Dis o d ium Ethy I Bis(5-tetraz o Iy Ia z 0)acetate T rihy d rate Colorimetric Reagent for Nickel and Copper HANS 9.JONASSEN, VIRGINIA C. CHAMBLIN, and VERNON
L.
WAGNER, Jr.
Department of Chemistry, Tulane University, New Orleans, la. RONALD A. HENRY Chemistry Division, Research Department U. S.Naval Ordnance Test Station, China lake, Calif.
,Disodium ethyl bis(5-tetrazolylazo)acetate trihydrate forms a watersoluble, highly colored complex with nickel(l1) ion that can b e used for a quantitative colorimetric determination in the presence of cobalt and iron. It also forms a stable, highly colored, 1 to 1 complex with the copper(l1) ion, which has a high absorbance value even at low concentration. Of the common cations of the first transition metal series, only cobalt(l1) interferes; however, this can be eliminated by calculations using simultaneous equations.
D
an investigation of the coordinating tendencies of disodium ethyl bis(5-tetrazoly1azo)acetate trihydrate (abbreviated tetra in this report) i t was found to form a stable, highly colored complex with nickel. Absorbance maxima were at 335 and 505 mp, and in these regions iron and cobalt do not interfere. Copper(I1) also formed a complex with peaks a t 268, 300, and 535 mp. Of all the cations investigated. only copper(I1) formed a 1 to 1 complex. URISG
Ea
I
ANALYTICAL CHEMISTRY
Preparation of Reagent. T o a cooled solution of 9.7 grams of 5aminotetrazole monohydrate in 40 ml. of a 20% solution of sodium hydroxide was added 28.4 ml. of a 20% solution of sodium nitrite. Crushed ice (60 grams) was placed in the solution and the slurry poured all a t once with vigorous stirring into 28.4 ml. of concentrated hydrochloric acid, 120 ml. of water, and 200 grams of crushed ice. After 20 minutes a cold solution of 50 grams of sodium acetate trihydrate in 100 ml. of water was added, followed immediately by 13.0 ml. of ethyl acetoacetate. The solution immediately developed an intense red color. The reaction mixture was then stirred for 2 hours while the temperature was maintained a t 0' to 5" C. by a n ice water bath. Kear the end of the stirring period 100 grams of sodium chloride was added; stirring was continued until the salt dissolved. The resulting solution was then plaied in a refrrgerator and allowed to stand a t 5 " C. for 2 days, during which time the product slowly crystallized. The orange-red solid was removed by suction filtration. It is desirable to free the product as thoroughly as poshT a
H I
Although a large number of colorimetric reagents of considerable sensitivity and accuracy (5) are knonm and used for determination of small concentrations of copper, separation processes for interfering metal ions are usually necessary. In the dithizone and the carbamate methods (5) for example, preliminary extraction processes are necessary. Many of the reagents for copper also lack specificity, iron being the metal most frequently found to interfere. Figure 1 s h o w that t x o of the three absorption peaks of the copper-tetra complex might be useful in analytical determinations; certain metal ions might interfere a t one wave length but not a t the other one. 1660
MATERIALS
sible of the occluded mother liquors, because washing even with cold water to remove salt and the like causes significant solubility losses. The yield of
the air-dried product was approximately 9.5 grams. Decreasing the amount of ethyl acetoacetate to 6.5 ml. so that the ratio of 5-aminotetrazole to ester was more nearly 2 to 1 lowered the yield to about 6 grams. The product can be recrystallized from a large volume of 95% ethyl alcohol. As tetra crystallizes slowly, several days standing at about 5' C. should be allowed for a reasonable recovery. The product can also be recrystallized wastefully from a small amount of water. The sample for analysis was recrystallized three times from alcohol. AXALYSIS. Calculated for CsH802iY12Na2.3H20: C, 19.05; H, 3.20; N, 44.44; "a, 12.16%. Found: C, 19.39, 18.98; H, 3.17, 3.15; N, 44.76, 44.46; Na, 12.04, 12.14%. The formula for the trihydrate requires 14.3% water; even after prolonged vacuum drying a t 100' C. in the presence of phosphorus pentoxide, the m-eight loss corresponded only to about 12%. This determination was complicated by the fact that the anhydrous salt was exceedingly hygroscopic. A 0.0001M stock solution of the reagent was prepared from the dried trihirdrate salt. Metal Ion Solutions. A 0.01M stock solution of nickel(I1) was prepared from C.P, nickel chloride and was standardized by precipitation with dimethylglyoxime (2). More dilute solutions for the spectral studies were prepared from aliquots of the standard stock solutions. A 0.01M solution of copper(I1) nitrate was prepared by dissolving 1.876 grams of the trihydrate in 1 liter of solution and standardizing by electrolysis.
