Table 111.
Titration of Divalent Metals in Mixtures Following Cupferron Extraction
EDTA, M1. Mixture Extracted Titrated Actual Theory Zr, Zn, Mg Zr Zn Mg 4.13 '4.14 Zr Zr, Zn, Ca Zn Ca 4.68 4.73 zr; zn; Ca Zr Caa 6.60 6.62 Zr, Zn, Ni Zr Znb 5.26 5.26 Zr, Zn, Ca, Mg Zr Zn $1 Ca M g 6.77 6.70 Zr, Th, Zn Zr Th Zn 2.05 2.08 Zr Th Fe Zn Zr, Th, Fe, Zn 2.05 2.08 Zinc masked with cyanide. b Nickel masked with cyanide; formaldehyde added to destroy zinc cyanide complex.
++
++
+
+
deviations for each divalent metal (without regard to which extractable metal was present) were as follows: Metal Zn Pb Mn
hfg
co Ni Ca Cd
Std. Dev., M1. 0.021 0.032 0.013 0.02G
0.040 0.033 0.041 0.017
No. of Detns. 12
9
9
6 8 10 10
3
Additional analyses were carried out on samples containing more complicated mixtures of met.al ions. These results are given in Table 111.
DISCUSSION
The separations obtained appeared to be very clean in most cases. Qualitative tests in which a zinc salt was used showed that in a typical extraction only about 5 p.p.m. of zinc was extracted into the organic solvent layer. I n such cases the accuracy of the analysis probably depends largely on the accuracy of the EDTA titration. Separations involving lead, cobalt, or nickel are more subject to error in the extraction step. I n separations of iron(II1) where a very acid pH is necessary, the aqueous layer should be removed from the nonaqueous as soon as possible after the
extraction. The longer the layers are left in contact, the more the ferric cupferrate will bleed back into the aqueous layer. Although an aqueous hydrochloric acid medium is recommended for the extraction, it is not always possible to dissolve samples of metals or alloys in hydrochloric acid. I n such cases the sample is diasolved in a satisfactory acid or acid mixture, which is then evaporated t o remove most of the excess acid. The residue is diluted with hydrochloric acid and water. Sulfuric acid may be substituted for hydrochloric acid if the sulfates of the metals present are soluble. Fluoride up to a t least 0.03X does not interfere unless thorium is present. LITERATURE CITED
(1) Furman, N. H., Mason, 11.' B.', Pekola, J. S., ANAL.(:HEM. 21, 1325 (1949).
(2) Schwarzenbach, G., "Die komplexometrische Titration," p. 33, F. Enke, Stutt,gart, Germany, 1955. ~
RECEIVED for review August 16, 1956. Accepted November 14, 1956. Contribution No. 492. W70rk performed in Ames Laboratory of U. S. At,omic Energy CommisEion.
Estimation of Melamine in Presence of Guanidine R. M. ENGELBRECHT
and H. E. MOSELEY
Research Departmenf, Lion Oil Division, Monsanto Chemical Co., El Dorado, Ark.
W. P. DONAHOO Inorganic Division, Monsanto Chemical Co., Sf. Louis, Mo.
W. R. ROLINGSON Plastics Division, Monsanto Chemical Co., Texas City, Tex.
b Three methods are described for the determination of melamine in the presence of guanidine. Although these methods were devised to determine the melamine content of crude melamine during the synthesis process, they can be used for any melamineguanidine mixture. An oxalic acidpotossium permanganate method is recommended for melamine assay work.
I
s THE manufacture of melamine many different compounds are formed, depending on the particular processused. Some of these compounds a r e guanidine, guanylurea, dicyanodiamide, a.mmeline, and ammelide. Methods have been described for the determination of melamine in the presence
of these compounds (1, 3 ) . In these cases melamine was precipitated from a 50% acetic acid solution with saturated, aqueous picric acid a t 5" C. Guanidine picrate was found to coprecipitate with the melamine. An oxalic acid-potassium permanganate method (2) for melamine is satisfactory for assaying purposes, but the above compounds interfere if present. This paper describes three methods fok the determination of melamine in the presence of guanidine. EXPERIMENTAL PROCEDURES
Method A. The melamine is precipitated as the picrate in the presence of guanidine with the temperature of the precipitation controlled. To about 10 grams of finely ground, crude
Inelamine sample in a 250-ml. beaker, add 150 ml. of distilled water, and heat to boiling on a hot plate. Stir the hot solution with a mechanical stirrer for a half hour. Filter into a 250-ml. volumetric flask and dilute to the mark. To a 10-ml. aliquot in a 250-ml. beaker, add 15 ml. of distilled water and 25 ml. of glacial acetic acid. Heat to near boiling and add 150 ml. of saturated, aqueous picric acid with stirring. Continue to heat to boiling. If the yellow hairlike needles of melamine picrate form a t this point, a snialler aliquot must be takea. If no precipitate has formed, remove the beaker from the hot plate and slowly cool to 30" C. with occasional stirring. Filter through a weighed, medium porosity glass crucible. Wash sparingly with cold, distilled water and finally with 10 ml. of ether. Dry a t 105" C. for an hour, cool, and reweigh. Note the total VOL. 2 9 , NO. 4, APRIL 1957
579
volume of filtrate, exclusive of the ether, for a solubility correction. The melamine is calculated as follows: %melamine
=
(mt. of ppt.
