V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1
1369
Analysis for Additive Elements. Using the same excitation conditions chosen for the lead and iron determination, the method m-as extended (Rotating disk and porous cup electrode methods) Average % Average % to include the additive elements. Satisfactory Deviation Deviation accuracies were obtained, although phosphorus from Mean from Input Standaid De\ialkio _ _ _ ~ Analysis Rotating Porous Rotating Porous Rotating Porous could not be accurately determined in samples Element Line, A. disk cup disk cup disk cup containing over 0.1% iron because of the inter2335 L O 6.1 Barium 5 6 78 . 8 ference of iron lines with the phosphorus line. Barium 2347 8.1 6.8 8 .. 21 6 .. 00 160.. 33 1 Calcium 4302 8.3 6.6 8.5 6.6 11.3 X !J Comparison of Methods. The results obtained Phosphorus 2532 5.0 4.2 5.0 4.4 6.2 3 !J Zinc 3340 6.5 4.2 6.6 5.5 8.7 ii.8 for additives using rotating disk electrodes were Average 6.6 5.4 6.7 R.7 8.6 7 3 compared with those obtained using porous cup ._ .. . electrodes. A comparison of the accuracies given by each technique, shown in Table VIII, indicated little difference betn een the two, although the porous cup technique appeared to give a slightly higher ratios from i i series of oils containing from 0.015 to 2.5% lead obtained using a disk speed of 7.5 r.p.m. were in fair agreement nccuiacy. The excitation condition used for the rotating disk a i t h intensity ratios obtained with a speed of 15 r.p.m., but those method was selected for the determination of lead and iron. The concentration range covered was from 0 0 2 to 0.20y0 for obtained with the higher speed gave a smoother analytical curve. Use of Air and Nitrogen Jets. The effect of air and nitrogen (*aIcium,phosphorus, and zinc, and from 0.025 to o.3Oy0 for barium. -4 viscosity effect giving low results for oils of S-4E jets directed a t the spark gap was observed in a series of moving film studies on both used and unused oils. With excitation con60 viscosity was found to be present for the rotating disk terhditions which heated the sample and without the use of air or nique as well as the porous cup method. nitiogen jets, line to background intensity ratios increased as When analyzing unused oils, these laboratories find the porous cup technique somewhat simpler and more rapid to use, but lesparking continued without reaching a constant value. The use of air or nitrogen brought the intensity ratios to a constant sults by both techniques appear to br rquallj satisfartory. value after about 30 seconds of sparking. With excitation conditions which did not heat the sample, intensity ratios came to a LITERATURE CITED constant value without the use of nitrogen or air jets, although the time required was somewhat longer. The use of a nitrogen (1) Applied Research Laboratories, Glendale, Calif., “Study of SpectronraDhic Determination of PhosDhorus in Oil.” jet gave increased film background, but the principal effect of (2) Calkins,‘L. E., and White, &I. >I N. h,. Petroleum News, 38, KO. both air and nitrogen jets seemed to be cooling the electrodes and 27, R519-30 (1946). sample. With excitation which did not heat the electrodes, (3) Clark, R. O., and Cooperators, COAR (API) Subcommittee on there appeared to be no particular advantage in the use of air or Emission Spectroscopy, Report on Quenched Electrode Procedure for Analysis of Lubricating Oils, API Meeting, Tulsa, nitrogen jets other than to shorten the prespark period required Okla., 1951. Accuracy of Lead and Iron Determination. Results of spec(4) Feldman, C., ANAL. CHEM.,21, 1041-6 (1949). trographic analyses of chemically analyzed used oil samples arc’ (5) Gassmann, A. G., and O’Keill, W.R., Ibid., 21, 417 (1949). given in Tables VI and \TI. Each spectiographic result is the (6) Gassmann, A. G., and O’Neill, W. R., Proc. Am. Petroleum Inst., 29M,79 (1949). average of a t least eight determinations. As shovm by Table (7) Pagliassotti, J. P., and Porsche, F. IT., ANAL.CHEM.,23, 198VI, the average accuracy of the lead determination was about 202 (1951). +loo/, of the amount present Table VI1 shows the average a w u r J r y of the iron determination to be about =!=l5y0. RECEIVED April 20, 1951.
