Rutheniumi 2,2‘-Bi pyrid ine Cornplexes as Fluorescent Oxidatio ri-Red uctio n Indicators BYRON KRATOCHVIL and DAVID A. ZATKO Department of Chemistry, Universify of Wisconsin, Madison, Wis. 53706
b Ruthenium complexes of 2,2’-bipyridine, 1,l 0-phenanthroline, and several methyl derivatives have been studied as reversible, fluorometric indicators for oxidat ion-reduction titrations. The ruthenium(l1) complexes have an orange-red fluorescence while those of ruthenium(ll1) do not fluoresce. Tris(2,2’-bipyridine)ruthenium(ll) was found best for titrations with perchloratocerate and tris(4,4’-dimethyl-2,2’bipyridine)ruthenium(ll) was most satisfactory for titrations with sulfatocerate and permanganate. A simple titration assembly incorporating a mercury lamp, condensing len!;, and glass filter was used for fluorescence excitation. Iodine, ferricyanide, and to a lesser extent iron(ll1) and uranium(VI), interfere by quenching indicator fluorescence; compounds causing turbidity in the titration solution also interfere by blocking fluorescence. Titration accuracy and precision are comparable to colored indicators, but greater sensitivity permits minimal indicator blanks in microtitrations, and titrations can be done in highly colored solutions. Application to the micrcidetermination of arsenic is described.
N
indicators are known for acid-base, complexometric, and prxipitation titrations, but applicatioiis to oxidationreduction reactions ani sparse. In 1938 Goto used a crude miiture of chrysaniline and homologs for titrations with permanganate (4) but did not isolate the fluorescent species. More recently Geyer and Steinmel zer (3) studied several 3,&dihydroxyphthalic acids as reversible fluore,xent indicators. Reduction potentials of the compounds investigated ranged from 0.85 to 0.95 volt us. the hydrogen electrode. These indicators should be useful therefore in the potential range where diphenylamine-sulfonic acid functions in colorless solutions, but thcy do not have a transition potential high enough to be used in many reactions with strong oxidizing agents such as sulfato- and perchloratocerate. Rao and co-workers suggested Rhodamine 6G as a fluorescent indicator for ,he titration of uranium(1V) with iron(II1) (6) and vanadium(1V) with dfatocerate ( I ) . UMEROUS FLUORIXCEXT
plexes of these ligands were prepared by a method similar to that of Dwyer ( 2 ) . A solution of 0.001 mole of RuC13 (Fisher Scientific Co.) in 10 ml. of 0.1M HC1 was added slowly with stirring to a slurry of 0.003 mole of the complexing agent in 10 ml. of 0.lM HC1. The mixture was held near boiling for a few minutes, 0.5 gram of sodium hypophosphite in 3 to 4 ml. of water was added, and the solution kept near boiling another 10 to 15 minutes, when it changed from greenish black to redbrown. The hot solution was filtered through a medium-porosity sintered glass funnel and a solution of 1 gram of potassium iodide in 2 to 3 ml. of water was added dropwise to the hot filtrate. The red iodide salt was removed by filtration when cool and recrystallized from distilled water. The compounds were analyzed for carbon and hydrogen (A. Bernhardt Microanalytical Laboratory). In two cases iodide was also determined. This was done by passing a solution of the compound through a column of Dowex 50W-X2 cation exchange resin in the potassium form to give a solution of potassium iodide. This solution was titrated potentiometrically with 0.002X silver nitrate, using a silver-silver iodide indicator electrode. Theoretical for Ru(4,4’ - dimethyl - 2,2’ - bipyridine)3T2: C, 47.64; H, 4.00; found: C, 47.47; H, 4.04. Theoretical for Ru(2,2’b i ~ y r i d i n e ) ~ C l ~C,: 56 2 5 ; H , 3.78; found: C, 55 97; H, 3.86. Theoretical for Ru(5,6-dimethyl-l,IO-phenanthroline)31~:C, 51.50; H , 3.70; found: C, 50.92; H , 3.85. Theoretical for Ru(5methyl - 1,lO - phenanthroline)Jz.4Hz0: C, 46.40; H, 3.79; I, 25.14; found: C, 45.49; H, 3.80; I, 25.12. Theoretical for Ru( 1,lO-phenanthroline)&: C, 48.28; H, 2.70; I, 28.34; found: C, 48.12; H, 2.74; I, 28.33. Indicator solutions, O.OOIM, mere prepared from weighed portions of the iodide salts; these were converted to the nitrates by adding a stoichiometric amount of silver nitrate solution, evaporating to dryness on a steam bath, taking up the residue with water, filtering off the silver iodide, and diluting to the desired volume. Ru(2,2’-bipyridine)J& Ru(BP), was EXPERIMENTAL obtained directly from the G. Frederick Smith Chemical Co., recrystallized, and Reagents and Solutions. 