Potentiometric, Amperometric, and Polarographic Methods for Microanalysis THOMAS D. PARKS’ AND LOUIS LYKKEN Shell Development Company, Emeryville, Calif. A review is given of various practical methods for the microdetermination o f common constituents in petroleum products by potentiometric, amperometric, and polarographic means. A brief description or mention is included of the equipment used in these methods, together with typical applications to microanalysis. Methods are outlined for the determination of acidity, basicity, saponification number, and halogens .by potentiometric titration; sulfate ion, halogens, silver, lead, iron, and copper by amperometric titration; and lead, aluminum, and sodium by polarographic analysis.
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ANY investigators have, recognized the value of electrometric methods in the determination of small amounts of material ( 3 , 6, 9, 10, 28, 89, 30). Benedetti-Pichler, for example, points out the general advantages of a potentiometric end point in microanalysis when dealing with colored or very dilute solutions ( 3 ) . Stock (89) recently reviewed microchemical applications of potentiometry with emphasis on special apparatus used for very small volumes (drop-scale analysis). Polarographic methods are inherently suited to microchemical VI ork bzcaiise they utilize dilute solutions (10-0 to lo-* molar). Majer has described various cells for the polarographic examination of very small volumes of solution (81). Amperometric titrations, either a t a dropping mercury or a rotating platinum electrode, are useful in the analysis of small volumes of dilute solutions (28).
Figure 1. Cell for Micropotentiometric Titration
Potentiometric, amperometric, and polarographic methods are closely related in their fundamental reactions, together forming 5 powerful tool for microanalysis. In general, they are rapid and sensitive methods which often permit the separation or simultaneous determination of ions which are very similar chemically. Representative procedures are outlined below to illustrate the mipmdstmmination. of various constituents in petroleum products. POTENTIOMETRIC TITRATIONS
Reduced-scale potentiometric titration methods are described for representative analyses carried out in these lahoratories. 1
Present address, Stanford Research Institute, Stanford, Calif.
The titrations arc mnde in the cell illustrated in Figure 1, which can be used with a vtiriety of electrode combinations and which permits titration in 10 ml. or less of solution (19). Acidity and Basicity. The potentiometric methods for the determination of acidity and basicity have two advantages over visual color-indicator methods: Dark colored oils give no difficulty and both strong and weak acids can be determined in one titration. The titration medium consists of equal volumes of C . P . benzene arid C.P. isopropyl alcohol, containing 0.5% water ( 1 ) . An outline of the pqocedure is given below; the detailed procedure and typical curves obtained are similar to those of the American Society for Testing Materials ( 1 ). The total acid and stronE acid numbers are determined on 10- to 50-mg. samples in a 10-ml. volume, titrating with 0.02 IV alcoholic potassium hydroxide. The total base and strong base number are similarly determined, except that the sample is titrated with 0.02 ,V alcoholic hydrochloric acid. In both cases, the buret used has graduations every 0.02 ml. and a blank test is made. Saponification Number. A . reduced-scale procedure for saponiflcation number, adapted from the A.S.T.M. procedure ( 8 ) , is used for 50-mg. samples of used oils. The saponification rack is equipped with water-cooled finger condensers ( 8 ) which fit the small potentiometric titration cell (Figure 1) described above. The oil samples are refluxed for 110 to 120 minutes in a known amount of potsssium hydroxide dissolved in a benzeneisopropyl alcohol medium. After cooling, the excess potassium hydroxide is titrated immediately with alcoholic 0.02 N hydrochloric acid by the potentiometric method (above). A blank test is made under the same conditions, omitting the sample. Halogens. The potentiometric determination of halide ions with silver nitrate solution is used after the microcombustion of organic materials in a quartz tube ( 8 7 ) or after solution or leaching of inorganic materials, especially when more than one of the halogens are present. The apparatus used for the titration is the same as that used for acid-base titrations, except that a silver electrode is used as indicator electrode and a glass electrode is used as reference electrode (80). The silver electrode is conditioned before use by polishing with emery cloth and titrating 3 drops of 10% potassium iodide with approximately 0.: K silver nitrate. The solution (10 ml.) containing 0.02 to 0.2 ml. of halide ion is acidified with nitric acid, 0.5 gram of barium nitrate is added, and the solution is titrated with small increments of 0.01 N silver nitrate. In titrating with silver nitrate, breaks for iodide, bromide, and chloride ions are easily distinguished from each other when present in comparable amounts (see Figure 2). Discussion. While it is possibIe to use more classical microequipment ( 3 ) ‘for these potentiometric titrations, it has been found advantageous to use scaled-down cells and procedures (27) based on the common macromethods because this permits ready comparison with results obtained by macro means and permits use of regular electrodes. The potentiometric titration of halides is a longer procedure than the amperometric titration ( I I ) , but it permits ready identification of the halides present. 1444
1445
V O L U M E 22, NO. 11, N O V E M B E R 1 9 5 0 AMPEROMETRIC TITRATIONS
Amperometric titrations are carried out in a cell having a fixed potential between a reference electrode and an indicator electrode. The current produced a t the indicator electrode is used to measure the increase or decrease of an electroaetive ion and the current is plotted against the reagent volume t o obtain a graphical end point. The dropping-mercury electrode was first used as an indicator in aniperometric titrations as a logical development of polarography (6); more recently, a rotating-platinum electrode was introduced ( I7). Together, the platinum and mercury electrodes permit the determination of a wide range of anions and cations. Where applicable, amperometric titrations are much more rapid than potentiometric titrations; the platinum electrode is more useful than the dropping-mercury electrode for titrations carried out a t a positive potential, because of oxidation of mercurous ions formed by solution of the mercury.
