Investigation of the Substoichiometric Extraction of 14 Metals from Sulfuric, Hydrochloric, and Perchloric Acids with ZincDiethyldithiocarbamate by Radiometric Extractive Tit ration Armin Wyttenbach and Sixto Bajo Swiss Federal Institute for Reactor Research, 5303 Wiirenlingen, Switzerland
The order found for successive extractions is Au3+, Hg2+, TI3+, Ag'+, Cu2+, Bi3+, Sb3+, Te4+, !doe+, Se4+, In3+, As3+, Pb2+, Cd2+. In all cases, except Au3+ and Mo6+, the number of diethyldithiocarbamate (DDC) ligands attached to the extracted complex is equal to the valency of the metal. Non-extractable precipitations are formed by Agl+ and Hg2+. From CI-containing solutlons, two complexes with the stoichiometry MeDDC, and MeDDC,. CI are extracted where Me = Hg2+, As3+, Sb3+, Si3+, or Te4+; their exchange constant was determined and their electronic spectra were measured. Extraction is complete within 15 sec., except for Mo6+, Se4+, Te4+, In3+, and As3+; these metals have a half-time of extraction of the order of 1 minute. In many instances, replacement reactions involving MeDDC are much slower than extractions with Zn(DDC)2. Possible analytical applications are discussed.
The diethyldithiocarbamate anion (CzH&NCSz- (in the following denoted by DDC) is a common reagent in analytical chemistry; with many elements, it forms complexes extractable into organic solvents. Sometimes DDC is preferred to the related and more familiar dithizone because of its greater stability against oxidation and acid decomposition and the greater solubility of its complexes. Extractions with suprastoichiometric amounts of DDC have been extensively investigated ( 1 - 3 ) and several reviews have been published ( 4 - 6 ) . The principle of substoichiometric extraction was introduced by Ruzicka and Stary ( 7 ) , and the subject has been recently surveyed (8, 9). Substoichiometric extraction involves the use of a chelating agent, the quantity of which is chosen so as to extract only a part of the metal present. The rapidity, simplicity, and high selectivity of this method make it very attractive. When applied to radiochemical work, it avoids the need to determine the yield of a separation. This is a distinct advantage in destructive activation analysis, for example. There is also the possibility, proposed by Elek et al. (IO), of doing multielement separations. One prerequisite to the application of substoichiometric extractions to radiochemical work is a detailed knowledge of the extraction behavior of every activity ( i e . , every element) which may be present in the sample. Information that is usually required includes the stoichiometry and the kinetics of extraction and the order in which the different elements are extracted. In the case of DDC, insufficient and even misleading information was found in the literature. We therefore decided to examine the extraction behavior of elements which can be extracted from solutions with an acidity of 0.1 or higher.
EXPERIMENTAL Reagent. Zn(DDC12, 5 X 10-3M in CHC13. Zn(DDC)2 was precipitated with NaDDC from a solution of ZnS04, then washed and dried. It was dissolved in CHC13 and recrystallized by addition of 2
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
CzHsOH and slow evaporation of CHC13. Zn(DDC)Z in CHC13 is preferred to an aqueous solution of NaDDC, because HDDCwhich is formed upon acidification-has a half-time of decomposition of only 7 sec in solutions of pH d. Curve IV is representative of the initial extraction of a complex with the stoichiometry DDC:Me = c and the transformation of this complex into a non-extractable compound when R > c. Mixed Complexes with C1. The frequent occurrence of curves of type I1 in extractions from aqueous phases containing C1- suggested the participation of C1 in the initially extracted complex with subsequent replacement of C1 by DDC. Evidence in support of this mechanism was obtained by monitoring the amount of 38Cl extracted into the organic phase as a function of R. In all cases tested (Hg*+,A$+,
Table I. Substoichiometric Extraction from HzSO4, HC1, and HC104a Metal
Ag"
HC104
HC1
H2W4
As in H2SO4
111 (d = 5/6, C. = 1). A precipitation i s formed in the aqueous phase, which begins to dissolve at R = 0.83; dissolution i s complete at R = 1. 6 Ag' 1 5 DDC- T X- -+[Ag,(DDC),X], for R 5/6.