1,lO-Pheanalyzed for carbon and hydrogen benanthroline, 5,6-dimethyl-l,lO-phenan- fore use. A 0.0OlM solution was methroline, 5-methyl-l,lO-phenanthroline, pared. and 4,4’-dimethyl-2-2’-bipyridine were Cerium(1V) solutions in perchloric obtained from the G. Frederick Smith and sulfuric acid were ureDared from Chemical Co. The ruthenium(I1) comeither Ce(OH)( or (fiH4j2Ce(N03)6. In the uranium titration, the first excess of iron(II1) quenched the indicator fluorescence, but in the vanadium reaction the indicator was oxidized to a nonfluorescent form. Two oxidation steps for Rhodamine 6G were reported, the first reversible and the second irreversible. The reduction potential was not given. Several ruthenium(I1) complexes with 1,10-phenanthroline, 2,2’-bipyridine, and their derivatives have been prepared and the reduction potentials of the ruthenium(II1)-(11) couples measured ( 2 ) . Ruthenium tris-2,2’-bipyridine has been used as a color indicator for titrations with cerium(1V) in perchloric acid ( 8 ) , but the color change, yellow to faint blue, is not so satisfactory as the more vivid red to blue of nitroferroin and so its application has been limited. Veening and Rrandt (9) have reported that rutheniuni(I1) complexes of 1 , l O -phenanthroline, 2,2’-bipyridine, and several methyl derivatives give an orange-red fluorescence when irradiated with violet light. The fluorescent maxima range from 575 to 590 nip and the activation maxima ‘Jary between 450 and 465 nip. The Ru(I1) complex with 5-methyl-1,lO-phenanthroline displays the most intense fluorescence and was applied to the determination of ruthenium traces in the presence of other platinum metals. The use of these chelates as fluorescent indicators for titrations with cerium(1V) salts and permanganate is described in this paper. The end point is marked by disappearance of the orange-red fluorescence. The indicator action is rapid, sensitive, and reversible, and permits titrations in highly colored solutions or under conditions where potentiometric detection of the end point is inconvenient. The only special equipment required is an excitation source for the fluorescence and shielding of the titration vessel from bright light.
VOL. 36, NO. 3, MARCH 1964
527
They were standardized against primary standard arsenious oxide, as was potassium permanganate. Other chemicals were reagent grade. Apparatus. Titrations were done in an open-top titration compartment with a black interior. An adjacent compartment contained a General Electric H-4 mercury lamp, cooling fan, condensing lens, and a blue filter (Corning No. 4303; maximum transmittance-470 mp). These were arranged so that a narrow beam of filtered radiation focused on the titration beaker. When used in bright sunlight or strong artificial light a cover with two openings, one for a buret and the other for observation, was placed on the titration compartment. Magnetic stirring was used. Procedure. I n preliminary tests, 5-ml. portions of 0.1000N sodium arsenite were titrated with sulfatocerate and perchloratocerate using the various indicators. For each titration 5 drops of 0.001.V indicator solution and 1 drop of 0.01M OsOa catalyst were used in a total volume of 30 ml. of 1M HpSO4. -4series of titrations comparing ferroin and Ru(4,4' - dimethyl - 2,2' - bipyridine)3(N03)2, ,Ru(DMBP), in clear and colored solutions were carried out in the following manner. Into each of two 100-ml. beakers was placed 5.00 ml. of 0.1000N sodium arsenite, 15 ml. of 1M H2S04,1 