Spectrophotometric Titration of Divalent MetaIs with 1 -Nitroso-2-naphthol in N,N-Dimethylformamide IRENE T. TAKAHASHI' and REX J. ROBINSON Department o f Chemistry, University o f Washington, Seattle 5, Wash.
b 1 -Nitroso-2-naphthol, an organic precipitating agent, can b e used as a volumetric reagent for the titration of micro amounts of divalent metals in a N,N-dimethylformamide solution. Spectrophotometric titrations of 0.0 1 prnole of copper(l1) and 10.0 pmoles of nickel(l1) in 40 ml. of solvent can b e performed with a precision of 2 parts per 1000. Lead(ll), zinc(ll), and uranyl ions also chelate with nitrosonaphthol in dimethylformarnide. The usefulness of nitrosonaphthol as a titrant in dimethylformamide is limited somewhat by its gradual decomposition in this solvent.
P
by Boylr anti Robinsoii (1) and hlarple, llatsuynma, and Burdett ( 2 ) havc presented spectrophotometric titrations of small amounts of metallic ions in nonaqueous solutions using organic chelating agents as titrants. Titration in nonaqueous medium offers the possibility of direct titration of metallic ious extracted and concentrated from aqueous medium by water-immiscible solvcnts without reintroduction into aqueous medium. The various advantages offered by spectrophotometric titrations in general have been noted by Reilley and Schweizer ( 3 ) . Organic chelating agents are especially suitable as titrants for metals in an organic medium hecause of their specific and sensitive reactions and their solubilities in organic solvents. The metal chelatrs formed, as well as the reagent itself, are usually more soluble in an organic solvent than in water so a wide choice of reagents is allowed. The forniation constants of metal chelates depend on the chelating agent, the metal, and the solvent system. Therefore, by the proper selection of the solvent, metals which react weakly with a chelating agent in a water medium often can be accurately titrated in a nonaqueous medium. I n this study l-nitroso-2-naphtho1, dissolved in dimethylforniamide (DMF), was used as a titrant for micro amounts of divalent metals. APERS
REAGENTS A N D APPARATUS
Metal Xitrate Solutions. Copper, Present address, The Dow Chemical Co., Midland, Mich. 1350
0
ANALYTICAL CHEMISTRY
nickel, and zinc solutions were prepared to be 0.1111 by dissolving Baker's reagent grade metals in a minimum of nitric acid and diluting to volume with distilled water. The exact' concentration of the copper solution was established by clectrolytic analysis and the concentration of the nickel solution gravimetrically by p1,ecipitation with dimethylglyoxime. The zinc solution mas prepared as a primary standard solution. Granyl and lead nitrat,e solutions were prepared by dissolving definite \wights of reagent grade nitrate salts and diluting to volume with distilled v-atci.. 1 -Xitroso-2-naphthol Solutions. Eastinaii Kodak, practical grade, I -nitroso2-naphthol was recryst,allized twice from petroleum ether (boiling range 60-90'' C.) , The reddish-Iirowii crystals, dried in air and stoi,ed over anhydrous calcium sulfate, melted a t 107.5108.5" C. Solutions, 0.001 to 0.01BI, were prepared by weighing thc reagent in a closed microweighing t.ube on R microbalance and diluting to 50 ml. with S.S-dimethylformamide. The coefficient of cubical expansion of din~ethylforinaniide.0.0009 per ' C. ( I ) , mas used iii calculating the molarities of the solut'ion to 22' C. This temperature was taken as the standard temperature because it was t,he average laboratory trniperatui*e. Likewise. all volumet,ric glasswarr was calibrated a t this tcmprrature. h',il~-Dimethylforniamide, Matheson, Coleman and Bell, hoiling range 1524O c . n-T3utylamine, Matheson, Coleman and Bell, boiling range 76-78' C., was distillcd from an all-glass system and stored in a glass-stoppered bottle. Glacial Acetic Acid. Raker's analvzed reagent, 99.5'3&. Metal Chelate Solutions for. Absorption SDectra. A dilutr water solution of tlie'metal ion was accurately measured into a beakei and evaporated to dryness in a vacuum desiccator attached to a TTater aspiratoi , The crystals of salt werc dissolved in the organic solvent, tieated with a measured amount of chelating agent. and diluted to volume x i t h the organic solvrnt The absorption spectia of the various solutions were taken with a Beckman hlodel DU quartz spectrophotometer and a lkcknian Model DK 1 spectrophotometer, hotli cquippctl n i t h 1-cm m a t c l i d silica crllq. The spectrophotomrtiic titrations n eie made with the DU specttophotometer using the modified cell compartment as designed by Royle and Robincon (1). The ti-