Hg2'
III (d = 1, = 2). The formation of Hg(DDC)2has been shown to proceed in two steps.d hence the extraction i s described by Hg2' + DDCHg(DDC)for R < 1. Hg(DDC)- I DDCIH~(DDC),~,,, f o r R > 1. +
-
The formation of an insoluble precipitation was already noted by Bode (3). Analysis of the precipitation formed in HC104 showed it to contain C1 and Ag in amounts consistent with the formula Ag,(DDC),C10,. Contrary to this, Kukulabfound no precipitation and an extraction curve of type I when extraction is made from an aqueous solution of pH 7- 11 containing NO3-, citrate, and EDTA. The complex extracted was shown to be a hexamer.c
Idem
Id et t 1.
IdC.I?l.
IdWl.
Conditions where [Cl] > 0. liV must be avoided, since complexation of Cd in the aqueous phase will prevent its complete extraction. Extraction i s complete at R = 2 only if [HI 5 0.01N.
I1 ( ( I = 1, h = 2). The greater [Cl], the more i s curve type I approached. This behavior i s explained by the simultaneous e x traction of Hg(DDC)Cl and Hg(DDC), ( 1 5 ) .
111 (d = 1, e = 2). A precipitation i s formed in the aqueous phase, which completely dis solves at R = 2 . The precipitate has been shown to have the composition Hg(DDC)ClO, (15). Hg2+ + DDC- + C10,- --t [Hg(DDC)ClO,], for R < 1. [Hg(DDC)C104js+ DCC---t [Hg(DDC)210,, + Clodfor R > 1.
Even traces of C1- in H2S04o r HClO, will change the extraction b e havior to the one given for HC1.
Extraction is complete a t R = 2 only if [HI 5 0.1Zv'.
Pb2'
InY+ I ( b = 3). In3' + 3 DDC[In (DDC),],,,
Remarks
Idern.
Conditions where [Cl-] > 0. liV must be avoided, since complexation of In in the aqueous phase will prevent its complete extraction. Extraction is complete at R = 3 only if [ H J 5 O.LV.
I1 (a = 2, b = 3). Simultaneous extraction of As(DDC),Cl and As(DDC),.
Idem
Formation of As(DDC),Cl is very small for [ c l ] = 5N.
11 (a = 2 , b = 3). Simultaneous extraction of Sb(DDC),Cl and Sb(DDC),.
Idem
-+
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
3
'TableI. (Continued) HC 1
H2S04
Metal
Bi3+
I (6 = 3). Bi3' + 3 DDC-
HC104
I1 ( U = 2, 6 = 3). Si mu It aneous extraction of
---t
[Bi (DDC),],,,
Ideni.
Increasing I C1-1 to >e\] or introduction of Clodchanges the extraction curve to type I with 6 = 1. In the case of Clod-, this is due to the formation of [ A U ( D D C ) ~ ] Cfor ~ OR ~ > 1, which is more soluble than its C1-homolog ( 1 4 ) .
IV ( c = 1 ) .
Extraction is due to Au (DDC)Cl,, which changes to the only partially soluble [AuDDC],Cl for R > 1 (14).
Idem.
The maximum formation of Bi(DDC)*Cloccurs at [Cl] 0.08N.
-
Bi(DDC)&l and Bi(DDC),.
Au3'
Remarks
1,V: I d a n . 0.1iV: I ( 6 = 3.5).
This reason for this unexpected stoichiometry was not investigated. Te4'
I (6 = 4 ) . Ted' + 4 DDC[Te (DDC),],,,
(0 = 3, h = 4). Simultaneous extraction of Te(DDC)&l and Te(DDC),.
I1 -P
Id em.
Formation of Te(DDC),Cl i s very pronounced if [Cl] P 1, the extraction curve being almost of type I with 0 = 3.
Irlert1.
The crystallized complex was shown to be M o o z (DDC)z.oThe color of the extracted complex is greenish yellow. but turns to reddish if R > 2. The complex i s said either not to extract into CCldf or to be only slightly soluble in CHC1, ( 2 ) , e but we had no difficulties to extract 6 mg of M o into 50 ml of CHC1,.
a Unless especially noted, results of extractions from 0.1Nand L V acids are identical. Roman numbers refer to the different types of extraction curves (see text). The subscript "org" denotes species in the organic phase, "so are solids; species in the aqueous phase have no subscript. In general, no distinction is made between mono- and polynuclear complexes. F. Kukula, Isotopenpraxis, 6, 303 (1970). s. Akerstrom, A r k . Kerni,14, 387 (1959). d Y. I. Usatenko and F. M. Tulyupa, Zh. Neorg. Khirn., 4, 2495 (1959); Chern. A b s t r . , 54, 16249 (1960). e F . W . Moore and M. L. Larson, Inorg. Chern., 6, 998 (1967). f J. Stary, Physical Chemistry, Series One, Vol. 12, Butterworths, London, 1973,p 279.