drop of OsOl catalyst, and 10 drops of 0.001.V Ru(DRIBP). To one of the beakers was added 10 ml. of water and to the other 5 ml. of a 0.01M solution of the cobalt(II1) complex of (ethylenedinitri1o)tetraacetate (KCoEDTA) and 5 ml. of 0.1M Cr(N03)3. Each solution was titrated to the fluorescent end point with sulfatocerate. The same titrations were then repeated substituting ferroin for Ru(DMBP). Interferences were investigated by a series of titrations of sodium arsenite with 0.1N sulfatocerate. The weighed salt was dissolved in 50 ml. of 1M HpS04. One drop of OsOl catalyst and 5 drops of 10-3M Ru(DMBP) mere used in each case. Where the salt added formed sulfate, arsenite, or arsenate precipitates, hydrogen peroxide in 2 N HClO, was titrated with perchloratocerate using Ru(BP) as indicator. For microtitrations of arsenic the following procedure was used. Stock solutions of sodium arsenite, 0.1000N, and sulfatocerate, 0.0853N, were diluted 100-fold. A blank of 20 ml. of 1 X H p S 0 4 , 1 drop of OsO, catalyst, and 1 drop of 10-51W Ru(D1CIBP) was titrated to the disappearance of fluorescence with sulfatocerate in a 5-ml. microburet. Then 4.00 ml. of 10-31kf sodium arsenite was added and the solution titrated to the same end point. RESULTS AND DISCUSSION
Ru(DhfBP) gives the sharpest and most accurate end points for sulfatocerate and permanganate titrations, while Ru(BP) is best for perchloratocerate titrations. Formal reduction 528
ANALYTICAL CHEMISTRY
Table 1. Comparison of Conventional and Fluorometric Indicators in Standardization of Cerate and Permanganate Solutions Material N of titrant Titrant titrated Ru(DMBP) Ferroin Sulfatocerate 0,04894" NarAs01 0.04892O Sulfatocerate Nar As03 0.04892* . . .b.c Sulfatocerate NazC204 0.1128d 0 . 112Sd Permanganate NarAs03 0 . 1067d 0. 1067d Ru(BP) Nitroferroin PerchloratoNasAsOa 0. 0660Zd 0. 06604d cerate NazC204 0. 06586d 0. 06584d Each value average of 10 titrations. In 0.02M Cr(NO&, 0.002M KCoEDTA. End point not observable. Each value average of 3 or more titrations. 0
potentials reported for these compounds, 1.07 and 1.26 volts, respectively (Z), correspond closely to values measured for ferroin (1.06 volts) and nitroferroin (1.25 volts), which are the most important color indicators for these titrants (5, 7 ) . To determine whether the fluorescent end points correspond visually to conventional colored end points, titrations comparing ferroin and Ru(DMBP) were carried out in colorless and colored solutions (Table I), Excellent agreement was found with colorless solutions. A mixture of red KCoEDTA and green Cr(N03)3was used for the colored titration medium because spectra of this mixture showed intense absorbance throughout most of the visible range. In a solution 0.002M in KCoEDTA and 0.02M in Cr(N03)3ferroin end points could not be seen even with a 5-fold increase in ferroin concentration, while the Ru(DMBP) end points were sharp and easily observed. Nitroferroin and Ru(BP) were compared similarly in colorless solutions by titrations of arsenite and oxalate with perchloratocerate. Standardization of permanganate with arsenite using ferroin and Ru(DMBP) also gave results agreeing within a part per thousand. Fluorometric and potentiometric end points agreed within 0.1% in all cases described in Table I. Effect of Temperature. Temperature variations from 5' to 80' C. do not affect the sharpness of the fluorescent end points, but above 80' fluorescence intensity diminishes and makes end point detection difficult. Ru(D1CIBP) can be used for titrations a t 80' only if the reaction is rapid, otherwise the temporary excess of oxidizing agent attacks the indicator. Ru(BP) is more resistant to oxidative decomposition and is not affected by transitory excesses of oxidizing agent a t higher temperatures. At room temperature neither indicator is appreciably decomposed, as was shown by successful back-titrations of sulfatocerate and permanganate with arsenite using Ru(DMBP) and perchloratocerate with arsenite using Ru(BP).
