INDUSTRIAL AND ENGINEERING CHEMISTRY
558
Pharmacopoeia (15) and i t is recommended that the prepared solution be used within 0.5 hour. Amyl nitrite dissolved in amyl alcohol or ethyl alcohol was found to remain stable for at least a week when the solution was prepared under artificial light and stored in a brown bottle. It is possible that amyl nitrite is decomposed by light of a specific wave length. Preliminary observations with ultraviolet and infrared light were made, but further investigation is necessary.
Conclusions A method for the quantitative determination of amyl nitrite, adaptable for vapor-air and liquid analysis, is based on the fact that a color is produced when a compound containing an -ON0 radical is combined with phenol and copper sulfate in acid solution. The method will detect amyl nitrite in air within an average deviation of 10 per cent. The advantage of this method is that the amyl nitrite vapor can be absorbed directly in the color-producing reagent. This eliminates the need for a separate solvent for amyl nitrite and simplifies the procedure because the reagent from the sampling vessel can be poured directly into a colorimeter tube and read; when determining unknown concentrations, the development of a color in the sampling tube indicates that nitrite is present and gages the length of time required to collect an adequate sample; and although the test does not compare in sensitivity with many of those already described (1, 5 , 7 , IO, I S ) , it has certain advantages for sampling a large volume of vapor a t a low concentration for long periods. Amyl nitrite has been determined a t concentrations of 5, 10, 20, and 100 p. p. m . Amyl nitrite is stable in air and relatively stable when ex-
Vol. 13, No. 8
posed to.artificia1 light, but decomposes within 2 hours when exposed to direct sunlight.
Acknowledgment The authors wish to express their appreciation to Otto Schales for determining the spectral transmittance of the color and for his many helpful suggestions.
Literature Cited Dennis, L. M., and Nichols, M. L., “Gas Analysis”, p. 220, New York, Macmillan Co., 1929. Drinker, P., and Snell, J. R., J . Ind. Hug. Tozicol., 20, 321 (1938). Evelyn, K. A., J . Biol. Chem., 115, 63 (1936). Feigl, F., “Qualitative Analyse mit Hilfe von Tupfelreaktionen”, 3rd ed., p. 379, Leipaig, Akademische Verlagsgesellschaft. 1938. Leffmann, H.. “bllen’s Conlmercial Organic Analysis”, 5th ed., Vol. I. p. 318, Philadelphia, P. Blakiston’s Son & Co., 1923. Liebermann, C., Ber., 7, 247 (1874). Liebhafsky, H. A,, and Winslow, E. H., ISD. ESG. CHEM.,ilnal. Ed., 11, 189 (1939). Manchot, W , Ann., 375, 308 (1910). Noyes, W.A , , “Organic Syntheses”, Vol. XVI, p. 7, New York, John Wiley Bi Sons, 1936. Snell, F. D., and C. T., “Colorimetric Methods of Analysis”, Vol. I , p. 644, New York, D. Van Nostrand Co., 1936. Spiegel, L., “Der Stickstoff”, p. 154, Braunschweig, Friedrich Vieweg & Sohn, 1903. Sundquist, H., and Mohlin, E., Svensk Farm. Tid., 23, 626 (1916). Treadwell, F. P., tr. and rev. by W. T. Hall, “Analytical Chemistry”, 7th ed., Vol. 11, p. 306, New York, John Wiley & Sons, 1928. Tyndall, J., Proc. Rou. Inst. Grt. &it., 5, 429 (1869). U. S. Pharmacopoeia, 11th revision, pp. 56, 458, Easton, Penna., Mack Printing Co., 1935. Viles, F. J., J . Ind. Hug. Toricol., 22, 188 (1940). Ware, -A. H., AnaZUst, 52, 335 (1927).
Cerate Oxidimetry Determination of Glycerol G. FREDERICK SMITH AND F. R. DUKE, University of Illinois, Urbana, Ill.
