Rapid Determination of Traces of Iron and Copper in Acrylonitrile ROBERT L. MAUTE, M. L. OWENS, Monsanto Chemical
JR., and
J.
L. SLATE
Co., Texas City, Tex. various standard iron solutions were added to acrylonitrile t o prepare the calibration curve. The Tr-ater used in making dilutions was passed through a mixed-bed ion exchange resin to ensure iron- and copper-free hlanks. The acrylonitrile used was made metal-free by distilling from an acidic 8-quinolinol solution after a reflux period of 30 minutes. Only the heart cut was used.
>lethods have been developed for rapid colorimetric determination of mutually occurring copper and iron in commercial acrylonitrile in concentrations as lo\+ as 0.05 p.p.m. The color reactions and absorbance measurements are carried out in the acrylonitrile phase, and thus prior treatment of the sample, such as decomposition, is not required. This approach precludes the introduction of contaminating metals, which frequently results in high blank corrections in the wet-decomposition methods generally applied to monomers. The specificity of the reagents (2,2'-biquinoline for copper and o-phenanthroline for iron) and the procedures used make it possible to determine either metal in the presence of a preponderance of the second, and there is no appreciable interference from a several hundredfold excess of the cyanide ion, present either as free hydrocyanic acid or as cyanhydrin--e.g., lactonitrile.
T
DETERMIS4TION OF IRON
HE influence of trace amounts of metals upon the p o l y merization rate of acrylonitrile is well known ( 7 ) . ,1 fex parts per million of metallic impurities: specifically cupric and ferric ion, cause a severalfold increase in t.he polymerization rate of acrylonitrile, when the persulfatc-thiosulfate redox system is used ( 1 ) . As the variability in the level of these trace metallic impurities affects the polymerization rate, a relatively high concentration of cupric ion is usually added to the polymerization system to swamp the natural variation of trace metal impurities. I n fact, it has been reported that a few parts per million of copper make it necessary to polymerize acrylonitrile a t a low temperature in order to maintain uniform niolecular weight (8). Thus, it became of interest to develop rapid methods for the determination of traces of copper and iron impurities in acrylonitrile. The customary dry-ashing and wet-decomposition metliods were briefly investigated. Both methods are slow, lark the desired precision and accuracy, and unless efipecially purified reagents are used, iron contamination is hard t.0 avoid. The wet-decomposition method for copper gave accurate, reproducible values with known amounts of copper; however, the t,ime ~ ' e quirement was still a major disadvantage. Therefore, ot,her methods for determining iron and copper T-vere sought. APPARATUS AND MATERIALS
Beckman Model H 2 p H meter. Beckman Model B spectrophotometer with 50-mm. cells. Hydroxylamine hydrochloride (Plfatheson), 10 grams dissolved in co per and iron-free water and diluted to 250 ml. o-l!henanthroline (G. F . Smith Chemical Co.), 0.40 gram dissolved in 100 ml. of C.P. methanol. Glacial acetic acid. Methanol, C.P. Ammonium hydroxide, C.P. 2,2'-Biquinoline (G. F. Smith Chemical Co.), 0.10 gram dissolved in 500 ml. of c . ~methanol. . Standard copper solution, 0.1965 gram of cupric sulfate pentahydrate dissolved in water and diluted to 500 ml. This standard, containing 100 p.p.m. of cupric ion, was further diluted with water to give various copper concentrations. Aliquots of the various copper solutions xvere added t o acrylonitrile to prepare the calibration curve. Standard iron solution, 0.1000 gram of analytical grade iron wire (99.85%) dissolved in dilute nitric acid. After boiling off the nitrogen dioxide fumes, it was diluted t o 1 liter. This standard, containing 100 p.p.m. of ferric ion, was further diluted with water t o give various concentrations of iron. Aliquots of the
