Colorimetric Determination of Small Amounts of Nitroprusside and Aquo- and Ammonopentacyanoferrates(l1) BRUNO JASELSKIS and JAMES C. EDWARDS Departmenf of Chemistry, University o f Michigan, Ann Arbor, Mich. Aquoand ammonopentacyanoferrates(1l) ore determined colorimetrically as azide complexes in the range of 5 >( to 5 X 1 O-4M by measuring absorbance a t 560 mp. The color is stable and easy to prepare, and Beer’s law is obeyed. Nitroprusside is determined colorimetrically as the isophorone complex in the range to 2.5 X 10-4M by of 2.0 X measuring maximum absorbance a t 495 mp. The absorbance is directly proportional to the concentration of nitroprusside. However, color is unstable and the measurements are carried out under carefully controlled conditions.
N
ITROPRUSSIDE and riquo- and ammonopentacyanoferrates(I1) have been suggested as spot test reagents for various organic functional groups. Nitroprusside has been used for the detection of active methylene groups (4) and aldehydes (7, 12) Aquo- and ammonopentacyanoferrates(I1) have been used for the detection of thioureas (6, 1 4 , @unsaturated aldehydes (9),aromatic amines, hydrazines ( 8 ) ,and nitroso compounds ( I ) . Horn wer, quantitative determination methods are unsatisfactory because of the fleeting nature of the color produced exrept for the determination of isophorone (3,5,54rimethyl2-cyclohexane-1-one) as described by Carpignano (6). Thus, a survel of the procedures for the determination of nitroprusside and q u o - and ammonopentacyanoferrates‘11) appears of interest for the possible development of indirect methods for determining other organic functional g_roups Titrimetric determination of nitroprusside with mercurous ion has been described b t Tomicek and Bubfk (16). Nitroprusside hay been decermined gravimetrically using dianti-pyrylphenrlmethan@ bv Gusev and Beiles ’ / ? ) and by Drecipitation of nickel h-droxide i~ the reaction of nicke; cvnnide with alkaline nitroprusside ~y Rose (13). 411 ions yielding insoluble mercurous salis will interfpre in the titrimetric method. The d i s n t i y v r v l p h e n v l m e t h ~precioitation ~~
is nonspecific. Rose’s method if more selective; however, cyanide, ferricyanide, and large amounts of ferrocyanide will interfere. Nitroprusside can be determined polarographically as described by Kolthoff and Toren (11) and Zuman and Kabat (16). Determinations of aquo- and ammonopentacyanoferrates(I1) in the presence of ferro- and ferricyanide have not been reported. Small amounts of nitroprusside are determined colorimetrically as the isophorone complex: aquo- and ammonopentacyanoferrates! 11) are determined colorimetrically as azide complexes. This paper describes conditions for the determination of nitroprusside and aquo- and ammonopentacyanoferrates(11). EXPERIMENTAL
Reagents and Apparatus. The sodium aquopentacyanoferrate(I1) and sodium ammonopentacyanoferrate(1I) were prepared from Raker & Adamson reagent grade sodium nitroprusside using the method described by Brauer (3). These compounds were recrystallized from alcohol-water mixtures and were analyzed. Practical grade Eastman Kodak isophorone and sodium azide were purified for analytical use. Isophoronf- was distilled and the fraction boiling a t 208’ to 211’ C. was used. Sodium azide was recrystallized from a water-ethyl alcohol mixture. Master solutions of ammono- and aquopentacyanoferrates(I1) and nitroprusside were prepared by weight and were stored in the dark. The isophorone solutior, was prepared by diluting 15 ml. of the redistilled isop9orone t c 250 ml. wit’- uhsolute alcoiio! Phop phat.2 and b,r:art,onate buffer solution: (1M 1 were made up using reagent gradf chemicals. liiydrogen peroxide, 0.5%, was prepare6 by dilution of a commercial stock. Tie. pH of trie solutions was measurec; ssmg 8 Beckxar- Model G p H meter. The nbsorpti-m spertrs were recordec! with a Gary sutqmatic recording spectrophotometer, Model XI, and absorhance measurements were made with s Beckman %i vue1 DU spectrophotometer using l-crc. ceiis.
