Spectrophotometric Determination of Microgram Quantities of Disodium Dihydrogen Ithylenediamine Tetraacetate OSCAR MENIS, H. P. HOUSE, and 1. B. RUBlN Analytical Chemistry Division, O a k Ridge N a t i o n a l Laboratory, O a k Ridge, Tenn.
4 rapid spectrophotometric method for the determination of microgram quantities of disodium ethylenediamine tetraacetate (EDTA) is based on the absorbance of the complex formed bg the reaction of EDTA with an excess of cupric ion. The copper-EDT4 complex is developed in a sodium dihydrogen phosphate buffer of pH 11. Absorbance is measured against a copper sulfate-buffer reference solution at a wave length of 250 mp. The effects of several variables were studied and optimum operating conditions established. Nickel, cobalt, and chromate interfere; cations that form weaker complexes with EDTA than copper do not interfere. The relative standard deriation for samples containing 125 to 500 y of EDTi was 89k; for 500 to 1500 y, 297G.
D
ISODIUM dihydrogen ethylenediamine tetraacetate (EDTA) has, in recent years, found wide application in analytical and other branches of chemistry. The quantitative determination of this compound is, accordingly, of considerable interest to the analytical chemist. A limited number of methods for the determination of EDTA have been reported ( 3 6 ) . Of these, a spectrophotometric method out,lined briefly in a bulletin issued by t h e Bersworth Chemical Co. ( 2 ) , xhich makes use of the color of a complex formed when EDTA reacts with cupric. ions, was selected for the determination of microgram quantities of EDT-4. As little or no detail is given, a number of variable factors affecting the procedure were studied. These included wave lengths best suited for measurement of absorbance, buffer concentration, reference solutions, and escess of chromogenic reagent. Studies were also made of interferences and of the stability of the complex with temperature. A satisfactor>- analytical procedure has been developed which incorporates a n u n Iwr of important modifications of the Bersworth procedure.
shown in Figure 1, the absorbance of this complex, when measured against a blank consisting of a solution of the reagents in a buffer solution, exhibits a maximum in the ultraviolet region of the spectrum between wave lengths 230 and 260 mp. Over this band, t h e absorbance is practically constant. Accordingly, a point near the center of this region, 250 mp, was chosen for use in the determination of EDTA. At this wave length, the absorbance was found to be much greater than a t the wave length recommended by Bersworth-280 and 340 mp ( 2 ) . Furthermore, small shifts in the wave length have little effect on the absorbance in the spectral region near 250 mp) whereas a t 280 or 310 mp the absorbance changes rapidly with small changes in wave length. Effect of Buffer Concentration. The complex is formed in a disodium hydrogen phosphate buffer solution adjusted to p H 11 with sodium hydroxide. The data presented in Table I indicate that minimum absorbance of the reagent blank and maximum precision are attained a t 0.1M disodium hydrogen phosphate. Calibration Graph. I t was established that the absorbance of the EDTA-copper complex, when measured against a copper sulfate-buff er reference solution, conformed to Beer's law. When water, buffer, or EDT;I solutions \yere used as reference solutions. the absorbance-concentration graphs m-ere nonlinear and
0 500
0.400 I-
EXPERIIENTAL
(1 0
Absorption Spectrum. EDT.1 react3 with cupric ion in a huffercvl wlution of pH 11, to produce a colored comples. .is
4"
+
03OC
w
u
z 4
m
a
0
Table I. Condition-.
B iifii>r, .if
:
0 2oc
.ibsorbance Index f o r EDT.4 as a Function of Buffer 3Iolarity Buffer eoliition. NazHPOI E D T h , 20. 30. 40, a n d GO Y per nil Final volume, 50 ml. PH, 11 Reference solution, -1 nil. of 0 . 2 5 ' ; C l i h O ~diluted tu 2.3 ui1. n i r i i bufrrr solution Reference Solution, Absorbance
Ab-orhance 1ndes.n -1v.. 0 -
0 IOC
Standard Deviation n /c
0 DOC
43 16
0
7
260
300
340
4 0
W A V E LENGTH, m p
?
17
Figure 1. Absorption spectra of copper-ethylenediamine tetraacetate in ultraviolet region
Abhorbance index = a s = -5' cb rT-rhere .4s = absorbanrr c = concentration. g . / l O O O p r a i i i ? (of solution b = length of light path, cni.
Absorbing Solution I 49 X 10-4 .TI Cu-EDT.4 €3. 1 49 X 10-4 31 Cu-EDT.1 c'. CUSOI - Sa?HI'Os .A.
1433
Reference Solution H20 CuSO~-Na~HI'O~ IinO
1440
ANALYTICAL CHEMISTRY
Table 11.