trant was measured with a 10-ml. microburet which had been calibrated at 1ml. intervals with dimethylformamide, using the densities previously reported (1). EXPERIMENTAL
Absorption Spectra. T o deteimine the propel \rave length and conditions foi the spectrophotometric titrations, spectra of I-nitroso-2-naphthol and its coppci (11), uranyl, nickel(II), zinc(II), and lead(I1) chelates in dimethylformaniidr nere taken from 320 to 625 mfi, Thc zolutions contained niiutures of 2 to 1 iind I t o 1 chrlatrq of the reagent and nictal. so that NTSS iragent n a s not present. Tht inctal chelates 4 1 0 cd ~ absorption niaunia in the region from 350 to 500 mw (Table I) Hecause tlic. iiranyl chelate absorbrtl in the same legion between 400 and 550 mpL, the tlitiiethylformamide nas made 0.0015M in acetic acid. In this slightly acid solution the metal chelates shoned only a small change in thrir spectra, but tlir spectrum of 1-nitroso-Znaphthol shon td a decrease in absorbance between 400 uiid 550 mfi. A 0.0015M acetic acid solution in dimethylformamide was ielected as the solvent mcdium for qome of thv titrations. I ' l ~ t s spectra of the metal chelates \verr also taken in a basic dimethylforinainidc solution t o ascertain if a bettw separation could be obtained betwec.ii the absorption maxima of the reagciit and metal chelates. -4s Royle and Robinson successfully used n-butylamine in dimethylformamide to shift the absorption spectra of the metal chelates of 8-quinolinol to longer wave lengths, n-butylamine n a s added to the dimethylforniamide for a basic solution of thr nitrosonaphthol chelates. The uranyl and lead(I1) ions n-ere kept in qolution by adding a small amount of glacial acetic acid before making the iolution basic nith butylamine. The addition of 2% butylamine increased the absoiption of the chelates and the reagent in the longer wave-length regions (Tablc I), I n the basic solution, the reagent absorbed very slightly in the same legion as the chelates of uranyl, lead(lT), and zinc(I1). Titration of these metals did not appear practical because a greater accuracy is obtained I\ hen a 11ave length I< splected at n hich
the absorptions of the chelate and the titrating reagent are greatly different. I n going from an acidic to a basic dimethylforniamide solution, a hypsochromic shift of the principal absorption maxinia of the copper, zinc, and nickel chelates occurred. The spectra of the uranyl and lead chelates and nitrosonaphthol showed a bathochromic shift of their principal absorption maxima. Stability of Nitrosonaphthol in Dimethylfonnamide. Solid l-nitroso-2naphthol was very slightly soluble in u a t e r b u t mas very soluble in diiiiethylforrnainide. h 0.005M nitrosonaphthol solution in dimethylformaniide, stored in a n amber glass bottle at room temperature, was stable without change over a 4-hour period, then decrcasrd in strength slowly; over a 9-day period it s h o m d a 5% decrcaqc in molarity. The absorption spectrum of the freshly prepared solution showed a maximum a t 368 nip with considerable absorption betryeen 400 and 480 inp, As the solution aged, the peak at 365 nip decreased and shifted to shorter wave lengths; the absorption betmeeii 400 and 480 inp decreased, nhilr an increase in absorption was ohserred beyoiid 480 nip. The solutions of nitrosonaphthol in dimethglfornianiide were acidified with acetic acid to retard the decomposition of the nitrosonaphthol. When the acetic acid was varied froiii 0.0015 to 0.3M, it was found that the 0.3M acetic acid solution gave the most stable solution of 0.005M nitroqonaphthol. HOWever, mort' dilute solutions of nitrosonaphthol in dimethylformaniidc dcromposed rapidly in spite of the addition of acrtic acid. The decompositioii wai evident from the change in the absorption sprctrum of the solution as tlic absorption shifted to the ultraviolet rcgioii, shomiiig the loss of the nitroso band a t 368 nip. It was suspected that the tliriiethylforiiiamidr was reducing thc nitrosonaphthol to an aminonaphthol. Because the atldition of :i large amount of acetic acid to the titrant slowed the rrnction between the metah and nitrosonaphthol and incrc,awl thc. acidity of the titration medium with each addition of titrant. a nitrosonaphtho1 solution in dimethy1forni:imic~e without acetic acid wts used as the titrant. This bolution nas freshly prepared before each ierim of titr a t'ions.