Sb3+,Bi3+, Te4+) curves of the type shown in Figure 3 were obtained. These curves can be explained by the following equilibria (formulated for the case of Bi3+ only): Bi3+ + C1and 3 [Bi(DDC),Cl],,
+
2DDC-
+==
[Bi(DDC),Cl],,,
2[Bi(DDC)3]Org+ Bi3'
(1)
+ 3 C1' (2)
T h e equilibrium constant K of reaction 2 is [Bi(DDC)2C1],3 [Bi(DDC)2Cl],,e3CY [Bi(DDC) 3]me2[Cl'] 3[ Bi3'] - [Bi(DDC) 3]oro2[C1-]3(Bi)(3) In Equation 3, (Bi) denotes the concentration of all forms of Bi in the aqueous phase, LY is the Ringbom's coefficient defined as CY
4
= 1
+
n
Cpn[Cl]" "=l
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
The overall stability constants 6, were calculated from the formation constants for successive chloride complexes given by Goldstein (12). By simultaneous measurement of the quantities of C1 and Bi extracted into the organic phase a t a given value of R and [Cl], all concentrations in Equation 3 were determined and K was established. Repeating this procedure for different values of R and [Cl] gave a series- of values for K , which for a given element should all be the same. The results for Hg, Bi, and S b are given in Table 11. As and T e could not be treated in this way because appropriate 3 /-, values are lacking. Mixed complexes are formed only if DDC is substoichiometric t o the metal ( i e . ,R < b ). Mixed complexes are not encountered in extraction under the usual (suprastoichiometric) conditions because for R > b we have (Me) 0 and hence complete transformation of the mixed complexes into pure Me-DDC complexes. Up to the present time mixed C1-DDC complexes were +
i
I -
*R cl-b-1
,I? a
b
Figure 3. Fraction of chlorine extracted ( F ) vs. molar ratio DDC/Me
b
Figure 1. Fraction of metal extracted ( F ) vs. molar ratio DDC/Me ( R )
(R)
Table 11. Constants for the Equilibrium between P u r e DDC and Mixed DDC-Cl Complexes (Equation 3) a Range covered by experiments Element
log K
Hg
14.410.1 8.5*0.3 7.5*1.0
Bi Sb V
-
R
No. of points
m R
28
0.2-1.7 0.6-2.4 0.5-2.4
13 28
in [ C l 1
0.002,\'-55 0.025-15 0.05,\'-0.5.Y
The error indicated is the standard deviation of an individual measurement. a
Figure 2. Fraction of metal extracted ( F ) vs. molar ratio DDC/Me ( R )
known to exist with Fe ( 6 ) , Pd (13), Re ( 6 ) , Pt ( 6 ) , Au ( I d ) , and Hg (15). T h e addition of As, Sh, Bi, and T e to this list shows that the occurrence of these compounds is quite frequent. It is interesting to note that extractions from aqueous phases containing c104- also yield extraction curves of type I1 with As, Sb, Bi, and Te. Although no direct proof (such as monitoring the extraction of C104- into the organic phase) was attempted, the formation of mixed DDC-C104 complexes in these cases seems to he a plausible hypothesis. The electronic spectra of the complexes of As, Bi, Se, and Te, extracted under substoichiometric conditions from HzS04, HC104, and HCl into CHZC12, were checked for possible differences between the pure DDC complexes and the mixed complexes. With each element, the hand positions were identical for the pure and the mixed complexes, and the band intensities proved to he proportional to the concentration of DDC ligands and not to the concentration of metal atoms. This is in agreement with the attribution of these bands to intraligand transitions ( 1 6 ) . The data are given in Table 111; bands I and I1 are x T* transitions (17). T h e agreement of the hand positions with those given by Nikolov (17) for the pure complexes in CHC13 or C,3HI4 is good. An exception is the band position for Se where hand I is found a t 233 nm and not a t 253 nm. E x t r a c t i o n Kinetics. The extraction kinetics of single elements were generally evaluated using a quantity of Zn(DDC)2 sufficient to extract 50% of the metal present. In the majority of cases (Au, Hg, Cu, Bi, Sb, Te, Cd, Pb), this expected extraction occurred within less than 15 seconds. However, Mo, Se, In, and As were extracted completely only after several minutes. In these cases, the rate of extraction is of pseudo-first order with respect to the metal; log (1 - F t / F m )decreases linearly with the extraction time, F , and F , being the fraction of metal extracted at time t and at equilibrium, respectively. Periods necessary to extract half of the expected amount ( ~ ~ 1 are 2 ) of the order of 1 minute for these elements. In the substoichiometric extraction of In from HC104, it was further found that 7112 depends on the initial composi-
-
Table 111.Electronic Spectra of P u r e a n d Mixed DDC Complexesa Central atom
As
Band I (log
Band I11 (log
Band I1 (log c )
E)
254 (4.23)
E )
3 60
237 s h Bi
-
258 (4.29) 236 s h 233 (4.20) 256 (4.30)
Se Te
. 275 (4. 10) 276 ( 4 . 2 2 ) s h
362
425 (2. 7)
a The position of the bands is given in nm and their intensity as log t with respect to the DDC concentration.