o-Phenanthroline is one of the most sensitive colorimetric ieagent,s for iron (4,17'). It's use in trace iron determinations is \vel1 described by Fortune and hIellon (4)and others. This reagent was used for all iron investigations. Because many of t,he metal-organic complexes-for example, dithizones and 8-quinolates-are soluble in organic solvents, it was aseumed that the ferrous-phenanthroline complex could be developed and its absorbance measured directly in acrylonitrile. Subsequent investigations revealed t,hat the determination of iron in acrylonitrile medium gave more rapid, reproducible, and accurate values than did the conventional decomposition methods. The variables affecting the determination xere also studied. The color development in acrylonitrile n-as found to be essentially complete in 25 minutes, after p H adjustment. Increasing the time for color development did not increase t'he accuracy. The p H range of 2 to 9 has been recommended for iron determination using o-phenant,hroline reagent ( 4 ) . This work indicated that an apparent pH of 4.5 to 6.5 gave a stable color and permit,ted maximum color development. *Asthe wave length of some colored coniplpses shifts in organic media, the maximum absorbance was investigated and found to be the same as in water-i.e., 508 mp. The literature (4)states that a fivefold excess of copper docs not interfere in iron determination if the pH is adjusted t o between 2.5 and 4.0. Copper is reported to interfere somewhat a t a pH of 5.86; laboratory studies confirmed this. Copper interferes, owing to a yellor color a t the higher pH. However, acrylonitrile containing a tenfold excess of copper did not give an interference beyond the precision of the method, as is shown in Talde I. .is all commercial acrylonitrile is manufactured with hydrogen cyanide, its effect on iron determination was investigated,
Table I. Effect of Copper and pH upon Iron Determination Fe Xnorm, P.P.31. 0 20
Cu Added, P.P..\I. 2 00
0 20
Table 11.
Fe Found, P P.M. 0 21 0 27 0 21 0 21 0 21
Effect of Cyanide upon Iron Determination H C S Addeda, P.P.M.
Fe Found, P.P.M.
0.10
200 300 275 b
0.02 0.02
0.20
5 10 25 50 275 b 500
0.21
0.40 0. 50
0.60
1614
3 5 5 5 5
Fe Known P.P.M.
0.30
6
plI
3 5 2 4 5
Cyanide added a8 NaCN. H C N added as lactonitrile.
0.10 0.22 0.22 0.20
0.30 0.07
500
0.08
275 b 500
0.60 0.20
V O L U M E 27, N O . 10, O C T O B E R 1 9 5 5 llellon and Fortune (4)had noted that cyanides interfered with the colored iron-phenanthroline complex-for example, with 2 p.p.m. of iron. 10 p.p.m. of cyanide was reported to give a 2% interferenre. Known :mounts of cyanide (from sodium cyanide and from lactonitrile) w r e added to acrylonitrile containing a known iron rontent. As seen in Table 11, a several hundred-fold excess of free cyanide ion over iron did not result in any appreciable decrease in tlic iron found. Lactonitrile did not interfere even with a 2000-fold excess over iron, evidently because of the failure of lartonit,rile to dissociate appreciably a t the p H used (14). The cyanide w a p added in all cases before the phenanthroline reagent. Procedure. T o a clean 250-ml. iodine flask, add 125 nil. of :rc*rylonitrile(free from suspended matter). Add 20 ml. of C.P. methanol, 10 ml. of 4yo hydroxylamine hydrochloride solution, and 5 ml. of 0.47' o-phenant,hroline solution, and shake vigorously. Use iron-free water (125 ml.) as a blank and t.reat it the game as the acrylonitrile sample. Adjust the blank and samples to an apparent pII of 5.0 to 6.0 with dropwhe addition of dilute ammonium hydroxide and glacial acetic acid, if needed. After 30 minutes read the absorbance of the solution a t 508 mp, using 50-mm. cells and the blank. as a reference. Convert the absorbance to iron concentration by means of a predetermined calibration curve. The calibration curve found for iron in acrylonitrile folloivs Beer's law in the range 0 to 2 p.p.m. DETERiMINATIOIV O F COPPER