Ti,
A, = A, =
2 00
IO0
3 00
400
5 00
Figure 1 , Titrations of nickel(l1) and copper(l1) with nitrosonaphthol 1. 10.1 9 pmoles of nickel(l1). 0.0091 6M nitrasonaphthol, 12.5% butylamine in dimethylformamide, a t 525 mp 2. 5.67 prnoler of copper(ll), 0.00468M nitrosonaphthol, 0.0015 M acetic ocid in dimethylformamide, a t 600 rnM
vent, which was either a mixture-of butylamine and dimethylformamide or acetic acid and dimethylformamide. The beaker, containing the metal nitrate and solvent, was placed in the microcell compartment of the Beckman DU spectrophotometer. After the solut'iori had been stirred for 5 minutes, t'he proper wave length \vas set, the sensitivity adjustment turned 0.5 to I .5 turns clockwise from the counter clockwise limit, and the spectrophotometer balanced. Then t'he solution was titrated with the 0.005 to 0.01X iiitrosonaphthol solution. After the initial 1 to 2 inl. of titrant) had been added, the spectrophotometer was rebalanced and the titrant was added in 0.1-ml. port'ions. using a drainage time of 10 seconds pel. 0.1 nil. The titration was continued until 2 ml. past the end point. The titration procedure of Boylc and Robinson (1) was followed. In this method, the spectrophotometer was rebalanced to zero after most of the cat,iori had been titratrd. Then 0.1- to 0.2ml. increments of titrant \vei'r added until well past the equivalence point. Thus, t'he region of maximum sensitivity of the inst~rrinieiit was used for the measurenient of the absorbance of the solution near the eyuivalciice point,. The folloniiig data w r e rworded during the titration : I: = volunie of solution, containing solvetit and cation only, at initial balancc point of vpectrophotomcter
Table I.
=
7'
=
HOlll t 1 o n
1 ' 0 2 ( 11)
Zn(I1)
Pb(I1)
Xi(II1 CU(I1)
volume of wlution, containing solvent, cation, and initial 1 or 2 ml. of titrant, a t rebalance point of spectrophotometer absorbance reading before spcctrophotometer wm rebalanced observed absorbance reading taken after addition of X In1 of titrant after spectrophntometer ww rebalanced tnilliliters of titrant added after spectrophotometer was rehalanced average trinpc~rsturr of titratit during tit ration
'l'ht ahsorbanc~c~I c,adings werc ( V I I rected for dilution using the equatiou derived by Roylc and Robinson (1).
The corrected relatiw absorbance real 1ings ( A c ) were plotted against the milliliters of titrant atldcd. The intersection of the best straight lines before :inti after thc brrak in t h r titration curve was used to cht,:thli-h the end point. The volume of titrant a t the end point, found from the titration curve, was corrected t o the \ohimc at 22' C., wing the coefficir,nt of cwbical expansion of dimrthylfortnmiirle, 0.0009 per ' C. ( 1 ) . From the voluine of titrant used tor the titration of t h r nietal and the millimoles of metal titratctl, the molarit\ of the nitroson:rphthol titrant n a i i d culated. The wave h g t h for a titration u : i k selected bo that the change in absorb:inr~ for each 0.1 r i d . of titrant was bet\\iwi 0.02 and 0.04 units. The relative .rhrorbance values w r ( kept ~ betwern 0 m d 0.400 throughout :I titration. RESULTS AND DISCUSSION
The titrations of copper(1l) 1011.; were perforniccl N t 600 mp, using the formation of t h r 2 to 1 chrllate of nitrobonaphthol with iwpper as the ciid points (Figure 1). Tlic, titrations, in niixturi'z of butylamine :mtl dinirthylformamiclc for dimethylforiiiaiiiitlc alone, indicated that either a vompc4ing rcartion 01 n decompositioii a$ occurring. Evon after stirring foi 1.3 miiiutes, the solution did not i m r w to rquilibrium A 0.0015RI arctic. ncwl wlution in c11methylfori~~aIrii(i( t h r h w t -olvent
Absorption Maxima of Metal Chelates of Dimethylformamide _--
-4bout 0.001 t o 0.01 mmole of metal nitrate was accurately measured with a calibrated pipet into a 50-ml. beaker. T h e solution was evaporated to dryness in a vacuum desiccator with t h r watcr aspirator The crystals of nietal nitrate in the beakrr were dissolvrtl i n 40 ml. of sol-
X
ML OF NITROSONAPHTHOL
-
~
SPECTROPHOTOMETRIC TITRATIONS
Titration Procedure.
=
Fe(I1) Nitrosoiiaphthol
I)>IF
3i7 439, 361 382, 446 377 ,399, 489
:3w
Wave Length. DMF. O . O O I ~iii aretic acid 372 443,384 372
--_
381
402 347,7Oi 5 368
1 -Nitroso-2-naphthol in Mp
-
~
IIMF and
2ch hutylamirit. 430,361 434 430,364 376 394 43.1
VOL. 32, NO. 10, SEPTEMBER 1960
1351
Table II. Analytical Results of Copper (11) and Nickel(l1) Titrations in Various Solvents
Copper, pmoles
1-Nitroso-‘%naphthol MI. Molarity
Dimethylformnmide 5 021
1.837 1.819 1 87T Av. .%v dev.