G
LYCEROL is ordinarily determined by oxidation, using a n excess of standard potassium dichromate in sulfuric acid solution. The reaction, 3C3Hs03 7KzCr20, 28HzS04= 9C02 7Cr2(SO& 7K2SO4 40Hz0, requires an excess of dichromate and heating for 2 hours a t 90” to 100” C. to complete the oxidation. The dichromate required for oxidation of the glycerol is determined by titrating the excess, using ferrous sulfate solution. For the backtitration the sulfuric acid concentration must be approximately 4 formal. The green color of the solution during the titration of excess dichromate makes necessary either an outside indicator “spot plate” ferricyanide estimation of the equivalence point or a potentiometric titration. The chief objection to the determination of glycerol by such procedure is the time required for the oxidation. The present work describes a procedure by means of which this required time is reduced from 2 hours to only 15 minutes and the temperature from 90-100” to 50” C. The reaction upon which the method depends is C3Hs03 8H2Ce(C104)s 3H20 = 3HCOOH 8Ce(C104)3 24HC1O4. The excess of perchlorato ceric acid is then determined by titration, using standard sodium oxalate with nitro-ferroin (nitro-ophenanthroline ferrous complex) as internal indicator. For this titration a 2 formal perchloric acid concentration is pre-
+
+
+
+
+
+
+
+
+
ferred. Oxidation b y dichromate requires 14 equivalents per gram molecule of glycerol and by perchlorate cerate ion 8 equivalents.
Previous Work The determination of glycerol by the dichromate procedure, using a potentiometric end point and the platinum-tungsten bimetallic electrode pair, has been described by the Chemical Division, Procter and Gamble Company (6). The same reaction using the same electrode system and an electronically operated automatic buret sto ping device, mas used by Shenk and Fenwick (’7). Stamm 8 3 ) determined glycerol, using excess permanganate in strongly alkaline solution with titration of excess permanganate after acidification by use of oxalic acid. Glycerol was determined by Malaprade (c), using periodic acid in excess, followed by titration of the excess oxidant by one of several methods. By this procedure glycerol is oxidized to two molecules of formic acid and one molecule of formaldehyde. This oxidation of glycerol differs from the perchlorato cerate oxidation in that the formation of formaldehyde as an end product is eliminated in the latter case. The Malaprade reaction has been further studied by Allen, Charbonnier, and Coleman (1). Using the sulfato cerate ion, the oxidation of organic compounds in general results in the formation of formic acid as shown by Willard and Young (14), Cuthill and Atkins (9), and Fulmer, Hickey, and Underkofler (4). The previous papers on cerate oxidimetry by Smith and coworkers (3, 8-19) should be consulted for information con-
ANALYTICAL EDITION
August 15, 1941
559
Preparation of Reagents TABLEI. OXIDATION OF FORMIC ACID USING PERCHLORATO CERATEION
Ce-+++
Time
A-
0 1032 AOxalate
HCOOH Jf
h Calcd.
.W
Mzn.
Temperature 51' C 0
0.0911 0.0846 0,0761
67
157
22.22 20.47 18.42
0.2254 0.2220 0.2178
....... .
1.13 x 10-3 1 . 1 6 X 10-3
Temperature 66' C 0 73
0.0914 0.0607 0.0407 0.0281
156 240
22.30 15.13 9.85 6.80
0,2264 0.2100 0.2000 0.1939
.....,.,
5.6 5.3 5.3
X 10-3 x 10-3 X 10-3
cerning preparation of reagents, preparation and stability of solutions, reaction potentials, and specific applications.
Rate of Reaction between Formic Acid and Perchlorate Cerate and Sulfato Cerate Ions Since the oxidation of glycerol by the perchlorato or sulfato cerate ion involves the formation of formic acid which niight be expected to exhibit some reduction action of measurable rate, it became necessary first to study the influence of such possible effects. It is particularly appropriate to do this, since the oxidation of glycerol takes place over a short time interval in contact with excess oxidant at slightly elevated temperatures. The rate of oxidation of formic acid by perchlorato cerate ion was measured by titrating a t given time intervals 25-ml. aliquot portions of a solution of a kno\\n original cerate and formic acid concentrations maintained a t constant temperature. The data are shown in Table I. The second order reaction equation is:
where T is the time, a is the original concentration of tetravalent cerium, c is the original concentration of formic acid in moles per liter, z is the number of moles of formic acid oxidized by time T , and n is the oxygen equivalent of formic acid. Since the reaction is 2Ce++++ HCOOH = CO, 2Ce+fT 2H+, we know that n = 2. From the constants obtained a t the two different constant temperatures k as a function of T may be calculated from the Arrhenius equation
+
+
log Iczlki = A ( l / T i
-
+
1/TP)
By such calculation A iyas found to be 1.1 X 10'. Iinon-ing the practical Concentrations involved in the case of an actual determination of glycerol, it is possible to calculate the percentage of perchlorato cerate ion in\-olred in the oxidation of formic acid. These working concentrations are approximately 0.03 molar for both the cerate ion and the formic acid. The calculated error per determination of glycerol is less than 0.05 per cent. However, since k r increases rather rapidly with increase in temperature, the reaction temperature employed must be kept below a definite maximum, not above 60" C. and preferably a t approximately 50" C. The oxidation of formic acid by the sulfato cerate ion, Ce(S04)3--, in sulfuric acid solution was found to be so slow that it is impossible of measurement without resorting to absurd reaction concentrations or extended periods of time. This might be expected when one considers the wide difference in oxidation potential exerted by the sulfato cerate and perchlorato cerate ions under the conditions outlined (9).