PROCEDURE
Determination of Aquo- and Ammonopentacyanoferrates(I1). Aliquots varying from 0.3 to 4.0 ml. of 5 X 10-3Jf aquo- or ammonopentacyanoferrate(I1) were delivered to 50ml. volumetric flasks. To each flask was then added 10 ml. of 1.044 sodium azide, 5 mi. of 1.OM phosphate buffer, pH 7.7, and 1 ml. of 0.5% hydrogen peroxide, in the order indicated. The solution was then diluted to volume with distilled water and, after standing for about 5 minutes, the absorbance was measured a t 560 mw against a water blank. The effect of interferences was followed by adding interfering ions to the aliquot of the master solutions. All solutions were kept in a dim light to prevent hydrolysis of nitroprusside and ferrocyanide ions in particular. Determination of Nitroprusside. Standard series of nitroprusside solutions were prepared by adding aliquots varying from 0.25 t o 2.50 ml. of about 5 x lO-3M nitroprusside to 50-ml. volumetric flasks. Exactly 10 ml. of isophorone-ethyl alcohol solution (15 ml. of isophorone and 235 ml. of alcohol) was then added from a buret to each of the flasks and the contents were diluted to approximately 40 ml. All solutions were kept in a dim light. Five milliliters of IN bicarbonatecarbonate buffer, pH 10.2, was then added to the flask and diluted to the mark. A stop watch was started a t the timL of the addition to the buffer, and approximately 7 minutes were required to reach maximum value of absorbance for this pH The absorbance stayed constant for about 3 minutes The meRsuremenu could then be made using an ordinary wrist watch and a Reckman DU spectrophotometer. Absorbance was measurec a t 495 mp againsi a blank contaming identica, concentrations of isoDhorony wid buff cr. m-ile absorbance wluez rm3 tc b i corrected for the absorption of the aquc. and ammonopentacyanoferrate(I1)ions, if present in large concentration:: Thev were determined separately and the absorbance was subtracted from the nitroprusside-isophororlc coilli)icx. Effect of incerferences wao ioliowed in addition of mterfering ions to t’;e master soktion. RESULTS AND DISCUSSlOh
Anuo- anci ammonoDentacyanoierVOL. 32, NO. 3, MARCH l P 6 C
381
I.ooc
pH l30l
a a
0 W
z 05oC
SI m a
I
I
4 00
500
1
I
Determination of Nitroprusside and Aquo- and Ammonopentacyanoferrates(I1)
Substance Analyzed Sodium nitroprusside
Interferences Bdded Compd. Molarity Iione None ... None ...
x 10-3 x 10-6 1 x 10-4 1 x 10-4 2 x 10-4 5 x 10-4 5 x 10-3
0,897 1.490 2.980 1.490 1.490 1.490 I.490 1.490 1,490 1.490 1,490 1,490
0.90 1.48 2.97 1.48 1.49
...
0.151
0.950 i ,900 3,800 5 .i o 0 3,800 3,800 3,800 3.800 3.800 3,800
13,945 1,890 3.795 5.682 3.793 3.610 3. i 9 3 3.590 3.790 3.860
1,540 4.620
4.575
1 X'io-4 x 10-4
... .
I
.
2 X 'io-4 3 x 10-3
x 1x 5x 1 x
4
Sodium aquopentacyanoferrate(I1)d None None
Absorb- Amount, Mg./5O MI. ancea Added Foundb 0.45 0.447 0.152 0.293 0.471 0.938 0,467 0.470 0,480 0.465 0,440 0.470 0.480 0.486 0.468
5 5 5
Sodium ammonopentacyanoferrate( 11) None None None None
10-4 10-4 10-4 10-3
0.30i 0.610 0.918 0.607
0.575 0.607
0.570 0,607 0.640
0,025 0.675
1.52 1.48 1.39 1.49 i .52 1.53 1.48
1.531
0 Absorbance of nitroprusside-isophorone complex measured a t 495 mp in buffer solution of pH 10.9. Absorbance of pentacyanoferrate( 11)-azide complex measured at 560 mp in buffer solution of pH 7.8.