tion of the system. When all other variables are held constant, 7 1 / 2 is proportional to (DDC)initial, (In)-O%itial, and to (H)-0.16. The situation is more complex when the aqueous phase contains more than one extractable metal (the amount of DDC still being substoichiometric to both of them). In this case, equilibrium is often not attained as expected from the extraction half-times of the single metals. This is due to the competition of both metals (e.g., A and B) for the ligand: A+ B'
+ +
DDCDDC-
--
(4) (5)
(ADDC),,, (BDDC),,,
If the rate of reaction 5 is similar or higher than the rate of 4, there will be simultaneous extraction of ADDC and BDDC. Subsequently a replacement reaction 6 (the extraction constant of A being higher than of B) will take place between the BDDC and the amount of A that is not yet extracted: A'
+
(BDDC),,,
-.+
(ADDC),,,
+
(6)
B'
It was found in many instances that reaction 6 is slower than 4 and 5 and is thus determining the overall rate of extraction of metal A. ANALYTICAL C H E M I S T R Y , VOL. 47, NO. 1 , J A N U A R Y 1975
0
5
I
24
10
32
50
40
t
z
(mm)
Figure 4. Fraction of In extracted ( F ) vs. shaking time ( t ) Curve 1 gives the extraction from a solution containing only In3$-, curve 2 from a solution containing equal amounts of In3+ and As3+. The expected extraction (F,) is 0.13 in both cases.
1 Table IV. Values of n log Kex Solvent
cc14a
Metal
CHC13b
15 11.9 12.6 7 6.6 5.6 4 4 3.4 2.2 2.9 1.4 1.3 DValues quoted by J. Stary (see footnote f , Table I). b E. Still, Finska Kernisteamfundets M e d d , 73, 90 (1964), gives two-phase stability constants, which have been converted into extraction constants using 2360 as the partition coefficient of HDDC in CHC13 and 4.5 X 1 0 - 4 as the dissociation constant of HDDC. Hg2' Ag" cu2+ Bi3' In3+ Pb" Cd2' Zn2+
An example of this phenomenon is given in Figure 4, where A = In3+ and B = As3+ and the extraction of In is shown. Curve 1 is obtained when In is the only extractable species (7112 0.7 min) and curve 2 when both In and As are present in the aqueous phase. In this latter case, the initial steep rise of the curve is caused by reaction 4, which however does not proceed to the expected value of F , because part of the DDC is used up by reaction 5 (71/2 0.4 min). The slow rising of the curve up to F is caused by reaction 6 (TI/* 18 min). Although not all possible pairs A/B were tested, several qualitative trends were noticeable for the rate of the replacement reaction 6: a) In most instances the rate is greater for (H) = 0.1 than for (H) = 1. b) The rate decreases with increasing proximity of the two metals involved in the order of extraction, e.g., As(DDC)3 Bi3+, Te4+, Mo6+, Se4+, I++*. 7 1/2 < 0.1 min, -0.8 min, -5 min, -10 min, -10 min, respectively. c) Complexing of metal B in the aqueous phase with C1- speeds up reaction 6, e.g., Bi(DDC)3 Cu2+: 71/2 < 0.1 min in 1N HCl, -1 min in IN HC104. d) Complexing of metal A in the aqueous phase with C1- slows down reaction 6, e.g., Mo02(DDC)2 Hg2+: 7112 < 0.5 rnin in 1N H2S04,5 rnin in 1N HC1. e) Se(DDC)4 seems to be a very inert species that does not react a t all within 15 min with T e or Cu; Te(DDC)4reacts only slowly with Cu (7112 20 min) or Bi (rl/z 1 min).