Effect of Excess Reagent on Absorbance of Complex and Reference Solutions
Table 111. Interference of Various Ions Conditions.
EDTA 50 y per ml. CuSO4 kolution, 0.25 w./v. % Absorbance Excess CUSOI, 0.25% S o h , Cu-EDTA Reference 311. complexa solution b 0.15 0.422 0.090 0.5 0,420 0.095 1.0 0.423 0.087 2.0 0.431 0.091 3.0 0.425 0.093 5.0 0.420 0.083 Relative standard deviation, 1% Conditions.
Q
b
Ion Added Cot+
a constant absorbance index could not be calculated. The data are presented in Figure 2. Accordingly, the copper sulfatebuffer solution was selected for use as the reference solution in subsequent studies and in the procedure that was finally adopted. Effect of Excess Copper Sulfate Reagent. I n the Bersworth procedure (2),only a very slight excess of copper sulfate is added over that required to complex all of the EDTA. Copper sulfate is added until a slight turbidity is observed by its Tyndall effect. An excess of 2 drops of 0.25% copper sulfate is then added. This method of regulating the excess copper sulfate is unsatisfactory, because the amount required to produce a detectable turbidity is dependent on the method of observation-Le., whether observed by means of a narrow beam of light or by diffuse lightand, more important, a number of interfering cations will form precipitates that obscure the end point of the copper addition, since they are displaced from their EDTA complexes by copper.
!4
BUFFER SOLUTION,
O.! Y
No,HPO,
p H , II
REFERENCE
SOLUTIONS: A-COPPER SULFATE- BUFFER SOLUTION b-EDTA- BUFFER SOLUTION 0-Bur 'FER SOLUTION 0 - WATER
\
I
I
I 0.020
Figure 2.
1
1
0.040 0.060 E D T A , gd1000 nL
I
-1
I
i
Error, CI
/o
0.032 0.13 0.16 0.32 0.024 0.12
49.5 49 47 45 47 33.5
Cr"+
0.32 0.64
Cat+ M g++++ Fe Fe+++
0.16 0.16 0.20 0.20 0.40
48.5 48.5 48.5 48.5 51
+ a
52 52.5
+ 5
Cr+a
0.14
83.5
+67
Ni++
Reference solution, 0.25% CuSO4 in 0.1M buffer solution. Reference solution, water.
EDTA, 50 y per ml. Reference solution, water Weight EDTA Ratio Found, Y Ion/EDk4
- 1
- 2 - 6 - 10 - 6 33
-
- 3 - 3 - 3 - 3
+ 4
copper sulfate solution was added in the analysis of all samples. The excess copper precipitates as copper hydroxide a t the p H a t which the color is developed and is removed by filtration or centrifugation, together with hydroxides of any other metals that have precipitated after displacement from their complexes with EDTA. Effect of Temperature. Tests were made to determine whether heating affected the absorbance of reference solutions and of copper-EDTA solutions after removal of excess copper as the hydroxide. The solutions were heated to 80' to 90' C. and cooled to room temperature, and the absorbance was measured. S o difference was observed in the absorbance of heated and unheated solutions. However, the reproducibility was lowered for the reference solutions that had been heated. Stability. Solutions of the copper-EDTA complex stored in polyethylene containers were stable over a 14-day period. Prior heating tended to increase the stability slightly. This was also true Kith the reference solution. However, the reference solution was moIe stable when stored in glass rather than polyethylene. Interferences. A limited number of tests were made of interferences Xickel and cobalt are more strongly complexed than copper by EDTA a t p H 11 (1, 6). Consequently, these two metals interfere in the determination of EDTA. Nickel interfered in all concentrations. Cobalt did not interfere a t concentrations below one tenth of that of the EDTA. Thc chromate ion interfered because of its strong absorbance in the spectral region used for measuring the absorbance of the copper-EDTA complex. Ions which are displaced from their complexes by copper do not interfere, because they are precipitated as the hydroxides and removed with the excess copper. The extent of interference by various ions is shown in Table 111.
