Some Properties of the Trace Heavy Metals Spectrophotometric Reagent, Di-P-Nap hthy1th ioca rbazone R. J. DuBOIS' and IIAMUEL B. KNIGHT Department of Chemistry, The University of North Carolina, Chapel Hill, N. C.
b The applicability (of the very sensitive di - fi naphthylthiocarbazone (DNZ) for quantitative separation and spectrophotometric determination of some heavy metals is compared with the commonly used reagent, dithizone (DN). DNZ offers inore than twice the sensitivity of DN with 25 metals investigated, Determinations can b e made in the parts per billion range. In this paper a liquid-solid chromatographic method of DNZ purification is presented. Investigation of those impurities separated suggests that the reagent need not b e repurified. M o d ified spectrophotometric titrations established the stoichiometry of the metal complexes. A chelon competition method allowe'j the determination of stability consttrnts for the Hg(ll) and Zn(ll) complexes.
-
T
spectrophotometric reagent D N liar been extensively used in trace niet,al malysis for many years ( 6 ); however, the more sensitive analog, DNZ. has been only partially investigated. The aiq~licabilityof D S Z for quantitative separirtions and spectrophotometric determination of trace amounts of the transi.,ion and platinum metals has been reported in many articles (%4> 7 , 3 , 10) but a thorough study of this reagent is lacking. The properties of IIKZ were similar in many respect,s to DiY. DXZ was soluble in organic solvents, such as chloroform and carhon tetrachloride. The reagent, howeve;.! was insoluble in water a t a p H of less than 11. Thewfore, two phase systems were necessary for analytical work. .Ibove a p H of 11, D S Z was partially sclluble in water, as evidenced by reddish coloration of the aqueous phase and a decrease in absorbance of reagent in the organic phase. Thus, unlike DK di-color spectrophotometric methods were used. The maximum molar absorptivity value for DNZ a t 650 mF was 67,000. This value was foun'31 by the indirect, method of Cooper and Kofron ( 6 ) . These values have now been verified with Hg(II), Z n ( I : ~ j %Cd(II), and Pb(I1) complexes. This value is a t'woHE
SESSITIVF:
1 Present address, Bacchus Works, Hercules Poffder Go., Magna, Vtah
fold increase in molar absorptivity over that of DK. The purit,y of the reagent as purchased from K & K Laboratories, Jamaica, S . Y., ranged from 30 to 50%. This was determined by comparison of the molar absorptivity of the impure reagent at 650 mp with that found indirect,Iy for the pure reagent. Several means of repurification have been reported (6, 8) but were unsatisfactory. In this paper, a n alumina liquid-solid chromatography column was used which gave faster and more efficient purjfication. The impurities were separated and collected. Investigation showed that these impurities were unreactive with all metal ions studied. Stability constants and combining ratios of the metal ion-DNZ complexes were determined. Modified spectrophotometric titrat,ioiis were used to ichiometry of the comt y constants were determined spect,rophotometrically by a method using a metal ion-metal ion complex system, such as Hg(I1)-Hg(I1) E D T * I ( I ) , in competition with DSZ. This method has been used in one-phase systems before but not in a two-phase system. This method was based on the competition of two chelons for the metal ion of interest. I n this technique, DNZ is used as one chelon competing for the metal ion. The second chelon has a known stability constant with the metal ion. This stability constant must be of the same order of magnitude as the metal ion-DXZ complex stability constant. By measuring t'he change in coricentration of the complex, MZn2 and the free reagent, Z, a t a wavelength where only these species absorb, K&B ' can be found. The wavelength of reagent maximum, A,, is used in t,his work. The relationship between Ki$%g and [MZ,] and [Z] can be shown. The two compet,itive reactions involved are:
MB e XI+.
+ F3
LIZn F2 ;\I+.
+ nZ
and
where €3 is one chelon and Z, the second chelon, corresponds to DXZ. Combining the two reactions,
MI3
+ nZ d XIZ, + 13
The equation for the stability constant for the reaction can be written as.