Table VlII.
~
Comparison of Accuracy and Precision
~
Testing Used lubricating Oils for n-Pentane- and Benzenehsolubles Proposed Semimicro Modijication of ASTM Method J. S. WIBERLEY, R . K. SIEGFRIEDT, AND L. J. DiPAOIA Socony-Vacuum Laboratories, Brooklyn 22, N . Y .
I
Y NORMAL use lubricating oil gradually accumulates small amounts of foreign materials, which are suspended in the oil and may eventually hinder lubrication and promote wear. In internal combustion engines, products of combustion, such : I soot, lead compounds, and partially oxidized pi1 and fuel, nlay find their way into the lubricating oil along with metallic wear particles and dust taken in from the surroundings. In order t o evaluate the condition of the used oil, it is generally desirable t o determine the total quantity of these insolubles. Filtration or centrifugation of oil-solvent mixtures is the conventional procedure for removing the insoluble8 from the oil, but many modifications of techniques and solvents are used by different labora-
tories (7-10). Committee D-2 of the American Society for Testing Materials has published a tentative method for determining n-pentane- and benzene-insolubles in used lubricating oils ( 6 ) . The authors, however, believe that there are a number of important disadvantages to the use of this method. The method is slow. I n the authors’ laboratory one operator can handle only about ten samples per day-Le., ten determinations of the pentane-insolubles and ten determinations of the benzene-insolubles. If the coagulant is added in increments, as directed, the number of samples per day is further reduced. The apparatus is bulky and expensive. Considerable bench space is needed for the 125-ml. centrifuge tubes and the racks to hold them.
ANALYTICAL CHEMISTRY
1370
The ASTM method for n-pentane- and benzeneinsoluhles in used lubricating oils has a number of disadvantages a s a routine test, and it does not permit the use of coagulant in the determinationof pentane-insolubles, so that all the inmluhle matter present in a detergent-type oil may not he ppeoipitated and measured. For these reasons a semimicro modification was developed. A 1-gram sample of oil i s used instead of a 10-gram sample, and the
centrifuge tubes and other apparatus are similarly smaller in size. A coagulant, n-butyl diethanolamine,isaddedinafixedquantity in the determination of both pentane- and benzene-insoluhles. This version of the method is faster, cheaper, and safer than the macroprocedure. Results obtained with coagulant are a better measure of total insoluhles and are more useful for evaluating used deterpenttypeoilsthanthoseohtained by the ASTM procedure.
removed by suction. In the determination of the Dentane-
misture.and once with beneene, dried, and weighed APPARATUS
growth in the use of oils containing hieh peroentages ai deterwnt
AdditTonof co&ulant is permitted in the detekination of the
A oentrifuge with carriers designed to hold 15-ml. centrifuge tubes is used. An International clinical centrifuge and an International oil testing centrifuge Model AE have been used in this work. The wire stirrer, made of approximately No. 20 B. & S. gage Nichrome wire, should he about 17 cm. long with a loop 1.5 cm. in diameter a t one end for a handle and a munded hook 3 mm. in diameter and 6 111111. long a t the other end. The suction apparatus is composed of a 250-ml. suction flask and an inverted U-shaped pieoe of capillary tubing (1.5 mm. in inside diameter) pulled to a tip a t one end. This end should be long enough to reach to the hottom of the centrifuge tubes. This apparatus is shown in the upper right quarter of Figure 1. Centrifuge tubes of heaeresistant glass with a rounded coneshaped b o t b m should base a gross c a p d t y of 15 ml. with wduations a t 1.2, 5, and 10 ml. A borosilicate glass tube, earning Cataloe No. 8060. maduated and numhprrd with R suitable ~~~~~ ahrasig tml has been found satisfactory. The oven should he capable of maintaining a temperature of 105' 3' C. The use of an explosion-proaf oven is strongly recommended. The analytical balenee should have a precision of somewhat better than 0.1 mg. ~~
~
~
~
*
REAGENTS AND SOLVENTS
*Pentane, commercial grade, should oonform to the following requirements: >istill&tion( d ) Initial boiling poi,o t , min., F. Dry point. mar.. &id vapor pressure.".E. 1.7, m a , . , ro./sa. men
..
