Metallo-Organic Complexes in Organic Analysis Colorimetric Determination of Alcohols FREDERICK R. DUKE, Frick Chemical Laboratory, Princeton University, Princeton, N. J.
A colorimetric method for alcohols based upon the reaction with ammonium hexanitratocerclte makes possible determination of relatively small amounts of alcohol in mixtures with approximately 5% accuracy. The disadvantages of the method are the low stability of the color and the interference of certain common reducing agents.
rhe percentage rate of decay was found to be substantially inlependeiit of concentration and a straight-line function for at .east 15 minutes for the higher concentrations (Table I). EXTINCTION us. CONCENTRATION CURVES. Working CUNW 1were prepared for the alcohols methanol through the butanols. The time of mixing and the time of reading in each c u e were r e :orded and the extinctions were corrected to zero time, using the iormula
T
H E red coordination complex produced upon the addition of an alcohol to ammonium hexanitratocerate solution has been used qualitatively to identify the alcoholic hydroxyl group (1). Primary and secondary alcohols slowly reduce the cerium to the trivalent state with resultant dissipation of color ( I ) . It was found, however, that with concentrations of alcohols in the colorimetric range, the extinction decay with time is a straightline function for 15 minutes or more, allowing extrapolation to zero time. Although the sensitivity of the method is lowered, the cerate solution is prepared without acidification to increase the color stability.
100 E , E, = 100 - A , where Ez is the extinction at zero time, E,,, the measured extinction, A the percentage decay per minute (from Table I), and t the time in minutes. The plots of molar concentration us. extinction b t zero time appear in Figure 2. PROCEDURE.To 2.5 ml. of the stock cerate solution in a 25-ml. volumetric flask is added the unknown alcoholic solution. After iilution to the mark and thorough mixing, the extinction is read
REAGENTS AND APPARATUS
AMMONIUM HEXANITRATOCERATE SOLUTION.The commercial product was dissolved in the minimum amount of water, and diluted so that 1 liter contained 667 grams of (NH?)&e(NO&. Filtration through sintered glass or fine glass cotton is necessary if the solution is cloudy. ALCOHOL SoLwrIoNs. Alcohols of the best commercial grade were diluted with water to 5 % alcohol by volume. A CENCO-SHEARD SPECTRQPHOTOMETER was used throughout the investigation. The entering and exit slit widths were held constant at 1 mm. and 10 mp, respectively. Table Alcoho hlethanol Ethanol n-Propanol n-Butanol
1.
Color Stability at Room Temperature D ecrea8e in Extinction yo per min. 1.1 1.0 1.1 1.0
Alcohol
Decrease in Extinctior
yo.per min. Isobutanol Is0 ropanol scc-Ltanol tert-Butanol
0.95 0.37 0.36
, 420 440 4 6 0 480 5 0 0 520 540
Stable
m)l Figure 1.
Butanol Curves
x. Carrtr u.. watrr 0 . Cwatr plur rlcohol urn. water e. Cerate plur alcohol.0. crrala
EXPERIMENTAL
OPTIMUMCERATE CONCENTRATION. To 25-ml. volumetric flasks were added in duplicate 1.O, 1.5, .2.0, 2.5, 3.0, and 5.0 m!. of the stock cerate solution. The duplicate in each case was diluted to the mark with water, and to the other flask were added 0.5 ml. of 5 % ethanol and water to the mark. Using the corresponding aqueous cerate as a blank, the transmission of each ethanol solution was measured at 475 mp. The decrease in transmission with increasing cerate concentration was very marked to between 2.5 and 3.0 ml. of cerate per 25 ml., but was small between 3.0 and 5.0 ml. of cerate per 25 ml. Therefore, 2.5 ml. of stock solution per 25 ml. were selected as providing the optimum cerate concentration. TRANSMISSION MINIMUM. tert-Butanol which produces a stable color waa selected for the measurements of the transmission over the ran e of wave lengths 400 mp to 700 mp. Five alcohol solution were added to 2.5 ml. of the milliliters of the cerate. solution, and the mixture was diluted to 25 ml. The reference was an identical solution of cerate without the alcohol. A transmission minimum was found in the range 460 to 475 mp; this minimum was duplicated using ethanol in place of terbbutanol. Because the transmission of the reference increased sharply from 460 to 475 mp, the latter wave length was selected for the determinations. The butanol curves are shown in Figure 1.
58
Millimoles of Alcohol
Employing the wave length and concenCOLORSTABILITY. trations selected as described, the decay of the extinction with time was measured for three different concentrations of each of the eight lower aliphatic alcohols, methanol through the butanols.
Figure 2.
Extinction VI. Concentration Curves
a. Irobuhnol b. n-Buhnol C.
n-Pmwnol
d. Ethanol
572
X f:
Methanol wc-~utano~ IIOW no1 tert-IPu;.nol
ANALYTICAL EDITION
September, 1945 Table Alcohol Methanol
Ethanol
Taken
Fourd
Error
MU.
MU.
60
58 79 41 42 123 38
% -3.3 -1.2 4-2.6
40
120
40 80
a3
+5.0
+a.s
-5.0
+3.8
Analysis d Aqueous Ethanol, Other Substances Present
Solvent Present
MI. Acetone, 10 Acetone 2.5
Pure Aqueous Alcohols
80 40
n-Propanol 1.0 ropsnol n-Jutanol Imbutanol rec-Butanol
Table 111.
II. h l y d s of
Ethanol Taken
Ethanol Found
Ma. 80
Ma.
