V O L U M E 21, NO. 5, M A Y 1 9 4 9 from 0.00 to 0.59 with an average of 0 . 2 1 5 . Results mav be duplicated on the same sample with a precision of about *0.05%. DISCUSSION
Because the intensity of the color system is affectcd by excess of kojic acid, it is necessary to measure carefullv the quantity used. Ten milliliters were found sufficient for 10 to 20 mg. of iron per liter ( 9 ) . The acid concentration may exert considerable influence on the intensity and hue of the color system and must be controlled within fairly narrow limits. The optimum pH range has been found to bc 5.5 to 7.0 (9). .It the lower value precipitation of iron compounds is avoided. Excessive acidity or basicity tends to destroy the color. Spectrophotometric study (9) has s h o w that the color system is stable for a t least 5 weeks at the p H range indicated above. According to Moss and Nellon (9) the most important interfering ions are cyanide, fluoride, iodide, nitrite, oxalate, phosphate, pyrophosphate, sulfite, aluminum, and zinc. The method is as rapid as a number of similar methods for iron (4-7) and is easily carried out. The color system reaches it. maximum intensity immediately and is stable for several weeks. The principal diverse ions which interfere are not normally present in iron ores. 4 s there is no sharp break in the spectral transmittancy curve
643 of the color system, the chance for error in the me: surement of the transmittancy is increased by even a slight error in the wave length setting. .Iny error arising in the measurement of 1 ml. of solution for the determination of the higher percentages of iron is lessened by using the same calibrated buret each time. Ailength of 20 cm. for 1 ml. is recommended. ACKNOWLEDGMEYTS
The writers wish to thank the General Research Council of Oregon State College, whose grant-in-aid made the work possible, and H. S. Barham of Kansas State College, who supplied the kojic acid. LITERATURE ClTED
Corbellini. d.,and Gregorini, B., Gazz. chim. ital, 60, 244 (1930). Mahin, E. G., “Quantitative Analysis,” 4th ed., p. 242, New York, McGraw-Hill Book Co., 1932. Mehlig, J. P., IND. ESG.CHEM.,AKAL.ED.,7, 387 (1935). I b i d . , 9, 162 (1937). Mehlig, J. P., and Hulett, H. R., I b i d . , 14, 869 (1942). Mehlig, J. P., and Shepherd, SI. J., J r . , Chemist A n a l y s t , 35, 8 (1946). I b i d . , 36, 52 (1947). EXG.CHEM.,AXAL.ED.,17, 86 (1945). iMellon. J1.G., IND. Moss, M. L., and Mellon, Jf. G., I b i d . , 13, 612 (1941). Tamiya, H., Acta Phytochim. ( J a p a n ) , 3, 51 (1927). RECEIVED September 10, 1947
Volumetric Determination of Iron Liquid Zinc Amalgam and Chrornous Chloride as Reductors WlLLI4RI D. COOICE, FRED HAZEL, AYD P A L L i C E \I. V c N h R B L‘niversity of Pennsylcania, Philadelphia, Pa.
C R I S G an investigation of a reaction involving pure iron
D salts, it became necessary to carry out approximately 400 iron determinations. Of the methods available, reduction by liquid zinc amalgam was rapid but inconvenient because an inert atmosphere had to be maintained for complete reduction (3, 4). I n the present work, the inert atmosphere has been eliminated and a small amount of chromous chloride used to complete the reduction. Solutions of ferric salts, acidified with either sulfuric or hl-drochloric acid, were reduced by treating with liquid zinc amalgam. After separation of the amalgam, .the residual ferric ion mas reduced by the addition of a few drops of chromous chloride solution. The reactions involving the chromous ion were followed by a low potential redox indicator, phenosafranine (oxidation-reduction potential -0.28 volt). Complete reduction of the ferric ion was indicated by the color change of the indicator from pink (oxidized form) to colorless. The reverse color change indicated complete oxidation of the excess chromous ion by atmospheric oxygen. S o evidence of oxidation of ferrous iron by air was observed under the conditions of the experiment (1, p. 390). The method was found to be more rapid and less cumbersome than reduction with liquid zinc amalgam in an inert atmosphere. Good precision and accuracy were obtained.
liters of dilute sulfuric arid ryere added. The mixture was placed on a steam bath for 1 hour, then washed with water. This amount of amalgam mi:found t o be sufficient to reduce about 100 samples containing 0.1 gram of iron.
Table I. Fe Taken Gram
Determination of Iron Fe Found Gram
Error
%
0.1508
0.1508 0.1509 0,1508 0 1507 N e a n 0.1508
0 0.1 0 0.1
0,0905
0,0905 0.0905 0.0904 0.0908 0,0905 Mean 0,0905
0 0 0.1 0 0
0,04323
0 04527 0,04523 0.04519 J f e a n 0,04523
0.1 0 0.1
0,00905
0.00905 0,00904 0,00901 0,00902 J l e a n 0.00903
0.0 0.1 0.4 0.3
REAGEUTS
Potassium Dichromate. Standard solutions of Kational Bureau of Standards potassium dichromate were made up by weight. Diphenylamine Sulfonate. A 0 32% water solution of diphenylamine sulfonate was prepared from the barium salt ( 7 ) Zinc Amalgam. Six grams of c P zinc (powdered, or 20- or 30mesh) were mixed n i t h 25 ml of clean mercury and a fev milli-
Chromous Chloride Solution. Approximately 0.5 S solution of chromic chloride in 0.5 t o 1 A- hydrochloric acid was placed in a dropping bottle Tyith 15 ml. of liquid zinc amalgam and shaken vigorously for a fen. minutes until the color changed to a deep blue. Occasional shaking was necessary to regenerate the chromous ion.
