal and ketal formation accompanied by production of water when carbonyl compounds are present (14,methanol was omitted and formamide was substituted. Formamide is able to perform the desired addition reaction nith the pyridine-sulfur trioxide compound (b and further serves as an excellent solvent, rendering most samples miscible, and causing sufficient conductance to permit a coulometric titration. HCONH,
+ C,HsN.SOs
-
HCONH.SOa,C.HJVH (3) Several combinations of pyridine, potassium iodide, formamide, and solfur dioxide were tried. The combination chosen consisted of 30 ml. of pyridine (dried by passing the pyridine through a column of 5A hIolecular Sieves, available from Linde Co., Division of Union Carbide Corp., Chicago)
Figure 2. Hofer fitting hypodermic connector to sample bottle 10 mi. of potassium iodide solution (25 grams of K I per liter of f o r m a mide), and 10 ml. of sulfur dioxide in pyridine solution (Karl Fischer Reagent Solution No. 1, available from E. H. Sargent and Co.). This combination did not become heated during i:oulometric generation nor did it become discolored, and the rate of the desired reaction in this solvent system vas much faster than with conventional solvent systems containing methanol. Bridge Solution. Seventy milliliters of titra.tion solvent were dried in the titration cell by adding a 10% by weight solution of iodine in pyridine through the septum with a syringe. When the meter indicstcd t h a t t h e solvent was dry, a portion of the solvent was transferred t o the cathode compartment t o serve as a bridge solution. Three-tenths gram of hydroxylaminc hydrochloridc was dissolved in the bridge solution t o provide an easily reducible cathodic system. Sample Bottles. A major source of contamination in trace water analysis is the sample container. Benzene containing 10 p.p.m.'water, when transferred t o a n unprepared glass bottle, will quickly remove water adsorbed on t h e vesscl walls. Several bottle-drying techniques were considered. The folloming proccdure provcd t o be the most satisfactory. Dry 100-ml. sernm bottles in an oven at 160" C. overnight. Stopper the bottles nith septa dried in the oven one hour and while both the septa and the bottles are still hot. Cool the bottles by inserting through the septum a needle connected by rubber tubing to high purity dry nitrogen (Linde Co., Division of Union Carbide Corp., Chicaso). When the hottles are pressurized to 5 p.s.i., insert a second needle into each septum to allow purging with nitrogen until the bottles are cooled to
room tcmpcrature. Withdran the cxit needle and then the nitrogen sourre needle. These prepared bottles may be stored for a week without redrying. Just prior to sampling, the bottle should be partly evacuated, again using the needle-through-septum technique to avoid opening the bottle and admitting n-ater-contaminat ed air. Partial cvacuation permits introduction of a sample into the c1os.d sample bottle through the septum using a hypodermic tube without undue pressure resistance. Transfer of SaIr.ple to Sample Bottle. Transfer of a sample t o u d r y bottle without evposure t o air may be accomplished by using a Hofcr fitting, silver r,oldered t o a 19gauge hypodermic needle (Figure 2). The needle-adapted fitting should bc dried overnight at '150" t o 160" C. j u q t prior t o use. The Hofer fitting needle is inserted through a septum into a test tube containing desiccant (covered n i t h glass wool to prevent dusting of the needle:l, and the drying tube is left attached while the fitting is connected to the sample line. The desiccator tube is removed and the Hofer fitting needle inserted through the septum of a partially evacuated dried bottle. About 50 ml. of the sample is purged through the needle at a rate of about 10 ml. per minute. The actual sample is collected in a second partially evacuated d r e d bottle. A benzene sample collected as described may be stored 24 hours, provided that contact of the lienzene with the septum is avoided (Table I). If the sample must be in trrtnsit or for other reasons stored longer than 24 hours, the benzene should be frozen by packing the bottle in dry ice. Frozen samples have been stored for more than a week n ithout significant contamination (Table I). Transferring Sample to Titration Cell. The transfer of the sample t o t h e cell was accomplished by using a U-shaped hypodermic, tube (Figure 3) dried and stored in a dcsiccator. For transfer of a sample, the long leg of the tube was inserted through the scytum of the sampll: bottle into the wmplc. T h e short leg was inserted through the septum of the titration crll. B y introducing d r y nitrogen with a hypodermic needle through the sample bottle s e p t u r , transfer of the sample to the cell !$as accomplished. The sample bottle was weighed before and after a portion of the sample n a s tranhferr cd to determine the amount delivered. Pressure xoduced in the titration cell by the sample transfer !vas released by venting with a hypodermic needle temporarily inszrted through the scytum for this purpoae. The hypodermic transfer tubing was not removed from the sample bottle until all replicate analyses were completed. By use of this technique, repeatability was satisfactory (Table I). Cell Blank. T h e dried cell solution will dry t h e air space above t h e titration solvent, as well as surfaces with which t h e solvent comes in contact. Water froin these sources is titrated
Table 1. Sam pit.
Days storage
Stability of Samples in Storage Av. H20 found, p.p.m.
H 2 0 found, p.p.m.
A. Samples in storage where septa were not dried 1 ?
0 1 2 0
1.5 7.4 14 1.8 7.5 15
1 4,l.S 7 2,i G 13. 14 2 . 0 , 1.8, 1 . 7 7.4, 7.5 14, 1 5
1 2
B. Sarnplcs iii storage where septa were dried 2.1, 2 0 17,l D
2.I 1.7 3.4 11.1 1.7 1 .9 3.7 15.5
3 3, 3 .5 10 9, 11.3 15,lO 1.8,19 3 5,3.9 15.1,15.8
C. Saiiiplcs frozen in dry ice c h e ~ t 2.2,2.9,3.3 2.4, 1 . 6 , 2 . 3 3.0, 2 . 9 "4.3.2 3.1, 2 . 9 2.3,2.9.2.G 2.8, 2 . 6 3.0, 3 . 1
concurrently with samples, and therefore this cell blank must be measured. T h e cell blank must be small and constant t o permit acceptable analyses. When the cell is thoroughly dried, t h e cell blank measured is negligibly small; otherwise, a correction should be made. T h e blank correction is determined as shown in Table 11. Titration Procedure. Add 70 ml. of titration solvent t o t h e cell using a 20-ml. hypodermic syringe. Insert a Tent, consisting of a hypodermic needle connected t o a drying tube. through the septum t o avoid pressure increasc inside the cell during the addition of sample. Turn on the magnetic stirrer; d r y the cell by adding a solution of I-gram of iodine per 10 ml. of pyridine with a syringe; and then, by generation, equilibrating and repeating the determination of the blank until t h e cell blank is less t h a n 0.5 meq. per second. Force a n amount of sample (1 to 5 grams, depending on the mater content) from the sample bottle into the titration cell, using a hypodermic needle connected to a nitrogen pressure source. Keigh the sample bottle, hypodermic tubing, and rubber plug for the hypodermic tubing, before and after transfer of the sample to obtain the exact neight of the sample transferred. Generate iodine coulometrically until the needle on the detection device returns to the predetermined end point. Note the reading of the timer in 0.1 second. Convert this reading to seconds and multiply it by the titration blank. Subtract this product from the timer reading in 0.1 second. The net reading in 0.1 second, multiplied by the multiplier settings, is the total microequivalents of water present.
Table II.
2.8 2.1 3.0 2.8 3.0 2.6 2.7 3.1
Determination of Titration Blank 111
Time in seronds from I I1 start of IT Time of Instru- generation Correcgenerament t o point tion tion, multiplier indiper sec., sec. X setting cating IV = lo-' used wet state I/III 104 51 51