D i o r a d , 10 Ethyl acetate" EtherU Acetaldeh de 2.5 Butyrald&dea Acetic acid, 0.2 Acetic acid, 2 0 20 ml. of saturated aqueous
40 40
80 40 40
80 80
40
72 39 39 81 40 45 81 73 4.3
Error
%
--2.5 10
-2.5 f1.2 0
4-12.5 f1.2 -9 90
-
solution preaent.
at 475 mp, the reference being an identical solution minus the unknown. The time of mixing and the time of reading are recorded, and one or two additional readings are taken at 2 to 3 minute intervals. The extinction extrapolated to zero time is compared with the workin curve to find the amount of alcohol present. If pure aqueous afcohol is being determined, the method of extrapolation used in the above para aph may be employed. The reference solution should be replarcyeach half hour, because in dilute solutions the cerate has a tendency to change color due to a hydrolytic reaction which finally results in the precipitation of basic ceric nitrate.
INTERFERENCES. Phenols, enols, and aromatic amines interfere by producing anomalous colors. 1,2-0xygen-containing
573
compounds are good reducing agents for tetravalent cerium (3) and therefore should be absent. Ethylene glycol is the only member of this series which yields a stable enough color for d e termination. Formaldehyde, acetaldehyde, and propionaldehyde interfere, the former by reducing the cerium, and the others by producing varying amounts of color. For example, pure acetaldehyde which has stood at 20" C. appears to have the extinction of about 1% aqueous ethanol, while that kept at 0" C. yields the color of a 5 to 10% alcoholic solution, and propionaldehyde behaves similarly to a lesser extent; the polymeric forms of these aldehydes probably contain hydroxyl groups. No common inorganic reducing agents, bases, or highly colored compounds may be present. The alcohol should be separated from all common anions by distillation of the neutral solution, because all common anions compete with the alcohol in complex formation. Very high concentrations of noninterfering organic .compounds cause deviation of the time extrapolation from a straight line. The method is not useful on mixtures immiscible with the reagent, although water extracts methanol and ethanol quantitatively from most mixtures of this type (9). APPLICATION OF PROCEDURE. Pure aqueous alcohol solutions were analyzed (Table 11). Solutions of ethanol containing large amounts of common nonalcoholic solvents were analyzed with the results listed in Table 111. LITERATURE CITED Duke, k . R.. and Smith G. F., IND. Fho. CHEM.,ANAL.ED., 12, 201 (1940). (2) Seidell. h.,"Solubilities of Organic Compounds", 3rd ed., Vol. 11, pp. 45ff, 1306, New York, D. Van Nostrand Co., 1941. 13) Smith, G. F., and Duke, F. R., IND.ENG.CHEM.,A N ~ LED., . 15. 120 (1943)
(1)
Colorimetric Determination of Molybdenum in Iron and Steel MITCHELL KAPRON AND PAUL L. HEHMAN, Packard Motor Car Co., Detroit, Mich. A photometric method for determining molybdenum in ferrous metals employs water-soluble solvent, of low volatility which produce a very stable molybdenum-thiocyanate complex color without the necessity for extraction. Interferences and their elimination, as well as the precision and accuracy of the method, are discussed.
W
I T H recent progress in the manufacture of precision electric photometric apparatus, colorimetric methods for routine analyses have greatly simplified many determinations. One of these is the determination of molybdenum by measuring the intensity of the color of the molybdenum-thiocyanate complex. This molybdenum complex is customarily extracted from solution, after reduction by stannous chloride, using ether (6,6), butyl acetate (I), or cyclohexanol (3) as extractants. The limitations of these solvents (8) and of the extraction method became a p parent when an attempt was made to standardize a routine method for use with a Fisher A.C. Model photometer using 23-ml. absorption cells. In an attempt to overcome these limitations and to develop a method characterized by the ease and simplicity of a direct determination the possibility of using water-soluble, less volatile solvents, the glycol ethers was studied. Those available for immediate considerations were Cellosolve, diethyl Cellosolve, butyl Cellosolve, Carbitol, methyl Carbitol, and butyl Carbitol obtained from the Carbide and Carbon Chemicals Corp. EXPERIMENTAL
Preliminary investigation showed that the molybdenumthiocyanate complex color was strongly developed in solution,
without the necessity of extraction. The method developed for carbon and alloy steels and cast irons consists of dissolving the sample in perchloric acid, adding the appropriate solvent, and developing the color suhsequently with potassium thiocyanate and stannous rhloride. I n high-chromium and stainless steels it way necessary to remove most of the chromium in order to get the required accuracy; the procedure outlined by Smith (8)for volatilizing the chromium &s chromyl chloride was found effective. As perchloric ,acid is unsatisfactory for dissolving tungsten steels, the methods of Cunningham (Z), Smith (9), and Poole (7) were investigated. The erchloric-phosphoric acid mixtures (7, 9 ) were further studiea, since they conformed more t o the procedures adopted for the other steels. S u b uent work revealed that the acid ratio of Smith (9) provide8 the optimum solvent for the sample weight taken and I t waa therefore chosen for more complete study. The transmittance characteristics of the color developed using butyl Cellosolve and butyl Carbitol were checked with a universal spectrophotometer and it was found that maximum absorption occurred a t a wave length of approximately 470 mp. Accordingly a Corning filter No. 430 having a maximum transmittance of light of this wave length was used for all subsequent work. The least amount of solvent that could be added conveniently and still ensure maximum color development was determined t u be 15 ml. Less than this led to a rapid fading of the rolllr, while more produced no increase in intensity.
Figure 1 shows the results obtained, using the six solvents mentioned above, on Tiationat Bureau of Standards steel 72b containing 0.223% molybdenum. Obviously, on the basis of color
