Determination of Iron in Used lubricating Oils by Spectrochemical Analysis JOHN HANSEN, PAUL SKIBA, AND C. R. HODGKINS Esso Laboratories, Standard Oil Development Co., Linden, N . J . A rapid and accurate method for the quantitative determination of a wide range of iron content in used lubricating oils is useful when extensive engine wear or performance studies are being undertaken. A spectrochemical procedure capable of yielding accurate analyses of 3 to 3000 p.p.m. of iron in used lubricating oils is described. A fixed quantity of internal standard is added to a weighed amount of sample and 10 drops of the resulting mixture ashed in the cavity of an electrode. The electrode is designed so that the cavity wall can be easily removed to expose the ash on a conical tip. Satisfactory results have been obtained using as little as 5 grams of sample. The over-all accuracy of the method is within approximately 4 ~ 5 %of the amount present.
T
HE quantitative determination of the iron content (together
with other chemical and physical examinations) of crankcase drainings is frequently a basis for estimating engine wear. The rather large quantity of sample required and the somewhat tedious separations which must necessarily be made prior to the wet chemical analysis of the iron content made a rapid and accurate spectrochemical method very desirable. By using any of the usual wet chemical procedures, approximately eight iron determinations can be completed per man day. The spectrochemical procedure described in this paper resulted in a t least a 100% increase in the number of determinations per man day, and considerable reduction in the elapsed time and in the amount of sample required. The precision and accuracy are equivalent to that of the conventional chemical methods. The procedure has been successfully used to analyze samples ranging from 0.0003 to 0.30% iron. This wide range is accomplished by the preparation of analytical curves for a large number of iion lines of varying intensity. However, only one internal standard line is used throughout, and no modification or variation of the technique or sampling is necessary. The paramount purpose for inaugurating the development of a spectrochemical method vas to reduce the cost per analysis and make it possible to complete a greater number of determinations. No consideration was given to any procedure which would require a preliminary ashing or digestion to destroy the organic matter to obtain a residue because of the time involved in this operation. Inasmuch as the major portion of the iron in the sample is in suspension, no effort was made to apply any continuous feed technique. For the same reason, the quenched electrode technique described by Calkins and White ( 1 ) was not deemed applicable.
3100 il. is recorded, The height-limiting apertures of the aperture plate on the stepped sector unit have been altered SO that one of the openings permits only the radiation passing the 25, 50, and 100% steps of the stepped sector to fall on the grating. Thus, four spectra can be recorded across 35-mm. film. By storing the exposed portion of the film in the transfer case and moving the film just enough to bring unexposed film in front of the mask opening, 36 exposures can be recorded on a single length of film. STANDARDS
Cobalt naphthenate was selected as the added internal standard because of its solubility in oil and the similarity in the excitation characteristics of the iron and cobalt. Inasmuch as the commercially available cobalt naphthenate contains a considerable quantity of iron as an impurity, the compound was prepared in the laboratory by the procedure described below.
I" D c _
I - -
I b
4
,"6'
1
APPARATUS
The following spectrographic equipment was used in the development of this method. Excitation: ARL-Dietert No. 2040 alternating current arc unit. SDectroprauh: ARL-Dietert KO. 2060 1.5-meter grating spec'trograph: Film: Eastman Kodak SA No. 1. Develouine unit: ARL-Dietert temuerature-controlled rocking developing machine. Shaker: International bottle shaker unit. Densitometer: ARL-Dietert No. 2250 projection comparator densitometer. In order to save film and time, a mask is inserted in front of the film a t the camera so that only the spectrum from 2900 to
CROSS SECTION
I1 2"
Figure 1.
'
L *CYLINDRICAL
WALL
REMOVED
Sample Electrode
A quantity of regular grade naphthenic acid (225 to 250 neutralization number) was diluted t o three times its volume with petroleum ether. This solution was thoroughly washed with 6 Ahydrochloric acid until the aqueous layer remained colorless. When a portion of the purified acid solution was checked spectrochemically and indicated the absence of iron, the neutralization number was determined.