n
Figure 1 . tion curves
Absorp-
Aqueous solutions, 1 10 - 4 ~
A. 6.
X
Tetra Copper-tetra complex
The solutions used in the spectral work 17-ei-e prepared by diluting aliquots of the standard solution. For the interference studies standard 0.01M solutions of cobalt(II), nickel(IT), iron(TI), manganese (11), cadmium (11), zinc(TT), iron(TTI), chromium(TI1) , aluminum(TII), and silver(1) were prepared and stanclardized by the usual analytical methods 13). 911 other chemicals were C.P. materials. APPARATUS MOLE FRACTION OF METAL ION
All of the spectrophotometric studies were made m-ith a Beckman Model B spectrophotometer using matched, 1em. Corex cells, except that a Cary Model 11 MS recording spectrophotometer with matched, cylindrical, 1em. cells with quartz windows was used to determine the absorption curves.
Fig. 2. Continuous variation curves a t 535 mp X lC-5M
Metal ion, ond tetra, both 2 . 5
0 Copper(l1) 0
0 A A
Nickel(l1) Cobolt(l1) Iron(lll) Iron(l1)
PROCEDURE
A sample of about 0.1 gram of the unknown is dissolved in hot concentrated acid. Concentrated sulfuric acid is added and the solutions are evaporated to drive off the oxides of nitrogen. The solution is diluted to 50 ml. and filtered. The p H is then adjusted t o about 6 and the solution is diluted t o 100 ml. A 1-ml. aliquot is mixed with 5 ml. of 0.0001M tetra and the whole is diluted again t o 100 ml. The absorbance of the samples is then determined at 535 and 410 mp for the copper ion and at 335 and 505 mp for nickel. Percentages of metal ions are calculated in the usual way. RESULTS AND DISCUSSION
Composition of Metal Ion-Tetra Complexes. From continuous variation studies i t is possible t o determine t h e chemical composition of t h e complexes. Table I gives t h e absorbance maxima for a series of metal ions. Figure 2 shons the results of continuous variation studies on the complexes of “tetra” with copper(II), nickel(II), cobalt(TI), and iron (TI) and (111) ions; the last three are the main ions n-hich might interfere. The maximum which indicates the approximate composition of the complexes occurs a t a 1 to 1 ratio of tetra to metal ion for the copper(I1) ion only. All the other ions investigated show a 2 to 1 ratio of tetra to metal ion. Although Figure 2 shows these studies only a t 535 mp. similar results were obtained a t several other wave lengths when suitable corrections were made for the absorbance of tetra. Copper(I1) Complex (1). The absorption curve of t h e tetra shows two sharp peaks a t 270 a n d a t 410 m p (Figure 1). The copper complex, on the other hand, s h o w three peaks: 268, 300, and 535 mp. Excess tetra in the solution shows up immediately by the reappearance of the peak a t 410 nip. The region from 620 to 1000 m p
Table
1. Absorbance Maxima Metal-Tetra Complexes
of
Wave Length Cation
Color Yellow-red Red CoiIII Blue Ni(1Ij Red Fe(I1) Brown Fe(II1) Brown Mn(I1) Yellowred Zn(1I)’ Yellow-red CdiIIi Yellow-red c~(‘III) Yellow Al(II1) Yellow
6$:!)
Table
II.
of Max.