92 105
17
Figure 1. Clinioal Centrifuge ConesI and Suction Apparatns
It is with thene objections in mind that the method described in this paper was developed. While the basic prinoiples of the ASTM method ( 6 ) are retained, practical difficulties are overcome partly by reducing the scale of operations and partly by following an unvarying procedure. The daily output of the operator is approximately doubled. A less expensive clinical centrifuge may be used (Figure 1) or a large centrifuge may be equipped with multiple trunnion carriers 60 that twenty-four tubes may be centrifuged a t one time (Figure 2). Savings me realized in mst of equipment, space, and solvents, and the hazards are minimized. A d e h i t e a m o u n t of coagulant is alvays used in the determination of both pentane- and benzene-insoluhles, so that the two results w e ohbined on B eomparahle and reproducible basis. OUTLINE O F MFTHOD
The sample of used lubricating ail is diluted with a pentanecoagulant mixture and centrifuged and the supernatant liquid is
Figure 2.
Large Centrifuge with Multiple Trnnnion Carriers
1371
V O L U M E 2 3 , NO. 10, O C T O B E R 1 9 5 1 Pentane-Coagulant Miuture. Add 10 nil. of n-butyl diethanolamine to 1liter of n-pentane and mix. n-Butyl diethanolamine is available from Sharples Chemicals, Inc., 123 South Broad St., Philadelphia 9, Pa. Alcohol, 95% ethyl alcohol or denatured alcohol (Formula 30 or 3-A). Benzene, industrial grade conforming to ASTM specifications (1).
Pentane-alcohol mixture, 1 to 1 by volume. Benzene-alcohol mixture, 1 to 1 by volume.
As all the above reagents are volatile and flammable solvents, normal precautions, such as adequate ventilation and the absence of flames and sparks, should be observed in the area where these solvents are used.
explosionproof oven is not employed, this precaution should be taken. Calculate the pentane-insolubles in per cent by weight. PROCEDURE FOR BENZEYE-INSOLUBLES
Starting with a fresh sample of used oil, precipitate and wash the pentane-insolubles as just described. Do not dry the insolubles in the centrifuge tube but immediately add 5 ml. of the benzene-alcohol solution and stir. Carefully remove the stirrer and centrifuge for 5 minutes, or until clear. Remove the supernatant liquid by suction and repeat the washing and centrifuging procedure, substituting benzene for the benzene-alcohol solution. Remove the liquid as before. Dry for 30 minutes at 105' f 3" C., cool, and neigh to the nearest 0.1 mg. Calculate the t,enzene-insolul)leP in per cent by weight.
% PENTANE INSOLUBLES
7 6 n
5
0 =E
+
s
4
/
0 CT
F3 -
I)ISCUSSION
Originally the intention \vas to duplicate the ASThl: method (6) on a smaller scale. A similar reduction had been practiced on another .4STM method ($), without changing the results obtained ( 5 , 11). Consequently, data were collected to compare results by the semimicromethod against those obtained with the -4STlI method ( 6 ) , following identical procedures in respect to the solvents and use of a coagulant. These data have been plotted in the form of ii scatter diagram in Figures 3 and 4. Included in these diagrams are only those samples which centrifuged clear either with or without the use of coagulant. Similar results are obtained by the two methods.