1362
1363
V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1 Cobalt chloride was prepared using cobalt metal C.P. (Eimer h Amend No. 363) which had been found to be free of iron. A Dortion of the Detraleum ether solution of the iron-free
... .~-.
~I
~~~~~
treated &th xylol to dissblve the cohaltnaphthenate and filtered to remove the precipitated salts. The xylol diluent was removed by evaporation. The cobalt content of the final product was determined ohemically. The final uroduet of the reaction was dissolved in reagent grade xylene'(Baker & Adamson) to prepare a stock solution equivalent to 13.5 grams of cohdt per liter of solution. This do& roliidion is fnrther diluted 1 to 10 with xylene before sddi..-.. tion t o the oil sample. ~~
~
~~
~~~
As an analysis of the clarified supernatant oil of a centrifuged sample indicated that the major portion of the iron in the drain-
@
ings was in a suspended fonn, the possibility of preparing standard samples by dissolving an iron salt of an organic acid in oil WRS discarded. Therefore, a series of chemically analyzed samples of varying iron content was selected for primary standards. The standards for the lower iron concentrations were prepared by appropriate dilution (u.ith new oil) of an analyzed sample. Special spectroscopic graphite electrodes (National Carbon C o . )uwe used in this work: a l / ~X 2 inch counter electrode and x 2 inch sample electrode. The counter electrode is prepitred X 12 inch electrode and breaking a t the score by sraring a mark. Inasmuch a? tho tip shape does not appear to he critical, no further Rhaping is necessary. In order that the oil might he nnhed direatly on the sample electrode without any possibility of the oil running over tho side of the crater, the electrode w a fahri~ eated 80 that a 60" conical tip wa8 surrounded by a '1, inoh deep cylindrical wall. The electrode and the cutter are illustrated in Figures 1 and 2. Lysing an electrode of this design and of the dimensiorix dven, it is possible to burn the sample without any loss. After the nshing is completed, the cylindrical wall is carefully broken am,y exposing the conical tip with the ash deposited on the surface. The wall is removed 80 that the arcing mill be lietween t,he counter electrode and the conical tip of the sample electrode. With an ordinary cup or platform-type electrode, much of the arcing is to the periphery of bhe electrode and not to t.hr sample.
p>
PLAN V I E W
SPECTROGRAPHIC TECHNIQUE
Inasmuch a8 the major portion of the iron present in crankoase draining8 is in suspension, i t is important that the sample he made homogeneous so that any portion taken for analysis will he representative of the entire drainings. The sample &B received is vigorously shaken on an International bottle shaker unit and 20 grams of the thoroughly mixed oil are weighed into a 2-ounce widemouthed bottle. Exactly 1 ml. of the internal standard cobalt naphthenate solution (1.35 mg. of cobalt per ml.) is added to bhe 20 grams of oil. This addition of internal standard can be done rapidly and aacurately with an automatic transfer pipet (Alfred Bicknell Associates No. P 2017). If less than 20 grams of sample are available, a preportionately smaller volume of int,ernd standard must he added. Tho woighed sample plus internal standard is shaken in the rnechmioill shaker for several minutes and then approximately 1 ml. is taken into a medicine dropper. This mixture is then ashed Fig-ure 2. Cutter for Making the Elert.de for dropn.isr in the rwvity of the electrode Dreviouslv described. Imn Determination The ashing operation can he carried out more rapidly if the electrode is mounted in a. heated block, as shown in Figure 3, and the ignition initiated with a micro burner. After 10 to 12 drops have been burned, the residual carbon is removed by more intense heat of the micro burner or a manifold burner. More reproducible spectrogranis areohtained when the electrode is strongly heated to burn off the carbon residue after the fourth and eighth drops have heen burned. When the ashing has been completed and the electrode hns coaled, the cylindrical wall is carefully broken away exposing the conical center tip. The prepared electrode is placed in the lower electrode Figure 3. EIectrode Heatec Rloek and Manifold Rurner
ANALYTICAL CHEMISTRY
1364 holder of the arc-spark stand and a l / g X 2 inch counter electrode is adjusted to a 6-mm. gap with the optical axis a t the midpoint of the gap. The sample is then arced for 40 seconds in the alternating current arc a t 5 kv. with 2 amperes current. A primary slit width of 50 microns is used. I n order that the analytical line may be read a t the optimum transmittance range, the energy passed by the 25, 50, and 100% steps of the 4 stepped sector is photographed. Eastman Kodak SA KO.1 film is used to record the spectrum. Inasmuch as the analytical lines used lie between 2954 and 3045 A. and as this region is in a rather flat portion of the gamma change versus wavelength curve of SA No. 1 film, a single emulsion calibration curve has been found to be adequate. The details of the photographic processing procedure are shown below.