1
0.080
.4bsorbance index as a function of reference solution
Therefore, tests were made to determine whether excess reagent was detrimental. Excess copper sulfate, up to 5 ml. of a 0.25% solution, produced no significant effect on the absorbance of either the copper-EDTA complex or the reference solution (Table 11). I n fact, in the presence of a number of cations such as iron, caIcium, or magnesium, an excess of copper sulfate was required to displace these cations from their complexes with EDTA; otherwise, the test results were low. I n order to ensure a sufficient excess of copper for this purpose, 4 ml. of a 0.25%
PROCEDURE
The following procedure is simple and can be carried out rapidly without the use of special apparatus other than an ultraviolet spectrophotometer. Transfer a test portion of sample containing from 125 to 1500 -/ of EDTA in a volume of 5 ml. or less to a 25-ml. volumetric flask. Add 4 ml. of 0.257, copper sulfate reagent solution, and then add 0.1M disodium hydrogen phosphate buffer solution to the 25-ml, mark. Filter the solution through a dry filter paper. Prepare a reference solution by diluting 4 ml. of the copper sulfate reagent solution to 25 ml. with the buffer solution and filtering the diluted solution. Use the reference solution to set the spectrophotometer to zero at 250 mp, and then measure the absorbance of the sample solution. For samples containing 500 y of EDTA or less, use 5-em. silica cells; for larger amounts of EDTA, use 1-em. cells. Measure the absorbance of a series of standard
V O L U M E 28, NO. 9, S E P T E M B E R 1 9 5 6
1441
E D T A solutions, processed in the same manner as the sample, and compute the average absorbance index. Use the absorbance index to calculate the concentration of E D T A in the sample from its absorbance. LITERATURE CITED
(1) Bersworth Chemical Co., Framingham, Mass., Tech. Bull. 2, sec. I, pp. 6-13, 1954. (2) Ibid., 8ec. 111, pp. 2-4. (3) Derby, 1., ANAL.CHEM.24, 373 (1952).
(4)
Furness, W., Crawshaw, P., Davies, W. C., Analyst 74, 629 (1949).
(5) Kerkow, F. W., 2.anal. Chem. 133, 281 (1951). (6) Plumb, R. C., Martell, A. E., Bersworth, F. C., J . Phya. and Colloid Chem. 54, 1208 (1950). RECEIVED for review November 29, 195% Accepted rMay 22, 1956. Southeastern Regional Divisional Meeting, ACS, Columbia, 9. C., November 5 , 1955. Work carried o u t under Contract No. W-7405-eng-26 a t Oak Ridge Xational Laboratory, operated b y Union Carbide Nuclear Co., a division of Union Carbide and Carbon Corp., for the Atomic Energy Commission.
Pyrohydrolytic Separation and Spectrophotometric Titration of Fluorides in Radioactive Samples J. E. LEE, JR., J. H. EDGERTON, and M. T. KELLEY Analytical Chemistry Division, O a k Ridge N a t i o n a l Laboratory, O a k Ridge, Tenn.
Apparatus is described which is suitable for the pyrohydrolytic determination of fluoride in radioactive liquids or solids. Fluoride in the radioactive distillate from the pyrohydrolysis is determined by a remotely controlled titration in an apparatus of special design. The apparatus was evaluated by determining the fluoride content of solid uranium tetrafluoride, a mixture of solid fluoride salts, and a solution of fluoride salts. The relative standard deviation of the data for 2.5 to 15 mg. of fluoride was 3'7,~ for uranium tetrafluoride samples.
T
HE procedure described by Warf, Cline, and Tevebaugh ( 5 )
for the pyrohydrolytic determination of fluoride has been adapted to the analysis of semimicro quantities of highly radioactive liquids and solids. Modified apparatus was developed for the pyrohydrolysis of the sample and for spectrophotometric titration of fluoride by remote control. The fluoride contents of solid uranium tetrafluoride, of a mixture of fluoride salts, and of a radioactive liquid that contained fluorides were determined
satisfactorily by use of the apparatus. The radiation level of samples was sometimes as high as 100 roentgens at contact, and radioactive fission products were present in the distillates. I PYROHYDROLYSIS APPARATUS
The unique feature of the pyrohydrolysis apparatus employed (Figure 1) is that it can be used for the analysis of either liquid or solid radioactive samples. It consists of a steam generator and superheater, a pyrohpdrolpsis unit, and a collector for the fluoride distillate. The steam generator is constructed of standard borosilicate glassware. The superheater is a silica tube which passes through a furnace made from standard 8-inch-length heating elements. The furnace can be heated to a temperature of llOOo C. Control of the steam supply is critical because of the small volume of the pyrohydrolysis unit. Control is maintained by leaving the steam generator open to the atmosphere through a condenser. Steam passes through the system as a result of the reduced pressure that is maintained in the distillate receiver, The pyrohydrolysis unit is constructed of platinum [or nickel (S)] ; it incorporates a standard 20-ml. tubulated Gooch crucible.
b1R INTbKE SILICb SUPERHEbTER T U B E
-
S l L l C b COUPLING
B A L L bNO SOCKET JOiNT
UM REbCTlON CRUCiBLE
S T E b M GENERbTOR I500 ml. F L A S K 1
-
POLYETHYLENE RECEIVER U N i T
VACUUM RECEIVER U N l T
HEbTlNG MbNTLE
o m o , " ,
Figure 1.
.4pparatus for pyrohydrolysis