2E W
% BENZENE INSOLUBLES
- 2 I
0 ASTM D893-48T Figure 3.
Per Cent Pentane-Insolubles
Preparation of Sample. The precautions recommended by the American Society for Testing Materials (6, Section 6) should be followed to ensure homogeneity of the oil sample. PROCEDURE FOR PENTANE-INSOLUBLES
Weigh a dry, clean centiifuge tube to the nearest 0.1 mg. Fill the tube to the 1.2-ml. mark with the prepared sample of used oil, then again weigh the tube to the nearest 10 mg. Fill the tube to the 10-nil. mark with the pentane-coagulant mixture and stir with an up and down motion of the wire stirrer until the sample is uniformly dispersed in the solvent. Do not allow the mixture to stand more than one hour. Remove the wire stirrer in such a manner that no significant amount of insoluhle material is lost. Centrifuge for 5 minutes. If not clear, centrifuge. Carefully remove the supernatant liquid by suction without disturbing or dispersing the precipitate, while leaving not more than 0.3 ml. of liquid in the centrifuge tube. Add 5 nil. of pentane to the tube and stir. Remove the wire stirrer carefully, centrifuge for 5 minutes more, or until clear, and again remove the supernatant liquid. Repeat this washing and centrifuging operation with 5 ml. of the pentane-alcohol mixture and remove the supernatant liquid as before. Dry the centrifuge tube containing the washed precipitate for 20 minutes a t 105" & 3' C., cool, and weigh to the nearest 0.1 mg. With relatively heavy precipitates, there may be some spattering if the tube is placed directly in the oven after removal of the supernatant liquid. I n such instances, most of the solvent may be evaporated by standing at room or slightly higher temperatures before the tube is placed in the oven. Whenever an
ASTM D893-48T Figure 4.
Per Cent Benzene-Insoluhles
The ASThI method does not provide for the use of a coagulant in the determination of pentane-insolubles, although in the determination of benzene-insolubles the coagulant, n-butyl diethanolamine, is added to the pentane, not to the benzene. Thus, it can easily happen that the pentane-insolubles are less than the benzene-insolubles, if coagulant is used in the latter determinstion (see Tables I and 11). Fundamentally, the authors hold that, for proper evaluation of the condition of the used oil with respect to insolubles content, all the insoluble material should be centrifuged from the oil-pentane mixture. The inadequacy of the ASTM method in this respect is illustrated by the data on pentane-insoluble* shown in Table 11.
1372
ANA LY TICA L
Table I. Insolubles by Semimicromethod with and without Coagulant for Benzene-Insolubles Diesel Oil Sample A B
C D E
F G H
Pentane, KO
Benzene
iY0
. .
Coagulant
coagulant
Coagulant
6.57 1.89 1 81 0 87 0 51
5.21 1 56 1 50 0 73 0.29 0.86 4.06 0.01
6.93 2 65 4.02 1.62 0 7i 2.29 6 52 0.63
0.Y3
4.71 0.07
Table 11. Insolubles with and without Coagulant for Pentane-Itisolubles Ihesel Oil Sainple I .J K I. RI
SSTM D 893-50T Pentanp, PenBenno tane, eene. coag. coag. coag. 0.24 0.33 0.6: 0.1,
0.45
2.11 2.64 2.31 1.35 3.09
i _ _ -
1.84 2 22 2.24 1.23 2.6;
_ _ _Semimicromethod ___~_
.
Pentane, no coag.
Pentane,
Benzene,
coag.
coag:.
0.72 0.87 1.82 0.44 0.83
2.14 2.62 2.43 1.37 3.00
2.21 2.21 1.10
1.60
2.41
~-~~--___~..~. _ - _
E M IS T-R Y
plus carbon. Here the use of the maximum amount of coagulant was necessary to obtain complete precipitation of the carbon dispersion. With less coagulant the supernatant liquid was black and opaque and obvious loss of insolubles occurred. I n the other three cases, increasing the amount of coagulant did not niateiially change the values found. The precipitated and washed insolubles were tested to see i i the high results might be due to the presence of adhering coagulant. These results, calculated from micro-Kjeldahl anal? qes appear in Table IV. Only small percentages of coagulant uere found, the quantity increasing but slightly with the higher coagulant levels. Fui ther examination showed that the insolubles precipitated from the oils which contained additives included appreciablr quantities of the detergent material. Apparently oxidation products may also be carried down with the carbon, even though the same oxidized oil may shon very little insolubles in the abwnce of carbon. As it n a s felt more important to determine all the insolubles, e\ en if some detergent and oxidation products were included, it \\as decided t o add 100 PI. of coagulant (1% by volume in the pentane) to all samples for both the pentane- and benzene-insolubles. Out of many hundred field samples which have Bince been tested, none has been observed which failed to clear after this treatment. The use of a fixed amount of coagulant has obvious advantages for production testing. A theoretical objection may be raised that use of greater amounts of coagulant than that just necessary to clear the oil will precipitate in addition excessive amounts of soluble “oxidized bitumens.” It is the authors’ belief that, while this objection may be true technically, the difference between results obtained with the minimum amount of coagulant and with the larger specified amount is negligible practically. In Table V some values of coagulated benzeneinsolubles using the minimum of coagulant added in increments according to the ASTAI method (6) are compared with valuw obtained using the fixed amount.