F.
Table I.
Intensity Ratio and Iron Content as Derived from Individual -4nalytical Lines 70Transmittance Wave Length, -4. Co3044.0 F e 2953.9 2966.9 2994.4 2999.5 3000.9 3008.1 3009.6 3020.6
Step l 34.2
Step 2 68:9
0:26
43:O 59.5
..
Co 3044.0 F e 2953 9 2966.9 2994.4 2999.5 3000.9 3008.1 3009,6 3020.6
36.2
'a"1p1e
Developing: Eastman Kodak D19 developer, 5 minutes, 68"
Sample 3
Shortstop: 5 ml. of glacial acetic acid in 400 ml. of m-ater, 15 seconds. Fixer: General Electric x-ray fixer, 1 minute. Water wash: 2 minutes. Drying: B.K.L. film dryer, 2 minutes. The spectra are measured % ith an ARL densitometer adjusted to 1 0 0 ~transmito tance for the background next to the line. The transmittance of the 3044.0 A. cobalt line and one or more of the following iron lines, 2953.9 A., 2966.9 A., 2994.4 A, 2999.5 A . , 3000.9 A., 3008.1 A., 3009.6 A, 3020.6 A. is measured. The analytical curves were constructed on 2 X 4 cycle logarithmic paper. Typical curves are shown in Figure 4. The intensity ratio is found by applying the transmittance readings of the several linps to a film calibration curve prepared for the region by any of the methods described by Churchill ( 2 ) . h per cent iron is found by applying the intensity ratio to the analytical curves. The average percentage as determined from the several element lines is reported.
0.0055 0.0052 0.0050
..
87:6 33.1 62.6
0.84 0.62 0.16 0.51 0.29
ii:3
..
2:i4
0.6031
.,.. 0.51 1.72 1.29 0.29 1.08 0.60 0.16
0.0120 0.0120 0.0115 o.nii8 0.0116 0.0114 0.0114
..
..
..
3k:l
..
16:6 25.2
6415
__
3i:l 63.0
88:s
, ,
0 0 0 0 0 0 0
2.87 2.15 0.49 1.87 1.05 0.27
42:9
..
.. ..
0.0051 0.0032
0.85
..
17:2 38.9
0.0058
..
..
, .
41.3 49.7 9,2 13.8
Co 3044.9 F e 2903,9 2966.9 2994.4 2999.5 3000.9 3008.1 3009,6 3020.6
Intensity Ratio c/a Iron
73:5
..
0218
0220 0210 0220 0208 0209 0213
~Table 11. Repeatability Study
c/a iron Chemical method 0.00396 Spectrochemical method Electrode 0.0044 0.0038 3 0.0040 4 0,0040
-t .F,
o. nmx
6 i 8 9 10
0.0041 0.0040 0.0038 0.0041
hlean Standard deviation
a
0.0044
0.00404
...
Sample 1 Deviation from ohemical analysisa
Deviation from mean
..
....
+ O . 00036 - 0.00024 - 0,00004 0.00004 - 0.00024
+0.00044
.