The literature contains many references to reagents pert,aining to trare copper analysis. Sandell (15) lists the common reagents and their limitations. A brief investigat,ion of the sodium diethyl dithiocarbamate method (16) for the desired range of copper gave poor results. -4s many improved colorimetric methods for copper havc been recently developed, other recent literature methods were examined-for example, copper in dyes and organic chemicals can be accurately determined wit'h zinc dibenxyldithiocarbamate after wet decomposition ( I S ) . The same reagent in carbon tetrachloride can be used to extract and det,ermine t,racrs of copper in acidified beer and cider (19,20). The reagent 2,2'-biquinoline is reported to detect 0.01 p . p m of copper and the cuprous-biquinoline complex is stable and unaffected by light, heat, or air ( 2 ) . In addition, t.he complex is soluble in acetic acid, benzyl and amyl alcohols, chloroform, and ethyl acetate, and the reagent, is specific for cuprous ion. The reagent is reported 10 times more sensitive than sodium diethyl dithiocarbamate and is more stable and has less interference than dithizone (9). Tho complex is unstable in a highly acid medium (pH a), :rnd ran he easily extracted from water with n-amyl alcohol. Cheng and Bray ( 3 ) determined traces of copper in soils by use of hiquinoline and found that it gave slightly more accurate and precise values on known standards than their improved sodium diethyl dithiocarbamate method. Gillis, Hoste, and ot,hers (5, 6, 11, 12) studied the cuprousbiquinoline complex and confirmed that biquinoline was specific for copper. They also found that the color is stable between p H 2 and 9 for a least 72 hours and that Beer's law holds up to 40 p.p.m. of copper. Biquinoline has been used to determine ap lit,tle as 0.02 p.p.m. of copper in plants, animal tissues, blood, water, lampblack, steel, and ores. Because of the success of the determination of iron directly in acrylonitrile, a similar method using biquinoline was applied for the determination of copper and certain variables were investigated. The color of the solution develops immediately aft'er pH adjustment ; however, in the case of acrylonitrile containing more than 5 p.p.m. of cyanide, the color develops more slowly. A period of 30 minutes will usually suffice for full color development. .4n apparent p H higher than 5.5 or less than 3.0 causes fading of the color (not shown in the table). A pH over 6.5 causes disappearance of the color. The most desirable pH found was 3.5
1615 'Iahle 111.
Effect of pH upon Copper Determination PH Cu Found, P.P.M
C u Known, P.P.M. 2.0
-
--
0 20
Cu Knoum, P.P.31. 0 40 0.20 0.30 0.40 a b
4 9
_~__-
Table I\'.
2.00 2.00 2.00 1 55 0 00 0 22
3.4 4.6 5.0 5 5 6 5
_ _ __ ~ Effect of Hydrocyanic kcid upon Copper Determination ~
H C N Added,
P.P.RI. 5a 25a 50"
C u Found, P.P.M. After Immediately standing 30 min. 0.39 0.41 0.35 0.41 0.28 0.40
1 l b 11h
0.21
l l h
0.40
0.29
Cyanide added as S a c ? . Cyanide added as lactonitrile
t.o 5.0 (Table 111) and the apparent pH was more stable if a slight excess of ammonium hydroxide was added and the pH brought into t.he proper range with acetic acid. The maximum absorbance for the complex in acvlonitrilemethanol medium was found to be 540 mp. The literature (IO)states that iron does not interfere in copper determination with biquinoliie. Laboratory data (not shown) confirmed that a tenfold excess of iron over copper causes no interference. Cyanide was found by Cheng and Bray (3) and Gillis ( 6 ) to interfere in the biquinoline method for copper; however., the interfcrence level was not given. Investigation of cyanide interference proved that a hundred-fold excess of free cyanide did not cause interference, provided at least 30 minutes were allowed for color development. A 50-fold excess of cyanide as lactonitrile did not affect the color development. Table IV gives the rr~ult?i found. The absorbance of copper-hiquinoline