-
-
-
Figure 5. Simultaneousextraction of Cd2+ and In3+
Order of Extraction and of Replacement. When a sample containing several metals is extracted with successive substoichiometric increments of reagent, the metals are extracted in a certain order. I t follows from the general theory of substoichiometric extraction ( 7 ) that this order is given by decreasing values of ( l / n ) log K',,, where the conditional extraction constant KIeXis given by
Unfortunately there is little information on the extraction constants K ex of DDC complexes in CC14 and very little in CHC13; the pertinent data are summarized in Table IV. Since these values are not sufficient to predict the extraction of all elements considered here, their extraction order was established by taking extraction curves for mixtures of two elements. The extractions were made from 0.1 N HzS04 (except for Pb), and great care was taken to avoid incorrect interpretation, arising from kinetic effects. The following order was found: Au3+, Hg2+, T13+,Agl+, Cu2+, Bi3-k Sb3+ Te4+ Mo6+ Se4+ In3+ Ass+ Pb2+ Cd2+ Zn2+. T i e daia of +able fV are' in aicord k t h h i s orier. I t should be borne in mind that the presence of complexing anions will sometimes change the order of extraction. For example, relative to the one given above, the order of the pair In/As is reversed in 1N HCl due to the large cy value for In-Cl complexes. The established order shows that within a series of homologs the values of ( l l n ) log K decrease with decreasing atomic number: Bi > Sb > As; Hg > Cd > Zn; T e > Se; T1 > In. When the values of ( l / n ) log K,, of two metals, Am+ and B"+, differ by less than approximately 2 units, there will be some extraction of B before all of A is extracted. This situation is described ( I O ) by
+
+
+
-
6
-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
I t is therefore possible to get some quantitative information on the K,, values from the extraction curves. Figure 5 gives the values of ( (Cd)/(CdDDC2)org)1/2 us. ((In)/ (InDDC3))1/3 for 4 extractions of a mixture of Cd and In in 0.1N HClO4 with different substoichiometric quantities of reagent. The slope of a straight line through the data and the origin gives ( K I ~ ) ~ /cd)ll2. ~ / ( KThe experimental value is 6.6, giving 1/3 log Krn - 1/2 log KC^ = 0.82, which compares favorably with the value of 0.74 expected
Table V. Apparent Deviations from the Order of Replacement Replacement occups m Replacmg Con p k x
Se(DDC), Sb(DDC)? Bi(DDC), In(DDC)?
H2S04
HC104
Hg2+ Bi"
yes yes
yes yes
Sb" As +
no
no
no
no
Lon
I + HC1
0.11HC1
no
no no
partiallv
yes
no
from the K,, values given by Stary ( 1 8 ) (log K I , = 10.34, log K C d = 5.41). It was not always possible to represent these data by straight lines through the origin. This was almost certainly due to the fact that for some pairs equilibrium was not completely attained even though prolonged extraction times were used. However, it can be said that the difference in the l f n log K,, values are of the order of 0.1 for In/As and 0.3 for Mo/As and for TefSb. The established order of extraction should also be the inverse order of replacement. T h a t is, a given metal A in the aqueous phase, when shaken with an organic solution of BDDC, should replace B from its complex if A stands on the left hand side of B in the order of extraction. This was checked by separate experiments and shown to hold. Some notable exceptions, specified in Table V, were caused by complexation either of A or B in HCl, others were caused by vanishingly slow reaction rates. A good example for inertness is Se(DDC)4 which, in HC104 or H2S04, can be replaced by Hg2+, but not as expected by Bi3+, Cu2+, or Te4+. Analytical Applications. A necessary condition for quantitative determinations by substoichiometric extraction as originally proposed by Stary ( 7 ) is the extraction of equal quantities of metal by equal quantities of reagent. It can be seen from the present work that in the systems studied ( H 2 S 0 4 ,HC104, or HCl), no difficulties arise for the extraction of Cu2+, Mo6+, In3+, Cd+*, and Pb+' with Zn(DDC)Z. However, the above condition cannot be met with metals forming mixed C1-DDC complexes without