,
-
+ O , 00006 - 0,00004 0.00024
-
+ O ,00006 + O . 00036
- 0.00016 + O . 00004 + O . 00004 0.00016 +0.00014 +O ,00004 - 0.000016 + 0 . 00014 + O . 00044
-
*0.00017 +0.00018 0.00023 ....
Sample 2 Deviation Deviation from cheiiiical f r o m mean analysis"
-___________ L,; iron 0.00375
....
.
- 0.00005
+o. 00005
+o ,00015
+ O . 00025 +o . O O O l B
0.0038 0.0037 0.0040 0.0039 0.003i 0.0038 0.0038 0.0041 0.0039 0.0038
- 0,00015 - 0.00005
0.00385
+0.00010
- 0.00015
+0.00005
-0.00005 f0.00025 +o. 00005
- 0.00005
...
14.570 +4.4y0 ... 8.9% 10.9% ... Deviation from iron content as determined by w e t chemical analysis
..
0.00013
- 0.00005
- 0.00005
+o. 00005
i0.00005 + O ,00035 f O ,00015
+ O ,00005 r0.00012
..,.
=t2.6Yo 6 5%
3=3.270 9 3%
1000
2000 XXK)
EXPERIMENTAL
The number of iron lines on uhich the transmittance can be measured is dependent on the iron concentration and the density of the spectrogram. The overlapping of the analytical curves permits a great deal of variation in the optical density except a t high or low concentrations. The __data shown in Table I are _typical of instances when seven of the eight element lines could be measured. The deviations in the results obtained from the several e l e ment lines of the same spectrogram can be attributed to an error in transmittance I measurement or in the film 3 calibration curve. The repeatability of the method was established by
I/
5
I IO
2030
50 Per Cent
Figure 4.
I I!l
// 100
200 300
500
Iron x IO-'
Working Curves for Iron Determination
V O L U M E 2 3 , NO. 10, O C T O B E R 1 9 5 1 ___
1365 __
Table 111. ~~
('lirtnical Iron l.\v.) 0 00230
0 00748 0 0124 0 055 0 108 0 125 0 156 0 199
than two pears in this laboratory to the determination of the iron cont,ent of crankcase drainings. The samples analyzed included crankcase drainiron Diff. de\-iation 0 0024 +o 0001 4 3 ings from both gasoline and Diesel engines lubricated with 0 022 -0 003 i i both additive and nonadditivtt 0 Ilb +O 008 7 4 . 1) 129 t o 00-1 3 2 type oils. 0 R I) 157 + 0 001 0 18'1 - 0 010 i0 While the basic techniyuc of the method could possibl\ -13 be applied to the dettlimination of other elements present in used lubricants, I I O inve-tigation of this sort has been made.
Comparison of Chemical and Spectrochemical Results _ _ Spectrochemical ______~_ Determination 1 -- - - -Determination 2 Determination 3 yo % 7c %70 %
iron
Diff.
0.0026 0.0073 0.0130 0.048 0.106 0.127 0.158 0.199
+o ,0003 - 0.00018 +0.0006 -0.007 -0.002 +0.002 +0.002 0.000
-
~~
deviation 13 2 4 12 2 1
0
4 8 7
0 6
1 3 0 0
iron
0 0 0 0 0 0 0 0
0025 0075 0130 056 126 131 159
20:
Diff.
+ a . 0002
+o
f0 +0 +O +O.