Only [MZ,], [Z], and Kg%B are unknown. The stoichiometr! numbrr, n, and KFTABABare knonn from previous work. The concentrations of the components of the metal ion buffer system, I3 and M13, are added in known amounts and remain relatively constant. This buffering system acts similarly t o a p H buffer. On rearranging Equation 1, so that
then K and finally Ki;%B can he found W Z n 1 and [ Z ] are obtained. if values of I% 1, Data from three iolutions are needed [LIZrA] for each -- Lalur obtained Each ~~
L-
[Z 1
of these three solutions contained an organic phase a i t h equihalent concentratioria of Z added To one iolution i b added only uater To the second solution is added an aqueous phaie a i t h known amounts of 13 and 1113 The concentrations of I3 and MB are such that some but not all of the reagent, Z, complexes 131th 13. To the third qolution is added nater equal in volumr to that of the first solution and containing an excess of metal ion, 11 If the absorbances of these three iolutions, designated -4?, .Ic. and . t b reyiectivell, are [LIZ ] measured a t A,, onc l a h e for -fl
If! 1
could be obtained The relationship betneen these abaorbanceq and [WZ,] and [Z] can be shoiin. If [MZ,] represents the total amourit of chelate n hen all the reagent is complexed, and if t equals the fraction of the chelate dissociated a h e n chelon I3 is added, then
[MZ,]= (1 - z ) [ N Z n l
(3)
and [Z] = nx[MZ,]
(4)
The absorbance of solution -4b a t h, results only from the chelate, LIZ,, remaining and the free reagent, Z VOL. 36, NO. 7 , JUNE 1964
1313
'
1
1
4
'
!
'
1
'
!
1
1
Figure 1 .
f
aZ[z]
(5)
where aWznand az are the molar absorbances of the complex and reagent, respectively, at A,. By substituting Equations 3 and 4 into Equation 5 ar,d subtracting A,,
and
A b
- -4,is
found proportional to
PI.
Similarly it can be shown that A , - .Ibis proportional to [MZ,] through the relation
24, - Ab
[lfz,](nu2 - U M Z , )
=
!
I
'
I
1
/ 11
'
'
1
1
1
I
I
DETERMINATION OF METAL ION-DSZ COMPLEX STOICHIOMETRY. Twentymilliliter aliquots of 1.00 X 10-5M DNZ
1
Infrared spectrum of impure DNZ
This may be expressed by: = aMZn[-\fZn]
1
7 8 9 1 0 WAVELENGTH (MICRONS)
5
extract the pure reagent with chloroform. -in alternative was to reflux the alumina with chloroform in a Soxhlet extractor. The chloroform was then evaporated a i t h a flash evaporator and the pure Dh'Z weighed. Ultraviolet, visible, and infrared (potassium bromide pellet) spectra were obtained for the red, blue, and pure reagent components. Hg(I1) solutions were shaken with the eluates and spectra run. Spectra were obtained for the impure reagent and the product of oxidation of the reagent with a 1 : 3 nitric acid solution.
700
I I I I I I I l l l . l ' l
1
3
03
1500
2000
4000 3000
(7)
and therefore,
Thus, through Equations 6 and 8, [Z] and [1IZ,] [Z] are related to absorbances. After converting Equation 2 to logarithms, log [Z] tis. log [ N Z , ] ' [ Z ] in terms of absorbances can be plotted. Equation 2 can now he used to determine K , the intercept, and through this
K$iZLB. I2xperimentally, it \vas necessarj to prepare the solution of the competing chelon and metal ion a t concentratioris high enough to remain constant in the range of reagent concentrations used. This solution was prepared by titrating the metal ion with the desired chelon in appropriate proportions to gain the desired effects. This method for determining KYTZB will work for a reagent or dye of unknown purity providing the impurities do not react with the metal ion in stoichiometric proportions differing from that of the complex being studied.