AMMOXICMPERCHLORATO CERATE. Dissolve 55 to 56 grams of (SH,)ZCe(SOs)sby adding the salt to a liter beaker containing 340 to 345 ml. of 72 per cent perchloric acid. Stir well during a half-minute interval and add 100 ml. of water. Again stir well for a half-minute interval and add an additional lo0 ml. of water. Continue this procedure until the volume attains 1liter. Transfer to a suitable glass-stoppered reagent bottle and store in the dark. It is preferable to store the perchlorato cerate solution in a black bottle, conveniently made by completely covering a reagent bottle with black electrical insulating tape. Such a solution requires standardization about every 15 days. Standardization is best carried out by use of a standard 0.1 N sodium oxalate solution according t o the procedure described by Smith and Geta ( I O ) , using nitro-ferroin as indicator. Carbon or dust if allowed to come in contact with perchlorato cerate solutions catalyze their slight deterioration rate. SCLFATOCERATESOLUTION.Dissolve 63 to 65 grams of (iYH4)4Ce(S04)4.2Hz0 in 1 liter of 0.5 molar sulfuric acid. This solution is stable in storage and need not be kept in the dark. SODICMOXALATEIN 0.1 N PERCHLORIC ACID. Dissolve 13.412 grams of Bureau of Standards standard of reference sodium oxalate, per liter of 0.1 formal perchloric acid. This solution is stable upon storage. It is used to standardize the ammonium perchlorato cerate solution and for titrating the excess of perchlorato cerate in the determination of glycerol. Nitroferroin is used as internal indicator in both cases. FERROUS SULFATE. Dissolve 40 grams of Mohr's salt, FeS04.(NH&S04.6HZ0,per liter of 0.5 N sulfuric acid. Store this solution under hydrogen and standardize, using the perchlorate cerate solution with ferroin as indicator. FERROIN AKD KITRO-FERROIS. The ferrous sulfate complex of o-phenanthroline (ferroin) and of nitro-o-phenanthroline (nitroferroin) are now reagents of commerce prepared in solution form ready for use in analysis. GLYCEROL TREATEDSOAPLYES, A N D SOAP-LYECRUDES. These samples together with analyses were supplied through the courtesy of the Colgate-Palmolive-Peet Go., Jersey City, K. J. Appreciation is hereby expressed for the courtesy thus extended. BASIC LEAD ACETATE. Dissolve 100 grams of pure lead acetate in approximately 750 ml. of water in a 1.5-liter Erlenmeyer flask. Add 100 grams of lead oxide (PbO) and boil for 1 hour, using the reflux condenser. Filter the solution thus obtained while still hot. Disregard cloudiness in t'he filtrate and store in a suitable container. 0.5 N POTASSIUM HYDROXIDE IN ALCOHOL.Dissolve 16 grams of pure potassium hydroxide in the least required amount of water. Add the aqueous solution to 1 liter of 95 per cent ethyl alcohol and mix thoroughly. Allow to age 2 or 3 days to permit any potassium carbonate thrown out of solution to settle.