Absorbance, hfp 410 268, 300, 535 275. 375. ,583 ’ 510’ - - 250, 518 500 435 490 ~
468
405 398
Cationic Interferences a t 535 M p
Foreign Cation None
Absorbance 0 . 8 1 f 0.01 0 . 7 8 f0 . 0 1 0 . 7 4 zt 0.01 0.80 f O . O 1 0 7 8 * 0 01
6%
Ni(I1) Fef I1I Cd(iY) Mn( 11) Zn(I1) Fe(II1) Cr( 111) AI(IIIj
0.si * 0.oi
0.80 + 0.01 0 . 8 2 + 0.01 0.80 f 0 . 0 1 0.81 zto.01 0 . 8 2 =t0 . 0 1 ~~
Table
111.
Copper
Ions Present
Determinations
Mmole Cu(I1) Found
Mmole of Each,
X 10-5
X 10-5
Solutions Cu(I1) Cu(II),Co(I1) Cu(II),Fe(II) Cu(II), Fe(II1)
2 2 2 2
0 0,2 0
0,2 0 0, 2 0
1 1 1 2
9 f 0 1 7 *o 1 8 A0 1 1 +0.1
Solids Per Cent Copper Present Found (Av.) 8.28 8.35 6.62 6.91 8.28 5
8 ,30a 8.55 6 ,72a 6.54 8.27
Spectrophotometric titrations.
is not shonn because neither the copper(11) complex nor tetra shows any absorption in this region. I n the low concentration range of this study the absorbance of hydrated copper ion becomes negligible. As the wave length best suited for colorimetric determination of small concentrations of copper is 535 mp, where the absorbance for free tetra is above zero, adherence to Beer’s law was tested a t this wave length. Conformity to Beer’s law (Figure 3) was as expected, since the continuous variation study showed straight lines going to the maximum. This is indicative of the absence of a n y complex ion species n-hich absorb in this wave length region. A test a t the absorption peak a t 300 mp showed adherence to Beer’s law if corrections for the amount of unreacted tetra were made. The disappearance of the peak a t 410 mp for free tetra also obeyed Beer’s law (Figure 3). INTERFERENCE STUDIES. -4series of metal solutions was prepared which was 1 x lo-5JP in both copper(I1) ion and the interfering metal ion, and 1 X l O - 4 M in tetra. Only iron(II\, cobalt(I1). and silver(I) show a slight interference (Table 11). T h a t of iron(T1) can be eliminated by oxidation to iron(TI1). Cobalt(I1) interference can be eliminated by using a method of successive substitution ( 4 ) . Similar experiments with anions showed no interferences for chloride, bromide, iodide, nitrate, sulfate, and phosphate ion. EFFECTOF p H CHANGE. It is important to control the p H between 5 and 8 because the absorbance characteristics of the acid form of tetra are different. Higher p H values interfere because of the precipitation of the metal hydroxides. SEKSITIVITY. The copper(T1) ion and tetra give an intensp red color. It a l l o m detection of copper(I1) ion with the Beckman Model R spectrophotometer down to concentrations of 0.2 p.p.m. a t 535 mp.
DETERMISATIOK OF UNKKOWNS.A number of copper ores and copper oxides were analyzed with tetra by the procedure outlined above. Mixtures of various metal ions were also prepared. Their abqorhances were compared with those of solutions containing only copper. Table IT1 shows the results obtained from these two sets of data. A number of the copper oxide samples Kere also analyzed by a spectrophotometric titration method. The samples were treated as described in the procedure. Results of these titrations are given in Table TI1 and Figure 4. Nickel Complex (6). As the spect r u m (Figure 5) indicates, tetra exhibits a sharp peak a t 410 mp, while t h e complex shoms a n absorbance VOL. 30, NO. 10, OCTOBER 1958
1661
ML.OF T E T R A
Figure 4. solutions Figure 3. complex
A. B.
Beer’s law studies for copper(l1)
Titration of 50 ml. of tetra, 1 1 X 1OV3M
A. 6.
Spectrophotometric
titrations
of
copper(l1)
410 m p 5 3 5 mp
X 10-5M, with copper(ll), Table IV. Interferences in Determination of Nickel a t 505 Mp [ 10 ml. of 0.000121.1tetra; 2 ml. of 0.OOOlX nickel(I1); total volume, 25 ml.]