complex in acrylonitrile did not change upon standing overnight, (wm if 50 p.p.m. of cyanide were present. Procedure. Place 125 ml. of acrylonitrile (free of suspended solids) in a clean 250-ml. iodineflask. Pipet 10 ml. of 4% hydroxylamine hydrochloride and 25 ml. of 0.02'70 biquinoline into the, flask, and shake vigorously. Vse 125 ml. of C.P. methanol as a blank and treat it the same as the acrylonitrile sample. Adjust the apparent p H to 3.5 to 5.0 with dropwise addition of ammonium hydroxide and acetic acid. Read the absorbance a t 540 m/z using 50-mm. cells and the blank as a reference. Convert the absorbance t.0 copper concentration by use of a predetermined calibration chart. The calibration curve for copper in acrylonitrile follows Beer's law froni 0 to 2 p.p.m. Work by Smith and Wilkins (18) indicates that 2,g-dimethyl4,7-diphenyl-l,IO-phenanothrolineis superior to biquinoline as it specific copper reagent. However, the reagent was unavailahle commercially at the time of this work and was not investigated. SUMMARY
Traces of iron and copper in acrylonitrile can he determined directly, and without prior treatment. Application of these procedures to routine samples indicates that the reproducibility of both methods is within f 0 . 0 3 p.p.m. These methods have been found applicable to the determination of these metals in other organic liquids, and with minor modifications should be readily adaptable for many other monomers and solvents. LITERATURE CITED
(1) Bacon, K.G. R., Trans. Faladay SOC.,42, 140 (1946). (2) Breckenridge, J. G., Lewis, R. W. ,J., and Quick, L. A.. Can. J . Research, 17B,258 (1939).
~
ANALYTICAL CHEMISTRY
1616 (3) Cheng, K. L., and Bray, R. H., AN.~L.CHEN.,25, 655 (1953). (4) Fortune, W. B., and Mellon, iM. G., IND.ENG.CHEW,ANAL. ED.,10,60 (1938). (5) Gillis, J., Bull. centre belye &/de et document. eaux (Lidge), No. 22,233 (1953). (6) Gillis, J., Hoste, J., and Fernandex-Caldas, E., A d e s edafol. fisiol. vegetal (‘Madrid), 9, 585 (1950). (7) Hill, Rolland, Ed., “Fibres from Synthetic Polymers,” p. 61, Elsevier Publishing Co.. Houston, 1953. ( 8 ) Ibid., p. 63. (9) Hoste, J., Ar~al.Chirn. Acta, 4, 23 (1950). (10) Hoste, J., Research, 1,713 (1948). (11) Hoste, J., Eeckhout, E., and Gillis, J., A d . Chim. Acta, 9, 263 (1953). (13) Hoste, J., Heiremans, A , , and Gillis, J., Mikrochernie uer. mikrochim. Acta, 36/37,349 (1951).
. 24, 991 (13) Rlarteiis, R. I., and Githeiib, R. E., A N ~ L CHEM., (1952). (14) Maute, R. L., and Owenh, .\I I... Jr., Ibid., 26, 1723 (1954). (15) Sandell, E. B., “Colorinieti ic Determination of Traces of Metals.” 2nd ed.. u. 87. Inter>cience. New York. 1950. (16) Ibid., p. 304. (17) Smith. G. F.. and Richter, F. P., “Phenanthroline and Substituted Phenanthroline Indicators,” p. 67, G. F. Smith Chemical Co., Columbus, Ohio, 1944. (18) Smith, G. F., and Wilkins, D. H., ASAL. CHEM.,25, 510 (1953). (19) Stone, I., Ettinger, R., and Sanz, C., Ibid., 25, 893 (1953). (20) Timberlake, C. F., Chemistry & I n d u s t r y , 47, 1442 (1954). I
.
RECEIVED for review April 23, 1955. Accepted July 13, 1955. Division of Analytical Chemistry,127th Meeting ACS, Cincinnati, Ohio, 1955.
Colorimetric Submicromethod for Determination of Ammonia P. G. SCHEURER and F. S M I T H Division o f Biochemistry, University of Minnesota, St. Paul, Minn. The blue color, formed when sodium phenate is added to a solution of ammonia that has been treated with hypochlorous acid, forms the basis of a method for the determination of submicro amounts of ammonia. The method has been used for the determination of the molecular weight of compounds containing nitrogen which is easily transformed into ammonia. Coupled with the cyanohydrin reaction it can be utilized for the determination of the average molecular weight of aldehydes, such as sugars and certain polysaccharides possessing a free reducing group. The procedure might lead to the simplification of protein determination by the Kjeldahl method.