Based on several determinations. c _Ibsorbance of nitroprusside-isophorone complex corrected for aquopentacyanoferrate(11). l q u o - and ammonopentacyanoferrate( 11)complexes of azide behave identically. 0
382
0
ANALYTICAL CHEMISTRY
I 10 0
I5 0
Figure 2. Effect of pH and time on absorbance of nitroprusside-isophorone complex
Absorption spectra
Pentacyanoferrate(ll1-azide complex, 7.2 X 1O-'M Nitroprusside-isophorone complex, 2.0 X lO-'M Nitroprusside and aquopentacyanoferrate(ll), 1 X 1 0-3M
Table i.
50
TIME, MIN.
6OW b 00
WAVE LENGTH, rnp
Figure 1.
I
0 .(
rates(II), on the addition of azide or thiocyanate, yield intensely indigo blw solutions having an absorption masimum a t 560 mp. This coior has been attributed by Baudisch ( 2 ) to the violet compound of aquopentacyanoferrate(111) complex. However, preliminar!. investigations indicate that this color is due to an azide or thiocyanate complex. The red comples of nitroprussideisophorone shows a strong absorption maximum of 495 mp. The color of this complex has been described by Carpignano (6) as being due t'o the interaction of the active methylene group with nitroprusside in a siniilnr nanner as shown by Cambi (4). The visible spectra of nitroprusside, aquopentacyanoferrate(II), and complexes of nitroprusside with isophorone. and aquopentacyanoferrate(I1) with azide are shown in Figure 1. Adherence to Beer's law was checked using standard series of nitroprussideisophorone, and of aquo- and ammonopentacyanoferrate(I1)-azide solutions a t 495 and 560 mu, respectively. The results are summarized in Table I. Beer's law is obeyed by aquo- and ammonopentacyanoferrate(I1)-azide compiexes in the range of 5 X 10-5X to 5 X 10-4M. The nitroprusside-isophorone absorbance is directly proportional to the concentration of nitroprusside in the range of 2.5 X 10-5M to 2.5 X lO%If, but the absorbance us. concentration curve does not pnss through t'he origin. However, this deviation is rather small. The apparent molar absorbance for the pentacyanoferrate(I1)azide complex under the experimental
conditioris I& approximatelj, 2700. Thil m o b a5sorbance of nitroprusside-isophorone is about 3800. Put this value is greatly affected by the experimental conditions. Tlie results for aquo- and ammonopentacyanoferrate(I1)-azide complexes can be reproduced to 1%; nitroprusside can be detrrmined t o about 1.5%, The slope of the calibration curve for the nitroprusside varics with pH, concentration of isophorone, and, to some estent; with trmpcrature. This neccssitatcs inclusion of standard mixtures in paralle! ivith ur,knowns for maximum accur:icy while :inalyzirig on a day-today basic Interferences. The hydrolysis o! ferrocyanide yidtis aquopentacyanoierrate!Ilj. Thus, the effect of ferrocyanide in the determination of aquopentacy~InoferrateiI1) has been of riajor interest. Aquopentacyanoferrate(11 can be determined easiiy :II tlir prc'sence of ferrocyanide if its mole ratir, does riot exceed 20. Kitronrusside iioes not interfere if i t is kep't R :ilia ;igI-,t, Iowever, ferricyanide Kili m t r r i c e rat seriously if presen I
p
i
c
no?. interfere ic present in about a tenf o b nxcess. Compounds containing active methylene groups and aldehydes interfere. Effect of pH, The cffect of p H on the formation of azide complexes with aquo- and ammonopentacyanoferrate(11) is rather small in the p H range of 7.0 t o 8.0. T h e optimum pH for this determination is 7.7. The isophorone-nitroprusside complex formation and decay are greatly affected by pH. The amount of the red compiex formed increases with pH, the time required to reach a maximum absorbance decreases, and the decay of the colored complex increases. The effects of pH on the intensity and the time required to reach maximum absorbance for the isopiiorone-nitroprusside complex