stringent control of the C1 content. Therefore, in the case of Hg2+, As3+, Sb", and Te4+, substoichiometric extraction does not seem very useful in practical analytical work. A more attractive application is the complete extraction of one or several elements from acid solutions by the use of suprastoichiometric quantities of MeDDC as reagent, where Me is a metal standing immediately to the right of the sought element(s) in the order of extraction. This mode of operation can yield very pure fractions with only one extraction and without the need to add masking agents or to adjust the pH. Some work along these lines was recently published for three ions ( 1 9 ) ,but lack of knowledge of the order of extraction prevented a more general application. It should also be remembered that applications to systems with many cations will entail replacement reactions, some of which are slow to occur. Finally, it will be possible in special cases (as with Se4+ and Te4+) to take advantage of the inertness of the DDC complex in order to arrive at specific separations. LITERATURE CITED G. Eckert, Fresenius' Z. Anal. Cbem., 155, 23 (1957). H. Forster, J. Radioanal. Cbem., 4, l(1970). H. Bode and F. Neumann. Fresenius' Z. Anal. Cbem., 172, 1 (1960). G. D. Thorn and R. A. Ludwig, "The Dithiocarbamates and Related Compounds," Elsevier, New York, N.Y., 1962. A. Hulanicki. Talanta, 14, 1371 (1967). D. Coucouvanis, Progr. lnorg. Chem., 11, 233 (1970). J. Ruzicka and J. Stary. "Substoichiometry in Radiochemical Analysis," Pergamon Press, Elmsford, N.Y., 1968. J. Stary and J. Ruzicka, Talanta, 18, 1 (1971). N. Suzuki, Jap. Analyst, 21, 532 (1972). A. Elek eta/.,J. Radioanal. Cbem., 4, 281 (1970). J. Joris eta/.,Anal. Cbem., 41, 1441 (1969). G. Goldstein, "Equilibrium Distribution of Metal-ion Complexes," ORNL3620, USAEC, 1964. G. B. Briscoe and S. Humphries, Talanta, 16, 1403 (1969). F. Kukula eta/., J. Radioanal. Chem., 3, 43 (1969). A. Wyttenbach and S. Bajo, Helv. Cbim. Acta, 56, 1198 (1973). C. K. Jprgensen, J. lnorg. Nucl. Cbem., 24, 1571 (1962). G. St. Nikolov eta/.,J. horg. Nucl. Cbem., 33, 1059 (1971). J. Stary and K. Kratzer. Anal. Cbim. Acta, 40, 93 (1968). A. Elek, J. Radioanal. Chem., 16, 165 (1973).
RECEIVEDfor review April 9, 1974. Accepted August 16, 1974.
Fluorometric Assay for 5-Hydroxytryptophan with Sensitivity in the Picomole Range K. H. Tachiki and M. H. Aprison Section of Neurobiology, The Institute of Psychiatric Research and Departments of Biochemistry and Psychiatry, Indiana University Medical Center, Indianapolis, Ind. 46202
Optimizing conditions for the reaction of 5-hydroxytryptoPhan (5-HTP) with o-phthaldialdehyde has provided a sensitive method for the fluorometric assay of 5-HTP isolated from brain tissue. Samples from tissue containing as little as 20 Pmol of 5-HTP gave fhorescent values which are greater than twice blank values, whereas in the case of standards, levels as low as 3 pmol can be assayed. The increased sensitivity has made it possible to measure the endogenous levels of 5-HTP in small areas of the brain of the rat.
The compound n- phthaldialdehyde (OPT) is known to react with various compounds to form colored and/or fluorescent products ( 1 - 5 ) . The reactivity of OPT to a given compound is dependent on the conditions of the reaction such as temDerature. acidicitv or basicitv. concentration of OPT, and reaction time. Mailkel and Mrller ( 5 ) found that by employing acidic conditions, the O P T reagent had a degree of specificity for 3,5-substituted derivatives of indole compounds. Using this fact, they published a fluorometric A N A L Y T i C A L CHEMISTRY, VOL. 47, NO. 1, J A N U A R Y 1975
7
