Determination of Furfural in Petroleum Stocks 0.I. MILNER
DAVID LIEDERMAN
and
Research and Development Department, Socony M o b i l Laboratories, Socony M o b i l O i l Co., Inc., Paulrboro,
Furfural can be determined photometrically in furfural refined lubricating oil stock by reaction with aniline in glacial acetic acid-benzene medium. The color is unstable, but the maximum intensity that develops can be measured and related to furfural content. The method is reliable for the determination of furfural concentrations as low as O.OOOlC&, and is applicable to both dark and colorless oils.
T
RdCES of furfural rvhich may remain in lubricating oil stocks that have been refined by furfural extraction can catalyze oxidation, causing discoloration during storage. It is t,herefore import’ant to maintain close control of the residual furfural content of the raffinate. A number of methods for determining small quantities of furfural have been reported (1-7). These methods, however, require a relatively expensive spectrophot,ometer ( 4 ) , or cannot be used directly on petroleum stocks, thus necessitating preliminary isolation of the furfural.
Table I. Sample
.Ibsorption Coefficient, Absorbance Units/Mg. Furfural
Furfural Foundh
P.P.11.
4
0 13 0
680 742 658
B
0
7
658 698
104 104
0
680
7
705
0 0
658
286 270 27R 272
C
a
Effect of Age of Aniline .icetate
- h i l i n e -icetat’% Prepn. Age, No. days
1 1 2 3
14.4 15.5 15.0
700
1 p.p.m. = 0.0001 w t . (7L
The method described in this paper is free from both of thr above objections, and has the following additional features. It is simple. It may be used on colored samples. An analysis can be completed in 15 to 30 minutes. Accurate results can be obtained on concentrations as low as 1 p.p.m. The method is based on the familiar reaction of furfural nith aniline to form a colored Schiff’s base according to the equation:
H-C-C-H H-C
ll
I1
C-CHO
\ /
+H
z
S
O+
0 H-C-C-H €I-C
I1
>’
II
C-C-N
ri
__
+ H20
However, in this case, the reaction is carried out in glacial acetic acid-benzene medium, rather than in aqueous medium as in the conventional method. This permits one to work directly with the oil and avoids the need for distillation, extraction, or similar preparatory ope1 ations common to most other available methods. COLOR CHARACTERlSTICS
It Fas found that when a hydrocarbon sample containing furfural was mixed with aniline acetate under the conditions de-
N. J.
scribed in the analytical Itrocrdure, :I red color devcloped. Examination of t,he spertral curve showed that the color \vas similar to that obtained iq- the convent,ional teat for furfural i n aqueous solution. However, in the :iiiliydroua medium, the color develops only slowly over the course of 3everal minutes to a maximum intensity, then fades gradually. Because of this instability, as well as the fact, that the reaction rxte varies n-ith furfural concent,ration, the customary procedure of allowing a fixed or minimum reaction t,ime before measuring color intensity cannot he followed. However, if t,he solutions are mixed and rapidly transferred to a colorimeter cell, the developnierit of the color can be fol1o:ved quantitatively hy mpidly and continuously ndiust,ing the dial of the instrument. The niasiniuni Lzhsohance that is att,ained hefore fading begills iy directly proportional to the fui,fur:tl content of the oil. REFRACTION EFFECTS
In attempting to correct for the initial color of dark oils hy considering the absorbance value of the unreacted benzene-oil solution, an apparent anomaly was obserred. The aniline ncetate, which has a slight color, gave a lower reading than benzene alone. This was shown to be caused by differences in the refractive indices of the two solvents. \Tit,h cylindrical colorimeter cells, the amount of light reaching the detecting surface of the photocell is altered significantly by a “focusing effect.” K i t h c ~ l l shaving plane-parallel faces, the effect is insignificant; ho\vmer, if ttlmrations of t8he optical path are produced-e.g., by marked displacement of the light source-a similar effect, is 01)tained even n-ith parallel surfaces. It is therefore recommended that parallel-faced cells be used: as a precautioii, tlip light source of the instrument should he adjusted so that equal readings are obtained with benzene and glacial acetic acid. Under these conditions, by taking into consideration the alnorbance of each component of the system, it is possible to make a simple correction for any color of the original oil solutim STABILITY O F REAGENT
The aniline acetate reagent gradually darkens on standing, but retains its effectiveness for a t least several weeks. I n fact, the intensity of the color developed for any given concentration of furfural gradually increases with the age of the aniline acetate. If exposure of the reagent to light is kept to a minimum, the increase in absorbance of the reaction product amounts to less than 1% for each day that the aniline acetate has aged (Table I). Hoxever, for precise work, it is necessary to analyze a standard furfural solution on the same day that the test samples are run, and to establish the absorption coefficient for that particular day. Despite this change in the reactivity of the aniline acetate ith time, results within the precision limitations of the method :tie obtained when the appropriate absorption coefficient is u w l (Table I). AN4LYTICAL PROCEDURE
Reagents. STANDARD FURFURAL SOLUTIOS, 0.01 mg. per ml. Reagent grade furfural, freshly redistilled, was dissolved in benzene and stored in the dark. AKILISEACETATESOLUTIOS.One hundred milliliters of reagent grade aniline was diluted to 1 liter with glacial acetic acid. Colorimeter. 9 Klett-Summerson colorimeter with No. 52 filter (maximum transmittance a t approximately 520 mp) and 2-cm. light path was used. 1822
V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5 Standardization. -4ppropriate volumes of the standard furfural solution containing 0.01 to 0.06 mg. of furfural were transferred to dry 30-ml. volumetric flasks and diluted to about 20 ml. with benzene. At the same time a reagent blank was prepared bjaddiiig 26.00 nil. of aniline acetate to a 50-ml. volumetric flask and diluting t o t,he mark with benzene. The colorimeter was adjusted with the blank. and each of the standards a-as treated in turn by adding 23.00 nil. of aniline acetate, then adjusting to voliinie and transferring to the colorimeter cell within 1 minute. Thc colorimeter reading was noted a t 10- to 15-second intervals. x i i d when maximum intensity was reached after 1 or 2 minutes thf. reading was recorded. Bn absorption coefficient, K , expressed :is milligrams of furfural per scale division, was calculated for each of the standards b!- dividing the instrument readin into furfural cwntent. Recaiise no individual K value deviateday more than 170from the average. the average value was used. Processing of Samples in Benzene. SAMPLES GIVIXG COLORXD SOLVTIOSS. The weighed .