are shown in Figure 2. The 'optimum range of p H to be used for determination of nitroprusside is 10.5 to 11.0, which is obtained by mixing 5.0 mi. of 1111 bicarbonate-carbonate buffer, pH 1G.l to 10.4, with approximately 20y0 ethyl alcohol solution of nitroprussideisophorciie , Effect of Reagents. The color intensity sf t i l e azide complex of aqriopentacyanoferrate(I1) increases with the concentration of azide ion. I i o w v e r , minor changes of azide concentration in the range of 0.1 t3 0.261 result, in less than a 1% change in dbsorhauice. Iiowever, t h e absorbance o! t'he red isophorone-nitroprusside compie:, is quite dependent upor: the con:ensrat;ion of isophorone. r,. ~ n i : ciiangp Ii. coricentratio:: of isoo 0.73M causes a p decrease in absorb-
Coiarimetric Identificatie, Phenolic Antioxidan
ance and at lower concentratims is evi more pronounced. The determinatioii of q u o - ar. ammonopentacyanoferrates(I1) is 1 producible to about 1%. However, t t colorimetric determination of nitr prusside requires carefully contro:hconditions. The results can be r e p duced to about 1.5%. It is hoped tha, these methods can be ailplied eventual 1 to the indirect determinations of organ functional groups. I
LITERATURE CITED
(1) Baudisch, O., Ber. 54, 413 (1921.1. ( 2 ) Baudisch, O., Sczenct: 108, 443 (1948'.
(3) Brauer, "Handbuch der Praparativen Anorganischen Chemie," pp. 1129, 1364, Ferdinand Enke, Stuttgart, 1954. (4) Cambi, L., Atti accad. Lincei 2 2 , I ; 376 f 1913).
8,
Rosarina, Ann. chim.
(8) 'Feigl, Mikrochemze I5,"lSk (1932,; (9) Feigl, F., ilnger, V., Zappert', R., Ibid., 15, 192 (1934). (10) Gusev, S. I., Beiles, R. G.! Z h w . Anal. Khim. 7,219 195'2;. (11\ Kolthoff. I. 31.. Toren. P.E., Y.Am. Chem. Soc. 75. 1197 (1953: (12) Pavolinl, Tito, RoLl chzm juarm. 6 5 , 713, 719 (1930) (13) Rose, P. H I Z. anal. Cherr,. i&$
195 (195.1) (14) Schwechten, h1. '6
Be.. 65, 173%
(1932 I .
(15) Tpmicek, 0 h u n i ~ r ,". Coliertzo Czecnoslot. Chenz rommuns. 9 37-
(161 Zuman, P., KaLat. 11, Chem. 48, 368 (1954)
RECEIVED for revien- nugust Accepted December 2 , lX&.
22,
lzstb
tc).?!
Estimation
C. i. HlLTOK Research Center, U. S. Rubber
Co., Wayne, M. l.
b k simpie method for the qualitative identification and quantitative estimation of many of the more common phenolic antioxidants requires less than 0.5 man-hour per determination. The method is accurate and precise. Anaiyses with known stocks showed an average error of less than 270 relative.
pp-
T v E R O U S papers have described the identification and estimation of amine antioxidants in various types of polymers (f ). Although several references mention phenolic antioxidants
and other compounding ingredients, none refers to a successful identification of a phenolic antioxidant. Zijp (6) obtained a definite identification of only one phenolic antioxidant-AgeRite Alba. Wadelin ( 4 ) describes a method for the quantitative determination based upon the wave-length shift and increase in ultraviolet absorption of phenols in alkaline solution. It is necessary to know which antioxidant is present. Newel1 (3) describes a colorimetric method involving reaction of the phenol with titanium tetrachloride in anhydrous acetic acid. This was successful
for five of the common phenolic antioxidants. Nawakoivski ( 2 ) determined tris(nony1ated phenyl) phosphite (Polygard) by alkaline alcoholys'iS and CUUpling with pnitrobenzenediazonium fiuoborate. Color was ob-caineci aitr; four additional antioxidants, but no further work was done. The present investigation involves coupling with diazotized p nitroaniline as is done with amine antioxidants (1); however, the strongiy acid conditions are not used. After coupling, the solution IS made alkahnc and the visible absorption spectrum 11 determined. VOL. 32, NO. 3, MARCH 1960
385