Suitable diquat's were then taken as necessary. The percent purity was obtained by comparison of t,he molar absorpt'ivity a t 650 mfi to that calculated for pure DXZ. METALIONSTOCKSOLUTIOSS.Solutions of 0.1Odf metal ion were prepared from C.P. grade reagents using deionized, distilled water. Ri(III), Hg(II), and Zn(1I) oxides were dissolved in perchloric acid. &(I), Tl(I), Pb(II), and Cd(I1) were prepared siniilarly from the metal nitrates. These solutions were standardized by accept'ed methods. Except,ions were hg(1) and Tl(I), which were weighed directly as the nitrate. Apparatus. T h e Cary Model 14 spectrophotometer and the PerkinElmer Infracord 137 were used for obtaining spectra and absorbance data. A l3urrell \Trist-.\ction Shaker with timer and adjustahle action 12-inch arm?: was w e d for extraction. Procedure. S E P A R A ~ ~ IOF O NIHP U R I T I E S A N D PURIFICATION O F I>NZ. Five-millilit er aliqiiots of a chloroform solution of :t known weight of impure D S Z were placed on 3 x l/;*-inch alumina rolrinins and eluted wit'h chloroforin. Four bands were collected. A red component arid a blue component werv rapidly eluted. The pure reagent and a trace brown cornponent were sei~aratetiwith difficulty. I t was necessary to remove the alumina mechanically, add water, and 4000 3000
2000
1500
CM.1
in chloroform were pipetted into separatory funnels. -1 1.00 x 10-5,v solution of the metal ion was adjusted to the pH of maximum extraction. Suitable aliquots of this solution were pipetted into the separatory funnels and the aqueous phases were brought to 20.0 ml. The samples were shaken for 15 minutes, the chloroform phase separated, and the absorbance measured a t 6jO nip Plots of absorbance us. milliliters of metal ion added gave the combining ratio of the complex. I)ETERMINATION OF STABILITY C O N STANTS. d solution 0.008M in Zn(I1)
and O.25OA11 in ethylene glycol-bis-(paminoethylether)-S,.2"-tetraacetic acid (EGTA\) was prepared. The pH was adjusted to 9.6 to 9.9. *\liquets of this solution were pipetted into separatory shaker< and the final volumes adjusted to 20.0 ml. Tnenty milliliters of 1.00 X 10-5M DXZ were added and the sample was shaken for 15 minutes. The chloroform phase was separated and the absorhance measured at 650 mp. .i second solution was prepared with 20.0 nil. of 1.00 X l O - 5 M DNZ and an excws of metal ion. The chloroform phase, of this solution and a blank wlution of 1.00 X 10-5M DYZ were measured at 650 nip. These data [-\lZn] provided a single - --- ratio. This
[Z I
proredwe was repeated, using 20.0 nil. of 2.00 X 10-551, 4.00 X 10-5A\l,6.00 x 10-5.v, and 8.00 x lO-5Jf solutions of DNZ. .A similar procedure was follonrd for H R ( D X Z )complex ~ except TITP-1 \\a$ w e d as the second chelon.
1000
900
800
EXPERIMENTAL
Reagents. D K Z STOCK SOLUTIONS. DXZ solutions were prepared by dissolving the appropriate weights of reagent in redistilled chloroform. 13 14
ANALYTICAL CHEMISTRY
Figure 2.
Infrared spectrum of alumina column purified DNZ
700
\
\
A
-
-D. ---E.
C
IMPURE REAGENT ALUMINA COLUMN PURIFIED REAGENT BLUE SUBSTANCE SEPARATED ON ALUMINA COLUMN REO SUBSTANCE SEPARATED ON ALUMINA COLUMN
I
W
v
z 4 m
r
0
Vl
m
a
WAVELENGTH, rnp
Figure 3. Typical spectra of the major impurities in DNZ separated on alumina column, impure DNZ and the pure DNZ
any change in spectra on shaking with a 1.O-W perchloric acid aqueous phase, If any metal complexes had formed, reversion to reagent would be expected on acidification. Stoichiometry of Several MetalDNZ Complexes. The combining ratios of Bi(III), Pb(lI), Zn(II), C d ( I I ) , H g ( I I ) , ’I’l(I)] and Ag(1) were determined by a modified spectrophotometric titration method. The results are listed in Table I. From the data obtained, there was no evidence of the sacondarj type complexes similar to those formed by D S (IO). The metal-DSZ complexes all appear to be of the primary type known for DX. For all reactive metals tested, the number of reagent molecules in the complex 1%as equal to the formal charge on the metal ion. Further work in this study was based on the assumption that the same relationship holds for all the simple metal cations complexing with
DXZ. Stability Constant Determinations. The method for determination of stability constants, described above, was attempted for Hg(1I) and Zn(I1). The K;’ZAng values mere determined for both. To calculate the Kk$2iB, the a factor was found from the p H values of the aqueous phases. This was used to calculate the ICE:? of the chelate