Interferences. The fluorescence of these indicators appears to be subject t o two types of interferences. The first includes substances that form precipitates. Because fluorescence excitation requires a separate light source, an excitation beam is present in the titration solution. Any turbidity in this path causes light scattering that blocks observation of the fluorescence. Therefore all salts insoluble in the titration medium, or reaction products that precipitate during the course of a titration, interfere. The second type includes substances that absorb energy in the region of fluorescence activation, 450 to 465 mp. The degree of interference by a particular species depends upon its molar absorptivity in this region. Sometimes dilution of the solution helps, while in other cases the absorption peak of a metal ion can be decreased or shifted by addition of a complexing agent. Iron (111) interference, for example, can be reduced by formation of the iron(II1) phosphate complex. Each of the following did not interfere when 0.5 gram of the salt was present in the titration of 0.500 meq. of sodium arsenite with sulfatocerate : Alz(SO&*18H20, NH4N03, BeS04, Cd(NO&, Cr(N03)3. 9Hi0, CoS04+7H?O, CU(N03)2.6H20,Lix03, Mg(N0&-6H20, Mn(N03)p 6Hz0, Hg(NO&. HzO, Zn(NO&. 6Hz0, n'aC2H3OZ,H3B03, KBr, n'aC1, and NaHzP04. r\'a2M00a and n'azW04 were insoluble in the titration medium and therefore interfered, as did fluoride (due to precipitation of CeF3). K$e(CK)c and I p interfered with indicator fluorescence even at the 10-mg. level by absorption of excitation energy. Fluorescence was not obscured by amounts of Fe(h'03)3.9Hz0 under 50 mg. and UOz(N01)2.6Hz0under 100 mg. Thus, iron(I1) solutions are titratable if the iron(II1) concentration a t the end point does not exceed approximately 0.01M. Of the following salts, 0.5 gram did not interfere in titrations of 0.1281 meq. of HzO; in 2M HClO, with perchlorato, cerate: AgN03, C a ( N 0 & * 4 H ~ 0 and Sr(N03)2. Pb(N03)2 formed a pre-
cipitate in the presence of Hz02under these conditions. General Applications. Below are listed results of severitl titrations with sulfatocerate using Ru(DMBP) as fluorescent indicator: FeC2H4(NH&(SOJ2-4HzB (Oesper's salt); meq. taken 0.4463; found (av. oj' 5 titns.), 0.4464; rel. std. dev., 0.08%). VOSO,; meq. taken, 0.2230; found (av. of 6 titns.), 0.2234; rel. std. dev., 0.47%. H202; meq. taken, 0.2197; found (av. of 3 titns.), 0.2195; rel. citd. dev., 0.1401,. Oesper's salt was E ~ titrated O with standard permangana#te; meq. taken, 0.4197; found (av. of 5 titns.), 0.4204; rel. std. dev., 0.22%.
Application to Microdetermination
of Arsenic. The sensitivity of the fluorescent end point is well suited to microtitrations. Using the procedure previously described, the following results were obtained for the titration of 3 arsenite samples: blank; 0.11, 0.10, 0.11 ml.; total ml. cerate required, 4.795, 4.795, 4.810; peq. arsenite taken, 4.000; found 3.996, 4.005, 4.009. Therefore quantities of arsenic as small as 0.15 Ing. can be titrated with an accuracy of 0.1%. LITERATURE CITED
( 1 ) Dikshitulu, L. S. A,, Rao, G. G., Talanta 9, 289 (1962).
(2) Dwyer, F. P., Proc. Roy. SOC.New South Wales 83, 134 (1949). (3) Geyer, R., Steinmeteer, H., Angew. Chem. 72, 634 (1960). (4) Goto, H., J . Chem. SOC.Japan 59, 1357 (1938); C.A. 33, 2062 (1939). (5) Hume, D. H., Kolthoff, I. M., J . Am. Chem. SOC.65, 1895 (1943). (6) Sagi, S., Rao, G. G., Talanta 5 , 154 (1960). (7) Smith, G. F., Richter, F. P., TND. ENG.CHEM.,ANAL.ED. 16, 580 (1944). (8) Steigman, J., Birnbaum, N., Edmonds, S.M., Zbid., 14,30 (1942). (9) Veening, H., Brandt, W. W., ANAL. CHEM.32, 1426 (1960). RECEIVEDfor review October 17, 1963. Accepted December 19, 1963. Presented in part before the Division of Analytical Chemistry, J45th Meeting, ACS, New York, N. Y., September 1963.