I
N THESE investigations into polysaccharides the authors
sought t o devise a method for determining the reducing group and, hence, the average molecular weight of these substances by a chemical method. The reaction between an aldehyde and the elements of hydrocyanic acid, known to proceed to completion under certain conditions according t o the law of mass action with simple aldoses, has formed the basis of these studies (4, 6). In the formation of polysaccharide cyanohydrins of high molecular weight the amount of combined cyanide is relatively small, and its determination requires an extremely sensitive method unless unlimited amounts of polysaccharide are available. The difficulty has been overcome by the use of ryanide l a h l e d with radioactive carbon-14 (5). The blue color produced by the action of phenol and hypochlorous acid upon ammonia ( 2 , 6-9) can be used t o determine fairly accurately extremely small quantities of ammonia. The density of color developed by dilute solutions of ammonium chloride (measured by an Evelyn colorimeter and filter 620) was found to be a linear function of the concentration of ammonia from 0 t o 8 x 10-8 mole of ammonia per ml. The slope of this linear function, moles of ammonia per milliliter per abaorbance, hov*ever, was best determined by saponifying an aliquot of a standard solution of purified acetamide containing about 0.0002 gram of acetamide with ammonia-free base. The solution was steam distilled with steam generated from a dilute solution of sulfuric acid and the ammonia-containing distillate collected in about 15 ml. of ammonia-free distilled water. About 75 t o 100 ml. of distillate was collected and the exact volume determined by weighing. This standard ammonia solution was then used t o determine the slope of the linear relationship between concentration of ammonia and color intensity. Bcetamide
as a primary standard was determined by saponification, steam distillation into standard acid, and back-titration with standard base using methyl red indicator. The slope has been found to lie independent of the concentration of either the phenol or the chlorine water reagent. The slope obtained, however, depends upon the success with which ammonia has been removed from the saponifying base. The problem of its removal has not yet been solved and consequently the slope relationship muat he I edeterniinrd every time fresh base is prepared. In contrast t o the phenol-hypochlorite reaction ( d , 6, 9) no heating is necessary, as the reaction takes place quickly a t room temperature. In addition, more color is produced with the phenol-chlorine water than with the phenol-hypochlorite reagent. The color disappears only very slowly, 1% approximately in 24 hours. I t is possible t o detect 5 X 10-lo mole of ammonia per nil. of solution. By comparison, Nessler’s reagent has only one tenth of this sensitivity and, moreover, the color must be read shortly after its development. The values of slope obtained by four acetamide determinations differed from their average value by about 2%. The accuracy of this method is therefore within &2%. PREPARATION OF REAGENTS
Ammonia-Free Distilled Water. This reagent is prepared by distilling distilled water from a dilute solution of sulfuric acid in a n all glass apparatus. Hypochlorous Acid Reagent. Chlorine is bubbled into icecold distilled water until solid chlorine hydrate forms. The a p proximate chlorine content determined by the iodide-thiosulfate method should thereby exceed the required minimum value, about 0.08M chlorine. The molarity should, however, be determined approvimately before use by adding 10 ml. of 5y0 potassium iodide solution and titrating the liberated iodine with 0.2M sodium thiosulfate. The required minimum chlorine content is determined by plotting the maximum color development obtained for a given ammonia solution against the strength of the hypochlorous acid reagent (Figure 1). A suitable ammonia solution which gives a maximum color development of about Soy0transmittance is prepared by adding one drop of 0.4M ammonium chloride solution to 500 ml. of ammonia-free distilled water. Potassium iodide, 5y0 aqueous solution. Sodium thiosulfate solution, 0.2M, 5 grams of sodium thiosulfate (hypo) per 100 ml. of solution. Sodium Phenate Reagent. A cool solution of sodium hydroxide, 7.2 grams (0.18 mole), in 300 ml. of ammonia-free water is added t o commercial phenol, 16.7 grams (0.0178 mole). and shaken until the latter is dissolved. Manganous chloride solution, 0.003-W.