Preparation of Sample for Glycerol Determination FATSAND FATTY OILS. Warm the sample on the steam bath until liquid at about 80" C., filter, allow the free water to separate out, and decant the supernatant liquid into a dry bottle. Weigh 10 grams of the prepared sample into a 250-ml. Erlenmeyer flask and add 100 ml. of 0.5 N alcoholic potassium hydroxide solution and several pieces of porous plate to prevent lumping. Boil the contents of the flask under a reflux condenser for 90 minutes. Transfer the solution to a 500-ml. beaker, rinse the flask thoroughly rvith hot water, add to the transfer beaker, and dilute to 350 ml. with water. Evaporate by careful boiling to approximately 50 ml., dilute to 250 ml., and re-evaporate to 50 ml. Add 100 ml. of a solution of perchloric acid (20 ml. of 72 per cent perchloric acid diluted to 100 ml.) and heat on the steam bath until the fatty acids separate. Filter off the fatty acid cake and precipitated potassium perchlorate, washing the filter and precipitates with cold water. To the filtrate add sufficient basic lead acetate solution t o precipitate any proteins which may be present. If no precipitate forms upon the first addition no more precipitant need be added; if a precipitate does form, avoid a large excess of reagent by stepwise precipitation. When proteins are completely precipitated, filter, concentrate the filtrate if necessary on the steam bath, transfer to a 500-ml. volumetric flask, and dilute to volume. Repeat the procedure, omitting only the sample, to make a blank determination as a control. SOAP-LYECRUDES,TREATED SAMPLE LYES, AND DISTILLED GLYCEROL.Weigh sufficient sample to contain about 1 gram of glycerol, using a 400-ml. beaker in the transfer. Dilute to 250 ml. and test the solution for proteins with basic lead acetate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
560
C = standard dilution of sample, ml. D = sample weight E = ml. of aliquot taken
TABLE 11. ANALYSIS OF SYNTHETIC GLYCEROL SOLUTIOKS USIKGPERCHLORATO OR SULFATO CERATE Sample 1
0.1036 N Ce(ClO4)s-M1. 25.00
2
50.00
No,
1
0,0767 N Ce(SO4)a MZ. 50
Vol. 13, No. 8
0.1032 N CzO4--
Glycerol Present
Glycerol Found
Error in
Results Obtained by New and Old Procedures
MZ.
Mg.
M g
Mg.
3.27 3.28 3.29 22.37 22.44 22.40
25.88
25.92 25.91 25.90 33.02 32.94 32.99
Both the previously described methods were applied to the determination of synthetic glycerol solutions, with the results shown in Table 11. The determination of glycerol from palm oil was carried out with the results shown in Table 111. The determination of glycerol from distilled glycerol was carried out with the results shown in Table IV.
32.96
.
CsHsOs fO.04
f0.03 f0.02
f0.06
-0.02
f0.03
0.0547 A' FeS04 M1.
28.86 28.93 28.88 17.77 17.71 17.64
25.88
25.94 25.90 25.03 32.93 32.97 33.01
Summary and Conclusions
+0.06
4-0.02 4-0.05
The determination of glycerol by the use of the perchlorato 50 33.96 -0.03 cerate ion in excess in perchloric acid solution, followed by +f 0 O .. O0 l5 back-titration using standard sodium oxalate, has a number of material advantages over the familiar dichromate-ferrous sulTABLE 111. DETERMISATION OF GLYCEROL IN PALM OIL USINGPERCHLORATO fate procedure. The time required for OR SULFATO CERATE oxidation of glycerol is reduced from 180 to (Sample O.fi732 gram. 10-ml. aliquot portions f r o m ~OO-ml.dilution of glycerol. Average result 15 minutes and the reaction temperature from 10 aliquots following dichromate determillation gave value 9.65 per cent glycerol) 0.1036 N o.io32,v C204 Glycerol 0.0767 A ' 0 . 0 5 4 7 ~ FeSO4 G I ~ ~ ~required ~ ~ ~ is! reduced from 90-100° to Ce(ClOa)e-Ct04-Blank Found Ce(Khh-FeSO4 Blank Found j0-600 Colorless solutions during titraM1. MI. MI. % Ml. M1. M1. R tion make available the use of an internal 25.00 8.56 9.64 50.00 39.16 1.32 9.64 8.59 0:75 9.66 38.99 1.35 9.60 oxidation reduction indicator-namely, 8.58 0.75 9.67 39.09 .. 9 .66 nitro-ferroin-and makes unnecessary the 8.59 .. 9.66 39.12 .. 