In the course of many determinations, it also became evident that, the optional use of coagulant for the determination of “coagulated benzene-insolubles” is impractical, because the required individual treatment prevents simultaneous handling of many samples. Furthermore, in many instances two operators differed as to the need for coagulating the same sample and ohtained significantly different results. I n view of this esperience it was dwided to investigate the gossibility of using a fixed amount of coagulant in the determination of both pentane- and benzene-iiisolubles. As a part of this investigation some synthetic samples of known composition \$-ere prepared for testing. Because the greatest difficulty has been experienced with samples of heavy-duty Diesel oil containing finely divided conihustioli wot, the samples were made up to studv such a bystem. The oil selected rtas a typical Diesel lubricating oil. For __.__ __ comparison purposes, the oil-base gtock Table 111. Insolubles Found in Sjnthetic Samples without an? of the additives was also Base Stock obtained. Base Stock Base Stock with Additives Coagnlan t Base Stock Oxidized with Additives (oxidized) Added, Portions of both the base stock and pl. Pentane Benzene Pentane Benzene Pentane Benzene Pentane Benzene the base stock with additives were oxi0 0.03 0.06 .. 0.04 0.02 0 01 dized in the Underwood oxidation test 10 0.05 .. 0.10 0.05 0.12 0 09 until neutralization numbers rose to 1.8 30 0.04 .. 0.08 ., 0.05 .. 0 20 0.15 60 0.04 .. 0.10 .. 0.05 and 1.5, respectively. Thus, four oil con0 2.5 0 19 100 0 . 0 4 . . 0 . 0 9 . . 0.0*5 . . 0 2; 0 23 ditions were represented: base stock; base stock oxidized; base stock with Plus 1.83% Plus 1.95% Plus 1 7QL;b additives; and base stock with additives Carbon Black Carbon Black Carbon Black oxidized, To some of each of these oils 0 2.00 2.01 2 74 2.33 0.31 2.42 2 22 were added weighed amounts of carbon 10 2.02 1.94 2.53 2.21 0.26 0:iO 2.66 1 44 black. I n order to match the degree of 30 2.02 1.89 2.71 2.21 0.48 0.29 2 79 2 55 60 2 . 1 6 1 . 9 9 2 . 4 5 2 . 2 5 1.50 1 . 3 7 1 71 2.48 dispersion found in actual service, a 100 2.05 2 46 2.13 2.18 2.00 2.74 ‘2 68 pigment black of fine particle size ~-___ (Binney and Smith’s Raven 11) was put through an ink mill five times in the form of a 12% oil paste, diluted to about 2%, and milled once more. The four samples without carbon and the During the investigation it was found that the use of coagulant four wit,h carbon were then subjected to analysis for pentanein the determination of the pentane-insolubles would reduire a and benzene-insolubles; varying amounts of n-butyl diethanolsecond modification of the procedure. When oil samples conamine coagulant were used a t the concentration levels recomtaining water were encountered, the amine coagulant separated mended for “coagulated benzene-insolubles.” from the pentane solution and was centrifuged to the bottom of The results appear in Table 111. the tube, where it formed a sticky mixture with the iiisolubles. Certain general observations can be made on these data. The As this wet amine was not washed out with pentane, it appeared base stock and base stock with additives show practically no in the weighed residue, producing fictitiously high insoluble reinsolubles. The same oils when oxidized show only slightlv sults. In the =1STM procedure for coagulated benzene-insoluhigher insolubles. Only the base stock with additives, ouibles, an alcohol-benzene wash is employed. Hence, an alcoholdized, showed any significant increase in insolubles with increaspentane n-ash was substituted for the last pentane wash. This ing concentrations of coagulant. HoJvever, the amount is below modification removed water and most of the amine coagulant 0.3% in all cases. from the insolubles without appreciably dissolving the insolubles Recovery of carbon “insolubles” was more or less high in all themselves. cases except for the tests run on the base stock with additives ~
~~
~~
1373
V O L U M E 2 3 , NO. 10, O C T O B E R 1 9 5 1 .__~ -~ ._
Table IV. Added, pl.