In the amperometric method, 25 ml. of solution, containing 0.02 to 0.2 me. of halide ion, are acidified with nitric acid and 25 ml. of acetone and 0.5 ml. of 1% gelatin solution are added. The solution is titrated amperometrically with 0.01 N silver nitrate, using a zero applied potential. For smaller amounts of halogen (less than 0.02 me.), the volume is reduced to 5 to 10 ml., 10 to 15 ml. of acetone are added, and the solution is then titrated with 0.001 N silver nitrate. Sulfate Ion. The procedure for the microdetermination of sulfate ion is similar to that described by Kolthoff and Pan (IS). It is particularly useful for the determination of sulfate ion in the presence of organic matter-e.g., sulfonates.
The sulfate ion is titrated amperometrically with 0.01 M lead nitrate solution (in the resence of isopropyl alcohol and a saturated solution of lead suPfate in 0.1 M potassium nitrate), using a dropping-mercury electrode in place of the rotating-platinum anode and an applied potential of - 1.2 volts. Silver. The amperometric method for the determination of silver is applicable to oils which contain aluminum, barium, calcium, copper, iron, lead, magnesium, tin, and zinc, as well as chlorine, bromine, sulfur, and phosphorus (26). Milligram quantities of silver are determined with an accuracy of 1%. An ammoniacal solution of silver ions is titrated amperometrically with 0.01 N potassium iodide solution, using the rotating-platinum anode and an applied potential of -0.23 volt. Copper and Iron. Kolthoff and May titrated very dilute solutions of dichromate ion wibh ferrous ion a t a rotating-platinum anode and suggested the possibility of the reverse titration of ferrous ion with dichromate solution ( 2 1 ) . Under the conditions used for the titration ( 2 5 ) , ferrous ions are osidized a t the rotating anode to produce a current. When dichromate sol9itiori is added, a diminution of current is found which gives a typical amperometric end point. Cuprous ions are not titrated directly but are used to produce an equivalent amount of ferrous ions before titration. As little as 100 micrograms of copper and 20 micrograms of iron are determined with an accuracy of 1 yo.
ELECTRODE : GLASS -SILVER MEDIUM
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NITRIC ACID - B A R I U M NITRATE
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MILLILITERS OF 0.01N SILVER NITRATE SALT BRIDGE
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Potentiometric Titration of Small Amoiints nf llalicle Inns with Silver Nitrate
Figure 2.
iW3-iA SENSITIVITY SHUNT
The apparatus rcquiretl for aniperometrir titrations is simple. The indicator clectrode is a piece of platinum wire sealed into ti-nini. sof‘t-glass tubing t o form an electrode 5 to 10 mm. long. The reference elrctrotlc~ is geiierally a saturated calomel-salt bridge ronibin;itio~i or :I mercury pool. The Fisher Scientific Company Elrctropotle is ronvenient for application of a fixed potc’ntial and nic~:~sui.c~rnont of t,hc current prodwed at the indicator c~lectrotlo. .I wh(,ni;rtic diagram of the rotating-platinum electrotic :ippai,:~tusis s I i o \ ~ nin Figure :3. The apparatus for titrations at a d r i ) p ~ ) i i i ~ - r i i c ~ r rlwtrode . ~ ~ u r ~ ~ differs in that, provision is made iii thc ccll for. the i*apill:ti,!~ elcc*trodt~ :mtl for. purging with iiitrogen. .\inpt~ronicti~icnic~tliotls,using the rotuting-~,l:itiiiuin c,lect,roc:e, are outlincd Lwlotv tot, the tit,terniinnt ion of hulitlc~,silvei,, lead, copper, lint1 iron iwis. Su1f:ite ion is drtermined using the dr.oppirig-mercui~yc~lcctrodeiii an uir-frw solutiori. Halogens. Amperonictric tit>rations are uscd in the niiwoanslysis of organic halogen compound% after combustmion( 2 7 ) or after It~achingor fusion of inorganic residues. Generally, it is possible to determine bromide ion amperometrically in the presence of chloride ion arid iodide ion in the presence of bromide ion and chloride ion ( 1 5 ) but, when more than one halogen is present, the potentiometric nicithod is preferred.