to the blue component. The reagent was very unstable in ultraviolet Purification of Reitgent. The yield light, the decomposed product being the for five samples of impure D S Z on blue impurity. The oxidation product alumina columns was 16.97, and the spectrum was analogous to that yielded purity averaged 9:1.87,. These reby DN (IO) under weak oxidizing consulth were superior to a n y obtained by ditions. Xone of the impurities showed available methods ( 5 , 8). However, chloroform solutions of very pure reagent were unstable. The purity decreaqed 20 to 30YGin 24 hours. ?;one of the impurities separated on Table I. Stoichiometry of Metal-DNZ Complexes the alumina colurnn xeacted with metal Xumber of ions complexing with the active DXZ. Metal Ion PH Combining Ratio Determinations Further, the absorbance of these im1 : 3 10 3 . 6a purities was negligible a t 650 mpC1, 1:2.03 3 0 where the most useful quantitative work 1 : 2 06 10 5 was done. Therefore, the reagent need 1 : 2 05 6.0 not be 1007, pure and its purity can be 1 : l 95 12.5 1 : l 02 10.7 eaqily calculated from the absorbance 1:1.02 3.0b spectrum of a known amount of impure DSZ. a 72% extraction. b Precipitates above 1.0 x lO-5M AgDSZ. The identification of the impurities proved difficult from the spectra obtained. The infrared spectra, including for the Zinc (11) and Table II. Stability Constants (KE(F2z’n) Figures 1 and 2, did not allow an unMercury(11)-DNZ Complexes equivocal determination of the impurities. Hon ever, the ultravioletA. Zn(I1) visible spectra, Figure 3, suggest that Reagent (DNZ) Run I Run I1 concentration added KgBA$,NZ’2(pH = 9.6) K ~ ~ . (pH ~ ~ = z ’9.9) 2 the blue and red components separated on the alumina coluinn are the major 2.24 x 1017 1 26 X lo1; 1,oo x 1 0 - 5 ~ impurities. ( S o attempt was made in 2 28 X 10” 3 . 7 0 X 10“ 2 . 0 0 x 10-%I 2 00 x 1017 2 . 2 7 x 1017 4 . 0 0 X 10-6.W Figure 3 to relate the true absorbance 1 05 x 1017 6.00 x 10-534 of one curve t o another.) The spectrum 2.18’ X 1017 1 02 x 10’7 8.00 x 1 0 - 5 ~ of the impure reagert shows peaks a t A ~ K . ; B ~ Z = =) 2 ~ . 1 3 x 1017 260 mp, 340 nip, and a shoulder a t apB. Hg(I1) proximately 600 mp. The blue component has a maxims a t 340 and 580 Reagent ( D N Z ) Run I Run I1 concentration added Kf$i.~vZ’2 (pH = 9.3) K~&%sZ’2 (pH = 9.3) mp, and the red componerit a t 260 mp. The purified reagent shows a definite 1 . 0 0 x 10-6M 1.10 x 1 0 3 4 3.50 X 2 . 0 0 x 10-6M 2.84 X 4.90 x 1 0 3 3 decrease in absorbance a t 260 and 340 4.00 x 1 0 - 5 ~ 2 . 1 4 x 1034 2.80 x 1 0 3 3 inp and a diminution of the shoulder a t 6 . 0 0 x 10-5’14 ... 2.80 X 600 mp. A ~ ~ .: + i : s ~ = ’ ~ 1.21 x 1054 The pure reagent in chloroform decomposed, yielding a spectrum identical RESULTS AND [)ISCUSSION
VOL. 36, NO. 7 , JUNE 1964
1315
formed betaeen the metal ion and competing chelon. For Zn(II), runs I and I1 were made using a buffer 0.250.V in EGT.l and 0.008.11 in ZnI3GT.I. The p H was 9.6 for run I and 9.9 for run 11, Table 11.1. Hg( 11) was investigated using diethylenetriaminetetraacetic acid (1)TP.i) ah the competing chelon. A buffered solution for run I of pH = 9.3 was prepared 0.100.11 in D W . I and 2.0 X lO-5JP in Hg1)TP.I. second buffer, run 11, also a t p H = 9.3, was prepared 0.100JI in DTP-1 and 5.00 X 10-4.1f in HgDTI’A. Stability constants
for Hg(DXZ), are reported in Table IIB. The stability constants obtained hold only for the chloroform-water system studied. .illthough D S Z is a weak dibasic acid, the stability constants have not been corrected for the effect of hydrogen ion competition effects. LITERATURE CITED
(1) ilikens, I). A , , I-niversity of North Carolina, Chapel Hill, X . C., Private
(3) Ibid., 18, 149 (1946). (4) Cholak, J., Hubbard, 11. Burkey, R . E., Ibid., 15. 754 (1943). ( 5 ) Cooper, 6 . S., Kofron, T‘. K., ASAL. CHEY.21, 1135 (194‘3). (6) Fischer, H., Angew. Chem. 47, 685 (1934) j - . _ -
(7) Grzhegorzhevsky, A. S., J . Anal. Chem. ( C S S K ) 9, 121 (1954). (8) Hubbard. D. M., AKAL. CHEY. 28, 1802 (1956). ( 9 ) Liartin, A . E., I b i d . , 2 5 , 1853 (1953). (10) Sandell, E. R., “Colorimet,ric Det,ermination of Traces of L\let,als,” Interscience, Yew York, 1959.
Communication, 1962.
( 2 ) Cholak, J., Hubbard, I). M., IXD. ENGCHEM., .ANAL ED 16, 333 (1944).