mash sparingly with cold, distilled water, and dry a t .i05" C. for an hour. Cool and reweigh. The filtrate should
+ 0.00055z)(nliquot fact>r)(35.5)
where 0.00055 is the solubility of melamine picrate per 100 ml. of water; 5 is the volume of filtrate divided by 100; and 35.5 is the per cent melamine in the melamine picrate precipitate. Method B. The guanidine is eliminated prior to the melamine picrate precipitation. Prepare the sample and dilute t o 250 ml. as described in method A. Bring a IO-ml. aliquot to a p H of 10.5 to 11.0 with ammonium hydroxide. Slowly add about 20 ml. of saturated, aqueous picric acid solution with constant stirring. Set aside for 20 minutes and then filter through a medium porosity glass crucible. This precipitate may be used for the determination of guanidine. Transfer the filtrate to a 250-ml. beaker and add an equal volume of glacial acetic acid. Heat t o boiling on a hot plate and add 150 ml. of the picric acid solution. The procedure from this point is identical to method A. Method C. In this method the melamine is extracted with caustic. Accurately weigh a 10-gram sample in a 250-nil. beaker. Add about 150 ml. of distilled water and heat t o near boiling. Cautiously add enough pelleted sodium hydroxide, with stirring, to make the solution 10% caustic. Heat to boiling and immediately filter into a 250-ml. beaker. Wash the residue with hot water. Place the beaker with the filtrate and vashings in an ice bath and maintain a temperature of 5" C. for 3 to 4 hours. Stir the solution occasionally. Filter through a weighed, fine porosity, glass crucible;
.___
sample wt.
be inensured in order to apply the solubility correction. The melamine is calculated as follon :;:
+ O.lOGz]100 7omelamine = --[wt. of ppt. sample wt. where 0.106 is the Eolubility in grams of melamine per 100 nil. of 10% caustic solution a t 5" C. arid 2 is the volume of filtrate divided by 100. ,
Table I. Comparison of Three Melamine Methods % Me1amine
%
Method'- Method Method C
Guanidine - ....~ ~ ~ . . A ~ ~ . . B 15.0 l.G 1.5 5.5 4.8 4.5 588 2.9 2.8 4.7 4.1 3.5 None 2.4 2.5 3.9 1.5 1.5 4.4 7.4 7.0 2.4 9.9 9.4 14.5 9.7 10.0 None 12.1 12.2 ~~
1.2 4.4 2.G
... ...
1.5
... ...
10.0 -
...
0.2288 gram a t 25" C., and 0.2429 gram a t 20" C.
DISCUSSION
CONCLUSION
The study of these three methods was begun when mateiial balance computations showed yieldr: greater than 100%. A picrate precipitate was dissolved in boiling water and permitted to recrystaIlize slowly to room temperature. When this was done, two distinctly different picrates were visible. Microscopic evidence clearly showed melamine picrate and guanidine picrate to be the two crysi :ils present. Table I shows the comparative results of the three methods. The data presented are reeults of single exp,erimeiits. Rememb~ringthat in method B the guanidin3 mas removed by precipitation prior to the melamine analysis, there is n o evidence of coprecipitation in the rcmlts for method A. The degree of gumidine coprecipitation may be seen from the following data on the sample: The Jveight of the precipitate was 0.0165 gram a t 30" C.,
Any of the three methods described in this paper may be used for the determination of melamine in the presence of guanidine. From the standpoint of time, method B is preferred, since melamine and guanidine can be determined on the same aliquot. However, the choice of a particular method will depend on the condition of the crude melamine sample. Some samples cannot be filtered when slurried with hot water; therefore, the caustic extraction method must be used. LITERATURE CITED
(1) IZorinfskiI, A. A., Zavodskaya Lab. 12, 418-21 (1946). (2) Steele, J. R., Glover, J. H., Hodgson, H. IT., J . A p p l . Chem. (London) 2, 296-8 (1952). (3) Zavarov, G. V., Khirn. Prom. 1945, No. 2,21.
RECEIVED for review October 17, 105G. Accepted December 10, 105G.
Detection of Traces of Iron FRITZ FEIGL and ALCIDES CALDAS laboraforio da ProduSdo Mineral, MinistGrio da Agricultura, Rio de, Janeiro, Brazil Translafed by RALPH E. OESPER, University of Cincinnafi, Cincinnafi, Ohio
Soluble or insoluble compounds of trivalent iron give the characteristic red color of bivalent iron when they react with a solution of 2,2'-bipyridine or 1,lO-phenanthroline in thioglycolic acid. To detect iron in solutions obtained from alloys containing copper, cobalt, nickel, and zinc, and also to reveal traces of iron in tap and sea water and concentrated mineral acids, it is best to make the solution ammoniacal after adding a small'quantity of an
580
ANALYTICAL CHEMISTRY
aluminum salt. 'The alumina precipitate acts as collector for the ferric hydroxide. The mixed precipitate can be tested directly with the thioglycolic acid solution of h e reagent.
T
HE red color reactions of ferrous iron with 2,:;'-bipyridine (3) or 1,lOphenanthroline (11) are so sensitive and characteristic that they permit the detection and cdorimetric determina-
tion of very slight quantities of iron (9). However, certain metal ions, which yield no, or a t the most pale, colors with these reagents, interfere in the reaction becmse they consume the reagent to produce amminelike salts and thus withdraw it from the reaction with the ferrous ions. These ions include cadmium, zinc, copper, cobalt, and nickel. The last three and also chromium(II1) prevent or impair. a positive response from