CALOMEL
Figure 3.
CELL
4mperometric litration Assembly
In tile procetiure for tkic (~ctcrniinationof copper xici iron, the metals are obtained in acid solution and the solution is passed through a silver reductor to give ferrous and cuprous ions. One aliquot of the solution is received under ferric alum and titrated with dichromate solution using a rotating-platinum anode at a potential of +1.0 volt to measure the t,otal iron and copper in the sample; anot,her aliquot is purged with air to oxidize the cuprous ion preferentially and the remaining ferrous ion is titrated as before; the copper is calculat,ed by difference. .Lead. Small amounts of Icad are determined by amperometric procedure.
311
indirect
The lead is precipitated as lead chromate and, after solution of the precipitate, the dichromat,e ion associated with the lead is
1446
ANALYTICAL CHEMISTRY
titrated with 0.01 N ferrous ammonium sulfate solution, using an applied potential of +1.0 volt. The method is particularly useful for the determination of lead in engine deposits containing large amounts of calcium. The a paratus used for the precipitation, filtration, and solution of t i e lead chromate is similar to that of Gordon and Burdett ( 4 ) . Discussion. Five minutes are usually required to complete an aniperometric titration at a rotating-platinum electrode, but this time is often reduced to 1 or 2 minutes per titration in a series of similar samples. Titrations using a dropping-mercury indicator electrode require 1.5 to 20 minutes, in any sequence. This increase in time is due to the nitrogen purge which is necessary between each addition of reagent to stir the solution and to remove dissolved oxygen. The reduction of silver ion at a rotating-platinum electrode is used as the basis for a number of amperometric methods. I n addition to the procedures given above for silver and chloride ions, methods have been reported for bromide and iodide ions ( 2 4 ) , cyanide ion (16), mercaptans (8),sulfide ions ( 5 ) , and disulfides (12). Alcohol or acetone is usually added t o suppress the solubility of the silver salts when titrations are made with 0.01 ,V or 0.001 N silver nitrate solutions. Oxidation of ferrous ions using a rotating-platinum anode at a potential of +1.0 volt is the basis for several titrations. Iron and copper are determined in the presence of aluminum, barium, calcium, lead, magnesium, potassium, silicon, phosphorus, and zinc ( 2 5 ) ; lead is determined by indirect titration of lead chromate. POLAROGRAPHIC METHODS
The value of polarographic methods in microanalysis wa3 recognized early by many workers in the field. The apparatus, techniques, and applications are summarized by Heyrovskf ( 6 ) , who points out that as little as 0.005 ml. of solution can be analyzed by the polarographic method. I n general, however, polarographic cells utilizing 5 t o 10 ml. of solution give sufficient sensitivity for microanalysis. A utilized cell assembly is used which employs interchangeable cells with standard-taper joints (18).