5 3 5 mp 410 mp
maximum a t 505 mp. As soon as excess tetra is present in the solution the peak at 410 mp reappears. The region from 620 t o 1000 mp is not shown because neither the complex nor tetra absorbs in this region. No absorbance curve is given for nickel because it is negligible in this concentration range. Although the continuous variation study indicated an absence of complexes of nickel other than the 2 to 1 species, a series of experiments was set u p to test whether Beer’s laiv applied for this complex a t the useful wave lengths of 335 and 505 mp. The results in Figure 6 indicate that, if corrections for the presence of excess tetra are made, the ware length of 335 mp can be used for
colorimetric work. However, the best wave length is 550 mp, because the absorbance of tetra is small and can be neglected in the concentration range for lvhich the complex is best suited in this determination. INTERFERENCES. I n order to study the effect of possible interferences, a series of solutions was prepared containing nickel(I1) and the interfering metal in about equal concentrations. The results in Table IV indicate that only copper(II), zinc, and cadmium(I1) interfere. Iron and cobalt, the two ions usually interfering in colorimetric nickel determinations, shoIv no such effect. Similar studies with anions (Table IT) indicate that most common anions do not interfere. EFFECTOF p H CHANGE. Control of
Foreign Ion M1. of
Absorbance
0.0001M
Cations 0.30 h 0 . 0 2
None
Fe(II1) Cu(I1) Co( I1) Crf 111) Zn( II ) ’
2 2 2 2 2
Cd(11)
0.30 h 0.02 0.38 1 0 . 0 2 0.31 h 0 . 0 2 0.29 1 0 . 0 2 0.40 1 0 . 0 2 0.34 f 0 . 0 2 0.31 3Z0.02 0.30 f 0 . 0 2 0.30 f 0 . 0 2
2 2 2 2 Anions ~
iG(f1)
c1-
4
BrINO3 -
0.30 f 0.02 0.30 f 0.02
4
0.31 10.02 0.29 h 0 . 0 2 0.30 3Z0.02 0.31 h 0.02
4
4 2 2
so,-PO,---
0.4 W
Z
Figure 5.
Spectral curves for nickel(l1)
Nickel(l1) and tetra, both 1 X 10-4M; final volume, 100 mi.
A. B. C.
D.
10 ml. of tetra 10 ml. of tetra and 10 ml. of nickel(l1) 20 ml. of tetra and 10 ml. of nickel(l1) 30 ml. of tetra and 10 ml. of nickel(l1)
1662
ANALYTICAL CHEMISTRY
!
0.3-
0
I
2
Figure 6.
3
4
I
I
5
6
ML OF Ni
(II)
7
I
I
,
I
8
9
IO
II
Beer’s law curves for nickel(l1)
10 ml. of 0.0001M tetra titrated with 0.0001M nickel(l1) A. 5 0 5 m p B. 335 mp
ACKNOWLEDGMENT
Table V.
Sample” Synthetic Alloy
4
Ni(I1) 2 ml. 2 ml. 2 ml.
Analyses of Unknowns
Co(I1) ... 4 ml. 2 ml.
Fe(II1) ...
2 ml. 4 ml. ...
% Xi ... ...
0.26% 0.50,’0.54 1.23% 0.36% ... 1.26 For synthetic mixtures nickel, cobalt, and iron were 0.0001M. 0.53%
Absorbance 0.25 & 0.02 0.26 0 . 0 2 0.25 0.02
**
The authors wish to thank Lawrence Dyer for his assistance in the preparation of disodium ethyl bis(5-tetrazolylazo)acetate trihydrate. LITERATURE CITED
(1) Chamblin, V. C., M.S. dissertation,
Tulane University, 1957.
( 2 ) Henry, R. A,, Dyer, L., unpublished
results.
p H is important in this determination since the acid form of tetra has different absorbance characteristics (2). p H should be controlled between 6 and 9; a t higher values some of the metal ions precipitate as the insoluble hydroxides. DETERMINATIONOF UNKNOWNS. Solutions mere prepared containing 30
ml. of tetra (0.0001M) and mixtures of nickel, iron, and cobalt. Nickel can be determined quantitatively from such mixtures (Table V). Alloy samples mere analyzed according to standard procedures and the amount of nickel was determined quantitatively Kith tetra; excellent results were obtained.