ANALYTICAL CHEMISTRY
644 Phenosafr mine. -40.0270 water solution. Iron Solution. The concentration of the iron solution was determined by reduction in a Jones reductor, and titration with standard dichromate. PRQCEDURE Ten milliliters of liquid zinc amalgam were placed in a 125-1311. separatory funnel, and aliquot portions of the ferric salt solution were pipetted into the funnel. The solutions were acidified and diluted to a volume of 50 ml., giving a final acid concentration of 1 to 7 X sulfuric or 1 to 3 N hydrochloric. The mixture was shaken vigorously for 30 to 60 seconds. One to 3 ml. of reagent grade carbon tetrachloride were added and the amalgam was withdrawn. One to 2 drops of phenosafranine indicator were added, followed by chromous chloride solution (usually 4 to 5 drops) until the pink color of the indicator disappeared and a clear green tint was visible. The solution was swirled 15 to 20 seconds or until the pink color reappeared. One milliliter of phosphoric acid and 0.2 ml. of 0.32% diphenylamine sulfonate were added and the solution was titrated with a standard solution of potassium dichromate to a deep purple color. An indicator correction of 0.03 ml. of 0.1 N potassium dichromate was suhtracted for each 0.2 ml. of indicator used.
occur until reduction of the iron has been accomplished and the indicator is oxidized by atmospheric oxygen only after oxidation of the chromous ion is complete. Results given in Table I shows that the procedure is capable of good accuracy and precision. The use of a separatory funnel and a small amount of carbon tetrachloride for separating the amalgam makes i t possible to carry out complete determinations more rapidly and with a8 good or better accuracy than by conventional methods ( 1 , 6).
Table 11. S t a n d a r d Oxidation-Reduction P o t e n t i a l s Volt-
Zn
% n - + * +26CrT- = Cr + + e blue green 3. HnIn-- = I n 3 H - + 2t.colorless pink 4. Fe-7 = F c ~ ~ cT5 . 2Hz0 = 0 2 4Ht 4e1. 2.
0.762 (A, 0 41 ( 2 ,
=
-
+
+
-0
28
-0,771 -1,229
+-
(6’
(1
DISCU SSIOh
LITER$TURE CITED
The favorable application of chrolnous chloride to the procedure is enhanced by the use of the indicator, phenosafranine. The color change of this indicator occurs a t an oxidation-reduction potential, -0.28 volt, which is between that of the chromouschromic ion potential aIld the potential corresponding to oxidation by atmospheric oxygen. These relationships are shown in Table 11, in which phenosafranine is designed as In. The potentials reveal that reduction of the indicator does not
(1) Hillebrand and Lundell, “.lpplied Inorganic Analysis,” p. HO(1 New York, John Wiley & Sons, 1929. (2) Latimer and Hildebrand, “Reference Books of Inorganic Chrrnistrv.” 4th ed.. D. 474. New York. Macmillan Co.. 1940. (3) Semosa, 2.anorg:Chem., 138,291 (1924). . 14,854 (1943 . (4) Smith and Kurtz, I N D . ENG.CHEY.,A X ~ LED., (5) Smith and Rich, J . Chem. Education, 7,2948 (1930). (e) Stiehler, Chen, and Clark, J . A m , Chem. sot,, 55,891 (1g33), . 5, 154 (19331. (7) Willard and Young, IND.ENG.CHEY.,A N ~ LED., R
~ .iugust ~ io, 1948. ~
~
v
~
~
Nomographs for Paraffin-Naphthene Split in the Type Analysis of Gasoline GEORGE W. THO3ISON Ethyl Corporation, Detroit, Mich.
N the type analysis of gasoline by the method developed by Kurtz and his associates ( 2 ) the relative proportions of paraffins and naphthenes in cuts which are free of aromatics and olefins .82
are obtained from the indrx of refraction, nLo, and density, diC of the cuts. The volume per cent of naphthenes in the mixture is obtained from a plot of the refractivity intercept, n g - d:’//2,
--
-18 -.76 -.74 -
I.O35 1.036
.80
1.037
f
150
, 1.039 W
0
z
&.72
1.040
, .70 --
>
-
p
.68-
t w
0
too
-
:
.6 6
E II
t
1.042
E
a 0
-
1.043
-
5
a
.62 .64
.60
?
0
1.038
1.044 USE DOTTED SCALE FOR VOLUME PER CENT NAPHTHENES IF BOILING POINT IS ABOVE 180’ C.
Figure 1. N o m o g r a p h 1
Figure 2.
Nomograph 2
1,045 1.046