0.00547 0 00552 0 00535 0 00545 0 00006
0.0015M Acetic Acid in I)hlF 5 036
5.645
2 839 2 831 2 831 2.829 3.171
0 003561 (J 003560 0 003558 0,003557 0 003561 Av. 0.003559 Av. dev. 0 0000016
0.0015M Acetic Acid iii DhIF with 5 pmoles of Zinc(11)1’remnt 5.021 1.952 (J.W51& 1,971 0,005095 Av. 0.005119 hv.dev. 0 000025
Nickel, pmoles 12.5% Butylaminc~iri 1)MF 10.19 2.308 0.00883 0.00886
2.300 2,30:3
0,00885
Av.
0.00885 Av. dev. 0.00001 1
for the titration of copper ions because the copper chelated strongly with the nitrosonaphthol even in a slightly acid solution, The titration of 5 pmoles of copper in 40 ml. of solvent gave a precision of 2 parts par 1000 (p.p.t.) or better (Table 11). When more dilute solutions containing 1or 2 pmoles of coppcr were titrated, the solution came to equilibrium too slowly €or a practical titration. On the other hand, the titration of 10 pmoles of copper in 40 ml. of solvent gave high results indicating a slow side reaction. Nickel(I1) interfered with the copper titrations as it was titrated with the
1352
ANALYTICAL CHEMISTRY
Table 111.
Comparison of Prepared and Experimental Molarities naphthol Solutions in Dimethylformamide
Nitrosoiittphthol Prepn. S o . I
IT 11
No. of Detns. against Standard Cu +2 Soh.
Exptl. Molarity
of Nitroso-
Prepared Molarity
70
Purity
2 2 2
0 . 0 0 4 5 7 5 ~ 0 . 0 0 0 0 0 3 0.004830 0.004441rt0.000001 0.004741 0 004873 f 0 000030 0.005088
94.7 h O . 1 93 6 7 h 0 . 0 2 95.8 3Z0.6
3 2 2
0 00126fi Zk 0 000004
0 004440
96 1
0.004469 f 0 . 0 0 0 0 5
0.004684
95.4 h O . 1
2 3
0.005122i0.000017 0.0039963ZO 000038
0.005334 0.001073
9G.02&OO.4 98 1 & O 9
+0
1
o.o04135~0:000005 0:0O?i99 SS:2 Z 0 : i
copper in tlie slightlj acid dimethylformaniide solution and a sharp change in slope did not occur where the 2 to 1 chelate of iiitiosonaphthol and copper was formed. However, in the presence of 5 pmoles of zinc, copper could be titrated with a precision of 5 p.p.t., but the results were 2% high. Because nickel did not chelate as strongly as copper, a basic dimethylformamide solution, containing 5 ml. of butylamine per 35 nil. of dimethylformamide, was used as the titration medium. Ten pmoles were titrated a t a wave length of 525 mp, with the formation of the 2 to l chelate a t the end point (Figure 1). The formation of a weak 3 to 1 chelate was suspected from the rise in slope after the end point. A precision of 1 p.p.t. was found for these titrations (Table 11). Concentrations smaller than 10 pmoles of nickel did not give a sharp break a t the end point. Though the spectra of zinc(II), lead (11), and uranyl ions showed chelation with nitrosonaphthol in dimethylformamide, the chelates absorbed too close to the region where the reagent absorbed for accurate titrations. Because their spectra in a basic dimethylformamide solution were very similar to the nitrosonaphthol spectrum, and because the titrations in butylaminedimethylformamide solutions a t 490 or 495 mp shon-ed no break in the titration curve,
it
\\ as conclutied that the addition of butylamine to the diniethylformamide inhibited chelation of the nitrosonaphtho1 with these three ions. I n 0.0015M acetic acid in dimethylforniamide or in dimethylforniamide alone, lead, zinc, and uranyl titrations shomed a slight break where the 2 to I chelate was formed, but these reactions cam(’ to equilibrium too sloir-ly for successful titrations. In Table 111, a comparison is made of the prtyared molarity values of nitrosonaphthol solutions and the experimentally determined molarity values obtained by standaidizing against standard copper solutions. The experimental results are low by 4 to 6% depending upon the purity of the solid nitrosonaphthol, notwithstanding that satisfactory precision was obtained. For maximum accuracy in analytical applications, standardization of the titrant against a primary standard solution of the metal baing determined is recominended.
LITERATURE CITED
(1) Boyle, Jr., W. G., Robinson, R. J., A N A L . CHEM. 30, 958 (1958). (. 2,) RlarDIe, T. L., RIatsuyama, George, B u r d e k L. IT’ Ibid., , IT 937. (3) Reilley, C. N.,Schi.eizer, B., Ibid., 26,1124(1954). RECEIVED for review February 23, 1960. Accepted June 28,1960.