RECEIVED for review Xovember 12, 1963. Accepted March 19, 1964.
APPl cation of Di-P-Naphthylthiocarbazone as a Quantitative Trace Heavy Metal Spec ro photo met ric Reage nt RONALD J. DUBOW and SAMUEL B. KNIGHT Department o f Chemistry, The University o f North Carolina, Chapel Hill, N. C.
b The use of di-b-naphthylthiocarbazone (DNZ) as a quantitative trace heavy metal spectrophotometric reagent rather than the commonly used reagent dithizone (DN) is described. Investigations of 2 7 metal ions, including those complexing with DN (all metal ions complexing with DN also react with DNZ) a r e presented. Molar absorptivities a t the metal ionDNZ complex spectra maxima a r e compared with the values for similar complexes of DN; in all cases, DNZ forms complexes with greater a b sorptivities. A method for the separation and determination of mixtures of Hg(ll), Zn(ll), Pb(ll), and Biflll) using DNZ is presented. This method is successful in identifying trace amounts of one metal ion among excesses of other metal ions. The theoretical lower sensitivity limits for several metal ions with DNZ a r e predicted.
T
H F GREATER S1,KSITIVITY O f D x Z over DS for spectrophotometric analyiis of certain heavy metals has been recognized by several researchers (8-4, 6, 7 ) , honever, a survey of the total icope of the uqeq of D S Z as a spectrophotometric reagent does not appear in technical literature. In thiz study, all metals complexing with D S were investigated. Other mctalu that border theye DS-reartives on the Periodic Table !\ere examined. Spectral data n e i c obtained for the
Present address, Hercules Powder Co ,
Racchue K o r k s , Magna, I’tah
1 3 16
ANALYTICAL CHEMISTRY
metal complexes. From these data, molar absorptivities a t the wavelengths of the complex maxima were calculated. Complex absorptivity was deteimined by using the change in absorbance a t 650 mw, the reagent maximum. The change in reagent concentration was used to calculate the complex concentration. The qtoichiometry of the metal ion-DKZ complexes was based on data previously obtained ( 5 ) . After correction for the absorbance of excess reagent at the complex maximum wavelength, the complex molar absorptivity was found. These data were obtained under pH conditions of maximum extraction. Per cent extraction us. p H characteristics was obtained for those metal ions complexing with DNZ. These metal ions were divided into two groups: the nitrates of I3i(III), Cd(II), Pb(II), Zn(II), Hg(II), Tl(I), and dg(1) were studied extensively, using only perchloric acid and sodium hydroxide to adjuct the pH; the second group was investigated less thoroughly. Two wavelengths could be used where quantitative measurements were most sensitive: the wavelength corresponding to the maximum of the complex, and the 650-mg maximum shown by D S Z . The former was less sensitive than the latter. The former wavelength required correction for spectral overlap and the presence of impurities. [When these corrections were made, much lower absorptivities were found than reported by Grzhegorzhevsky (S)]. The 6 5 0 - m ~ readings
showed small overlap from the complex spectrum and none from the impurities; therefore, the per cent purity of the reagent need not be known for eithcr quantitative or molar absorptivity work. Standard working curves uere prepared for the metal ions of interest, using 650 mw. EXPERIMENTAL
Reagents. Di - p - naphthplthiocarbazone was purchased from K 8: K Laboratories; Jamaica, Long Island, ?;. Y. D S Z solutions (in chloroform) were prepared from a reagent of known purity. .1 5.00 X 10-*51 (8.43% w./w.) stock solution was prepared and appropriate dilutions were made depending on the purity of the reagent. Metal ion stock solutions were prepared, 0.10.V in the metal ion and 1.00M in the perchloric acid, diluted with deionized distilled mater, and standardized. Ammonium citrate buffer solution, O.lOMj adjusted to a pH of 9.5 to 9.7. Procedure. PER CENTMETALI O N EXTRACTED us. p H . Stock metal ion solutions of Ag(I), T l ( I ) , Hg(I1). P b ( I I ) , C d ( I I ) , Zn(II), and Bi(II1) were diluted to provide 1.40 X 10-6 1.20 X lo-&, 1.00 X loc, and 8.00 X 10-6M solutions. The pH was adjusted with perchloric acid and sodium hydroxide. Twenty-milliliter aliquots were taken and added to 20.0-ml. aliquots of the 5.00 X 10-6M DNZ chloroform solution. A blank was also prepared. Samples were shaken for 15 minutrs with a Burrell automatic shaker. The chloroform phase was separated and scanned using a Cary Model 14 recording spectrophotometer. The pH