9.65 use of a potentiometric method. The Derchlorato cerate Drocedure in Derchloric acid medium has been checked by Filter if necessary, transfer the filtrate to 500-ml. volumetric the use of sulfuric acid solutions and sulfato cerate oxidaflask, and dilute to volume. tion without much saving of time but retaining the other advantages over existing procedures. Procedure in Determination of Glycerol The internal oxidation-reduction indicators used in these This procedure is a modification of standard soap manureactions are nitro-ferroin for the perchlorato cerate profacturer's methods with minor modifications. cedure and ferroin for the sulfato cerate scheme. The end Transfer samples of 10.00-ml. aliquots of the prepared solution points in all reactions are sharp and distinct, and the acto 400-ml. beakers. Add from a pipet a measured portion of curacy attained in all cases is believed to compare favorably standard perchlorato cerate solution which is known to be a sufwith existing procedures. Samples of commercially supplied ficient excess. Dilute to 100 ml. with 4 formal perchloric acid. materials of a variety of types have been used to illustrate the Warm on the steam bath to 50" C. and allow the reaction to proceed for 15 minutes at this temperature. The reaction temperamethod. ture should not be allowed to exceed 60". Cool and titrate the No hazardous reactions were encountered in the progress excess of perchlorato cerate ion with standard sodium oxalate of this work. solution, using 2 drops of 0.025 M nitro-ferroin as indicator. If for any reason the use of perchlorato cerate standard solutions and perchloric acid is objectionable the usual procedure (6) TABLE IV. DETERMINATION OF GLYCEROL DISTILLATES USING may be employed, in which case a sulfuric acid solution is subPERCHLORATO OR SULFATO CERATE stituted for the perchloric acid solution of the glyerol and the (Sample 2.4437 granis. 10-ml. aliquot portions from 100-ml. dilution of sulfato cerate solution in sulfuric acid is used as oxidant. The sample. Average result from 6 aliquots following dichromate procedure sample and excess oxidant must be heated to 90 t o 100" C. for gave value 97.81 per cent glycerol) 90 minutes before back-titration, using standard ferrous sulfate 0.1036 N 0.1032 N Glycerol 0.0767 N 0.0547 N Glycerol to determine excess sulfato cerate, with ferroin (in place of nitroCe(Cl04)~-czo4-Found Ce(SOda-FeSO4 Found ferroin) as indicator. 'Ill. M1. c In this case the chief advantage of the proposed procedure is lost-namely, a lower reaction temperature and shorter reaction time-although the advantages of ceric-ion oxidation are obvious as compared to dichromate oxidation, since there is no interference from the green color of the chromic Literature Cited ion formed in the latter case. T h e determinabion of glycerol (1) Allen, Charbonnier, a n d Coleman, IND. ENG.CHEM.,Anal. E d . , 12, 384 (1940). by use of dichromate and ferrous sulfate need not be carried (2) Cuthill a n d Atkins, J . SOC.Chem. Ind., 57, 91 (1938). out potentiometrically ( 6 ) , since excess dichromate can be ENG.CHEM.,Anal. Ed., 12, 201 (1940). (3) D u k e a n d Smith, IND. titrated with ferrous sulfate in 4 formal sulfuric acid solution, (4) Fulmer, Hickey, a n d Underkofler, Ibid., 12, 729 (1940). using ferroin as indicator. The reverse titration of ferrous (5) Malaprade, Compt. rend., 166, 382 (1928). (6) Procter & G a m b l e Co., Chemical Division, IND.ENG.CHEM., sulfate b y dichromate is not similarly satisfactory.
c,
1
Calculation of Results Glycerol compositions are calculated following the formulation: Per cent CsHsOs = 92C(A - B)/80 DE where A = ml. of cerate solution taken, multiplied by its standard factor B = ml. of back-titrant required, multiplied by its standard factor
Anal. Ed., 9, 515 (1937). Shenk a n d Fenwick, Ibid., 7, 194 (1935). S m i t h , F r a n k , and K o t t , Ibid., 12, 268 (1940). S m i t h a n d Getz, Ibid., 10, 191 (1938). Ibid., 10, 304 (1938). Ibid., 12, 339 (1940). Smith, Sullivan, a n d F r a n k , Ibid., 8, 449 (1936). S t a m m (Oesper), "Newer M e t h o d s of Volumetric Analysis", P a r t 111, p. 53, New York, D. V a n N o s t r a n d Co., 1938. (14) Willard a n d Young, J . Am. Chem. SOC.,52, 132 (1930).
(7) (8) (9) (10) (11) (12) (13)