A polarographic method is often chosen for the microdetermination of a metal or organic compound even when other micromethods are available. The choice of method used depends on many factors, including the number of replicate analyses, the time per determination by each method, the equipment available, and the accuracy required. When the amount of material for analysis is limited-cg., samples from engine deposits or corrosion strips-a microbalance is used t o weigh the sample for the determination. On the other hand, trace amounts of metals in used .lubricating oils are determined on the residue from a large sample. Of the large number of metals which can be determined by the polarograph, three typical procedures are outlined below. Lead (10) and sodium (SI)are determined directly by measuring the diffusion wave of the metal ion in solution. Lead gives a well defined wave and generally requires little sample preparation; on the other hand, the procedure for sodium is difficult because it is necessary to remove many interfering metal ions and because it is difficult to obtain a good electrolyte. Aluminum is determined by an indirect method ( 2 4 ) ; the procedure illustrates a means of circumventmg poor polarographic characteristics of an ion by using a secondary reaction in the determination. I n this case, the aluminum is determined by measuring the decrease in wave height of a standard 8-quinolinol (oxine) solution upon the addition of the unknown aluminum solution. For the determination of lead in organic samples, the organic material is removed by wet oxidation, the precipitated lead sulfate is dissolved in 1 A’ hydrochloric acid, a n 4 the lead ion is determined polarographically using a potential range of -0.2 to -0.6 volt. I n the procedure for the determination of sodium, most of the interfering elements are removed by means of the mercury rathode (7, 23),and the remainder are removed by treatment with
ammonium carbonate and ethyl alcohol. The sodium is then determined polarographically using tetraethylammonium hydroxide as the supporting electrolyte (SI). ACKNOWLEDGMENT
The authors wish to acknowledge the work of a number ot their colleagues who, over a period of years, have assisted in the development of many of these methods. LITERATURE CITED
(1) Am. Soc. Testing Materials, “A.S.T.M. Standards on Petroleum Products and Lubricants,” D 6G4-49, p. 958 (1949). (2) Ibid., D 939-47T, p. 1091 (1949). (3) Benedetti-Pichler, A. A., “Microtechniques of Inorganic Analysis,” New York, John Riley b- Sons, 1942. (4) Gordon, B. E., and Burdett, R. A,, ANAL.CHEM.,I?: 137 (1947). (5) Hansen, K. A., Parks, T. D.. and Lykken, L., Microdeter-
mination of Sulfur in Organic Compounds by the Hydrogen Reduction Method,” Symposium on Microchemistry in the Petroleum Industry, 115th Meeting of AMERICANCHEMICAL SOCIETY, San Francisco, Calif. (6) Heyrovskg, J., “Polarographie,” Vienna, Springer Verlag, 1941. (7) Johnson, H. O., Weaver, J. R., and Lykken, L., ANAL.CHEM., 19,481 (1947). (8) Kolthoff, I. M., and Harris, W.E., I N D .ENG.CHEM.,ANaL. ED., 18, 161 (194G). (9) Kolthoff, I. M., and Laitinen, H. A., ‘ pH and Electro Titrations.” New York. John Wilev & Sons. 1941. (10) Kolthoff, I. M., andlingane, J: J., “Polarography,” New York, Interscience Publishers, 1941. (11) Kolthoff, I. M., and May, D. R., IND.ENG.CHEM.,ANAL.ED., 18,208 (1946). (12) Kolthoff. I. M., May, D. R., Morgan, P., Laitinen, H. A., and O’Brien, A. S., Ibid., 18,442 (1946). (13) Kolthoff, I. M., and Pan, Y. D., J . A m . Chem. Soc., 61, 3402 (1939). (14) Laitinen, H. A., Jennings. W. P,, and Parks, T. D., IND.ENG. CHEM., ANAL.ED.,18, 355 (1946). (15) Ibid., p. 358. (16) Ihid., p. 574. (17) Laitinen, H. 4.,and Kolthoff, I. M.,J . Phys. Chem., 45, 1U79 (1941). (18) Lykken, L., Pompeo, D. J., and Weaver, J. R., Ibid., 17, 724 (1945). (19) Lykken, L., and Rolfson, E’. B., Ihid., 13, 653 (1941). (20) Lykken, L., and Tuemmler, F. D., Ihid., 14,67 (1942). (21) Majer, V., Mzkrochemie, 18, 74 (1935). (22) Muller, 0. H., “Polarographic Method of Analysis,” Ea$t(~ll, Pa., Journal of Chemical Education, 1941. (23) Parks, T. D., Johnson, H. O., and Lykken, L., ANAL.CHEM., 20, 148 (1948). (24) Parks, T. D., and Lykken, L,, Ihid., 20, 1102 (1948). (25) Ibid., “Determination of Copper and Iron in Lubricating Oils
by Amperometric Titration,” in press. (26) Ibid., “Determination of Small Amounts of Silver in Lubricating Oils,” in press. (27) Parks, T. D., and Lykken, L., Petroleum Refiner, 29, 85 (1950). (28) Stock, J. T., Analyst, 71, 583 (1946). (29) Ibid., 73,321 (1948). (30) Stock, J. T., and Fill, M. A., Metallurgia, 33, 219 (1946). (31) Weaver, J. R., and Lykken, L., ANAL.CHEM.,19, 372 (1947). RECEIVED April 8, 1949. Presented before the Divisions of Analytical and Micro Chemistry and Petroleum Chemiqtry. Symposium on Microohemiat r y and the Petroleum Industry, a t the 115th Meeting of t h e AMERICAN C H E M I C A L SOCIETY, San Francisco, Calif.
T H E ORCHESTRA