Chemiluminescent Indicator Titration of Cadmium with Potassium Ferricyanide FREDERIC KENNY, RUSSEL 8. KURTZ, ALICE C. VANDENOEVER, CAROLE J. SANDERS, CAROL A. NOVARRO, LUCIA E. MENZEL, ROSEMARY KUKLA, and KATHRYN M. McKENNA Hunter College o f The! City University of New York,
b In this new ond rapid determination of cadmium, the chemiluminescent property of siloxene indicator and a photomultiplier photometer are employed in titrations of cadmium with potassium ferricyanide. The photometer is previously standardized against cadmium solutions of known concentration. Metcils forming precipitates with ferricyanide interfere. Nitrate solutions of cadmium are generally used. However, chloride solutions can be employed, if the chloride concentration is approximately the same in both the standardization sample and the unknown.
I
titration, cadmium is precipitated as ferricyanide: 3Cd+a 2Fe(CN)6-3 Cd3[Fe(CN)8]* and hence, in an ideal titration, 30.00 ml. of 0.100OM cadmium solution containing 0.3372 gram of cadmium would require the use of 20 00 ml. of 0.1000M ferricyanide solution A 0.1OOOX ferricyanide solution was employed in this work. Siloxene indicator, prepared as directed by Kenny et al. (11) except that the washing with ether was omitted, was added in a 100-mg. portion prior to each titration. This indicator emits light in the presence of exeess ferricyanide because cf the increase of potential in the solution. The effect of potential change on light evolution has been discussed ((?,9). The conditions in the solution, after each increment of ferricyanide and at N THIS
+
-
695 Park Ave., New York, N. Y.
the stoichiometric point, must be as near to equilibrium as possible. By stirring rapidly and always a t the same rate, and adding the ferricyanide slowly a t a constant rate, an attempt was made to give the reaction the time and opportunity needed to approximate equilibrium conditions closely. Titration rates slightly under 4.0 ml. per minute were found to be satisfactory when the solution was rapidly stirred a t a constant speed with a mechanical stirrer. Samples requiring approximately 20.00 ml. of ferricyanide could be titrated in slightly over 5 minutes. When conditions approximating equilibrium are maintained, the change of intensity of light evolution becomes a function of the change of potential in the solution (10) and, hence, a t the stoichiometric point an end point can be observed. The pronounced change in light intensity a t the stoichiometric point was observed by means of a Photovolt Multiplier Photometer 520A set a t its highest sensitivity. This instrument a t this setting is sensitive to less than 0.002 microlumen. X 0.1OOOM cadmium solution was prepared by dissolving 22.482 grams of C.P. cadmium in approximately 150 ml. of 8 M nitric acid and quantitatively diluting to 2 liters. Since intensity of light emission of the indicator depends in part on the pH of the solution, care was taken to use cadmium solutions having roughly the same pH-approximately 0.5. A 0.10CUM potassium ferricyanide solution was prepared by drying the C.P. salt a t 110' and dissolv-
ing 64.830 grams quantitatively in 2 liters of solution. The photometer was standardized by titrating known amounts of cadmium in three or four samples of 0.1000M solution. The average deflection of the instrument was used as the end point deflection in subsequent unknown titrations. The response of siloxene indicator is not instantaneous, but increases with increase of potential. Under the conditions of the titration a faint light may appear prior to the stoichiometric point. Because of the gelatinous precipitate formed in the reaction, some of this light may be obscured. Furthermore, the potential jump a t the stoichiometric point is relatively small compared, for instance, with the jump for the titration of iron with ceric sulfate. Hence, it was considered preferable to determine the deflection experimentally a t the stoichiometric point rather than attempt to locate with exactness the point of maximum rate of change of light emission. Since, as pointed out (11), the indicator loses light-emitting capacity with time, any batch of siloxene indicator used in a standardization must be used on the same day in the corresponding unknown titrations. The usable life of a batch of indicator has been found in this and previous researches to be a t least 3 days. All samples in this research had a pH, prior to titration, of approximately 0.5. The unknown samples of Table I VOL. 36, NO. 3, MARCH 1964
529