_
.
Base Stock Pentane Benzene
Base Stock
_ _Oxidized Pentane
Benzene
Base Stock
Additives _with __ ___ Pentane
Benzene
10
30 60
100
Base Stock with Additives (Oxidized) Pentane Benzene 0.4 0.6 1.9 1.1 22 . 64 11 . 82 2.6 9.1 _.
Table V.
Neut. No. ASThZ D 664-49 0.6 1.0 0.8 0.8 0.7 0.7 0.9 0.8 0.7 0.9
handled by one operator to about 30 per day.
Per Cent n-Butyl Diethanolamine Found in Precipitates
CONCLUSIONS
The seniiniicroprocedure for pentaneand benzene-insolubles described in this paper is now the standard method in use a t the Socony-T'acuuni Laboratories. It is recommended in place of the $STY1 method (6) for the following reasons:
~
Coagulated Benzene-Insolubles
(Heavy-duty Diesel oil) BenzeneBenzeneInsolubles Insolubles with with Coagulant Fixed Min. .4dded, Pi. Coagulant Coagulant 30 0.61 0.56 3.03 10 2.94 10 0.70 0.68 10 0.32 0.31 1.11 30 1.10 30 0.59 0.49 1.25 1.49 10 0.86 30 0.75 10 0.70 0.58 1.75 10 1.58
Coagulant Added, PI. 100
100 100 100
100 100
100 100 100 100
FVithout this alcohol-pentane wash, an oil containing 0.2570 water yielded a sticky precipitate from pentane. Kjeldah! analysis of this precipitate showed that 60% of its weight was the amine coagulant. When the alcohol-pentane wash pfocedure was employed, a dry precipitate containing 13% amine was obtained. The amount of amine retained by a precipitate after one alcohol-pentane wash seems to vary with the sample and niay be as high as 20% of the insolubles if the original oil contains water. If the oil is dry, only very small amounts of Coagulant will be retained on the precipitate, as was seen in Table IT'. FUTURE WORK
It is felt that the method described is more acceptable for routine analysis of used oils than the ASTM method (6). One requirement, howFver, seriouslj- reduces the number of samples that one operator can handle in a work day: the requirement that two wl)ai':~tesamples be taken, one for the pentane-insolubles and one for the benzene-insoluhlrs. I t is a common belief that, drying of the pentane-insolubles significantly decreases their solubility in benzene; hence, :iPepalate sample, m-hich is not dried previous to benzene treatment, is specified. A rrccsnt re-examination of this requirement has shown that it niay be possible to eliminate the second sample, and to determine the benzene-insolubles on the dried and weighed pentane-insolubles alrrady- available. Tahle 1-1 gives comparative figures on sixtern samples of used oil, which wei'e carefully selected to represent :i vwiety of oil conditions. They are run in triplicate bJthe s t a n d w d and by the short-cut version of the method and the averages :ire reported. If anything, t,he standard version gives the highel, result, although it is obvious that all pairs check. Of course. it is the nature of the insolubles which is significant. One would not expect to alter the herizenr solubility of metallic iron b y drying it. Comparisons on :i very large number of usual sarnpl