+o
+o
00002 0006 001 018 006 003 006
deviation 8.i 0.3
4.8 1 8 16.7 4.8
1.9 3.1
~
1wparing and arcing 10 electrodes for each of two samples, iron1 (liferent sources, having approsinlately the same iron vontPtit. The, results of each spectrogram, together with the deviation l ' w i i i the mean and from the iron content as determined 1 ) ~ . \ \ e t chemical analysis, are shown in Table 11. One of the principal attributes of the method, particular1~-as :I pplied to the samples received in this laboratory, is the wide i:irigr of concentrations that can be determined without any iiiodification or variation of technique. The data shown in l'atile I11 indicate the accuracy a t different levels of concentrat i ~ nits compared to the average of two or more wet chemical :I iialJ.ses. The method described has been successfully applied for niore
ACK~OWLEDGNIENT
The authors uish to express appreciation to T. J. Devlin anti
S D . D'Adarno for their assistance in running samples and
fol
t l i ~ i rcontribution to the improvement of the physical techniqucl of the sample preparation. LITER4TURE CITED
(1) Calkins, L. E., and White, h l . lI.,N a t l . Petroleum News,38,S o . 27, 519 (1946). 12) Churchill, J. R., ISD. ENG.'?HEM., - 4 s ~ED., ~ . 16, 653 (1944).
RECEIVED April
20, 1951.
Spectrographic Analysis of New and Used lubricating Oils C. RI. GARIBRILL, A. G. GASSMANN, AND W. R. O'NEILL Ethyl Corp., Detroit, Mich. The work was begun because rapid, accurate niethods for determining metal content of lubricating oils are needed for controlling the blend of additive oils and for examining used oils to aid engine performance studies. Ordinary methods of chemical analysis are usually too cumbersome and tedious fur this purpose and attempts have been made to utilize the speed of spectrographic methods. A spectrographic method using the porous cup electrode technique is described for the determination of the additives barium, calcium, phosphorus, and zinc in new lubricating oils, and its variables are discussed. An over-
S
I'E.CTROGRAPHIC determination of metals and phouphorus i n lubricating oils presents special problems because of the difficulty of introducing the sample into the excitation discharge ant1 liecause the type of base oil and additive may differ from sample to sample. Three different techniques have been used to introduce the sample into the spark. One technique employing electrodes prepared by quenching hot graphite rods in the oil *ample ( 3 )has limited usefulness (5). A second technique (5, 6) wing porous cup electrodes similar to those described by Feldman (4)has a more general application but is not suitable for anslj zing used oils containing suspended solids. A third technique employing a rotating disb electrode has recently been used for defrrniining phosphorus in unused lubricating oils (1,7 ) . The porous cup technique has been successfully used in these laboratories for routine analysis of new lubricating oils. Its use for determining phosphorus has been described (5, 6). The present paper describes an extension of the method to include the additive metals barium, calcium, and zinc, and discusses further qtudies of some of the variables involved.
all accuracy of 10% is claimed. A second technique uses a rotating disk electrode for the determination of lead, iron, and the additives in used oils. Results for the additives are compared with those obtained using porous cup electrodes. Accuracies of the order of 10% are claimed for the determination of lead and iron and the additives. Film background read adjacent to the element line is used as internal standard in both techniques. The rapid, accurate, and economical spectrographic method described is a specific addition to the literature in that i t applies to the analysis of both new and used lubricating oils.
.4s the porous cup technique cannot be used for determining the total amount of an element in used oils containing suspended solids, a method using the rotating disk electrode technique has been developed for determining lead, iron, and the additives in used oils. EQUIPMENT
The equipment used included a 2-meter grating spectrograph, PIIultisource excitation unit, densitometer, and solution excitat ion attachment manufactured by the Applied Research Lahoratories. POROUS CUP ELECTRODE METHOD
h porous cup electrode is filled with a sample of the unused
lubricating oil to be analyzed. The electrode is made from Sational Carbon Co. special spectroscopic graphite rods 6.35 mm. in diameter. A section of rod is cut to a length of from 22.6 to 22.8 mm. and faced off to an exact length of 22.2 mm. -4. cavity 3.96 mm. in diameter is drilled in the rod, leaving a floor of about 1 mm. The cup is finished by drilling the cavity by hand t o leave a floor thickness of 0.64 f 0.05 mm. using a flatbottomed drill and a stop-collar. The cup is used as the upper electrode; a graphite rod 6.35 mm. in diameter with a 45'