(3) Kolthoff, I. AI., Sandell, E. B., “Textbook of Quantitative Inorganic An-
alysis,” Macmillan, Xew York, 1952. (4) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 71, Interscience, Ken; York, 1944. (5) Ibid., pp. 279, 304. (6) Wagner, V. C., Honors thesis, Tulane University, 1957. RECEIVED for review December 16, 1957. dccepted May 12,1958.
Determination of Maleic Anhydride in Polyesters PAUL
D. GARN
and HAROLD M. GILROY’
Bell Telephone laborafories, Inc., Murray Hill, ,This work was undertaken to provide a reliable method for the determination of maleate moieties and maleic anhydride, all reported as maleic anhydride, in polyester resin formulations. Homogeneous alkaline hydrolysis cannot b e used in this determination because maleate esters and polyesters dimerize to butene-l,2,3,4tetracarboxylates. These esters and polyesters can b e hydrolyzed in a two-phase system, benzene or chloroform with aqueous potassium hydroxide. A portion of the aqueous phase is acidified and the maleic acid is determined polarographically. A method of general applicability is useful not only for process control in preparation of polyesters but also for control of resin formulations b y those who purchase, rather than make, the polyesters. N o suitable method has been available.
A
to determine maleic acid groups in polyesters have not yet been very successful for several reasons. One is the difficulty of separating maleic or fumaric from other dibasic acids, and another important cause is the anomalous behavior of maleate esters under certain conditions. Other observers, notably Stafford, Shay, and France1 (5), have reported serious deviations from expected behavior. There are, in the literature, a numTTEMPTS
1 Present address Btomics national, Canoga Park, Calif.
Inter-
N. J.
ber of ways to determine maleic or fumaric acid. Few are applicable to this problem. Of those of interest here, one method is colorimetric (6), and the other polarographic (4). The first involves a saponification and filtration followed by a colorimetric determination with bromine. The operator time is about 2 hours and the total elapsed time a t least 2 days. A trial of the method (3) and the results as described indicate that the method is quantitatively applicable only to the anhydride or acids. The second method was designed for process control, but was tested in the hope that it could be modified for this work. The method apparently requires standards very similar t o the samples. It is not suitable as a general method for determining maleate unsaturation. I n 1955, Garn and Halline (9) described a polarographic method for determining phthalic anhydride in alkyd resins. The procedure consists of Kappelmeier (1) saponification, followed by polarographic determination of the acid in aqueous sulfuric acid. They stated that maleic or fumaric acid could be determined in the same solvent system. This is true, but when another observer ( 7 ) tried to apply the method as described to the determination of maleate groups in esters, anomalous results mere obtained, Specifically, it was found in this laboratory (S) that if maleic acid or anhydride was used, satisfactory gravimetric (1) and polarographic results were obtained; but if a maleate ester was
used, a gravimetric yield about 12% high was obtained, similar to the findings of Stafford, et al. A polarographic yield of about 44% was also found. From these and other results, this behavior is ascribed to a dimerization of the maleate ester in the presence of alcoholic potassium hydroxide to form the tetraethyl ester of butene-1, 2, 3, 4tetracarboxylic acid. The double bond is probably in the 1-position. On hydrolysis, the tetrapotassium salt of this acid precipitates with an alcohol of crystallization. The same reaction does not occur in maleic acid or anhydride, probably because the saponification is too rapid. These results show that the Kappelmeier saponification, whether or not followed by polarographic analysis, cannot be used for determining maleate groups in polyesters. However, a twophase saponification can be used. The polyester is dissolved in benzene or chloroform and shaken m-ith lil’ aqueous potassium hydroxide a t room temperature. After shaking, the water layer is neutralized with sulfuric acid, then diluted to volume. A sample is added to the electrolyte solution and a polarogram obtained. ANALYTICAL PROCEDURE
A 0.2- to 2-gram sample is dissolved in 100 ml. of benzene or chloroform. This solution and 200 ml. of 1.ON potassium hydroxide are put in a 500-ml. glass-stoppered flask and shaken mechanically for 3 hours. Chloroform is used for samples with high maleic conVOL. 30, NO. 10, OCTOBER 1958
1663
