Flame Photometer Techniques Determining Typical Additives in Petroleum Oils A. L. CONRAD AND W. C. JOHNSON The Standard Oil Company (Ohio), Cleveland, Ohio A flame photometer method for determining alkali and alkaline earth elements in inhibited lubricating oils is described which permits atomization of samples in organic media. This greatly reduces the time required for sample preparation and eliminates many possihle causes of contamination. Working in low concentration ranges permits samples to be diluted to the point where viscosity effects become uniform. Analyses of typical lubricating oils show reproducibility of the method. Techniques for improving accuracy are included and information is given concerning expected interferences.
T
HE flame photometer methods for determining alkali and
alkaline earth metals in solutions have been described repeatedly. With few exceptions, these methods involve atomization of a water solution of the s a h , although in several methods biological materials are diluted with water and analyzed by the flame photometer method (4,7 , 9). Other solvents with viscosities not greater than water may be used ( 2 ) . In attempting to apply the photometer to the determination of these elements in lubricating oils containing metal additives, an ashing procedure could have been employed to reduce the sample to common salts. Ashing procedures have been found tedious, however (I), especially when ash constituents include the less soluble barium and calcium sulfates. Contamination easily occurs during the sample preparation (4, 7 ) .
per cent emission as read on the instrument transmission dial ivas recorded. Small amounts of the two standard solutions were atomized, and the increased or A emissions over that of the blank were recorded (70E of sample -% E of blank). Finally the A emission of the unknown solution was determined. A plot of A emifisionsagainst concentrations was used to determine the concentration of the unknoa n solution. In developing this procedure, the accuracy which might be obtained was investigated, as well as optimum conditions for obtaining this accuracy. STANDARDS
Of primary importance in establishing accuracy was the selection of standards. Whereas spectrographically pure salts could be used as standards for water solutions, these salts were esscntially insoluble in oils, Consequently, commercial grade metal naphthenates xere used instead. The metal contents of the naphthenates were determined by sulfated ashes, and the naphthenates were then blended by weight with base oils containing no metallic additives. These standards were retained, diluted, and atomized with all subsequent determinations. The standards remained stable for several months, although they were frequently checked against freshly prepared blends. Several calibration curves obtained from thrse standards are shown in Figure 1. CONCEYTRATION RANGE
Figure 1. Metal Naphthenate Calibration Curves
To eliminate the need for ashing and subsequently redissolving the ash constituents, a procedure was folloived whereby the sample could be dissolved directly in an organic solvent and atomized. This greatly reduced both the time required for sample preparation and possible cont,amination. PROCEDURE
A weighed amount of the unknown sample was dissolved volumetrically in a suitable solvent. I n most cases a 50-50 mixture of benzene-isopropyl alcohol was found to be satisfactory, although other solvents could possibly be used. .4t the same time a similar amount of a blank oil containing no additive, was diluted, as were similar amounts of two known standards. The instrument settings were adjusted to previously determined optimum conditions, the dissolved bank was atomized, and its
A study of these curves disclosed the fact that the straight-liric relationship between concentration and A emission could be extrapolakd through the zero point. Theoretically then, one could determine very low concentrations. Using a A emission of 10 as the lowest reading permissible for satisfactory accuracy, a study was made of instrumental adjustments which would give maximum emission for each element to be determined, thus permitting readings to be made on very low concentrations. .is reported by Bills et al. ($), who describe a modification of the Perkin-Elmer' instrument involving increased amplification to obtain greater sensitivity, excellent reproducibility was obtained in the lower concentration ranges. S o modifications were neccssary to ohtain these results on the Reckman spectrophotometer. The sensitivity knob was simply adjusted to its most sensitive or counterclockrvise position with a corrasponding slit width; and all variables such as gas, oxygen, and air pressures were investigated, so that masimunl A emission could be obtained. Dcterniinations were also made in higher concentrations by decreasing the sensitivity and slit width. Table I shows the conditions used to obtain the calibration curvcs of Figure 1. These conditions are reported for information only, because their applicability would be limited to specific concentrations used, and in some cases to a specific inst.rument.
1530
I
V O L U M E 22, NO. 1 2 , D E C E M B E R 1 9 5 0
'1 I
determining the 1 emissioris. Methods have been reported in which a flame hlank arid a distilled water blank were used (I, 7 ) . I n atomizing organic solvents, accuracy can be improved by using an organic solvent blank with a viscosity as similar as possible to that of the diluted unknown sample. The effect of various blanks can he seen in Figure 3. Identical calcium dctermiiiations were made, calculated o n the Imis of three different 1)larik~. \\-hrn the flwme h h n k was used, curve -4, the line did not pass through the origin. ITse of a pure solvent, Iilank, curve B , iniprovrd the, position of the line. Best results w r e obtained Lvtien a solvent tilank containing dissolved base stock oil w:is used, w r v e C, \\-here the line passed through the origin, indicating that compensation had been niitde for all blank c.mission. The, amount of base stock oil did not vary by mort: th:in 20% from t,he amount of oil in the unknown sarnple being :tn:ilyzcd. The clifterericcxs between the lines indiciitrd the amount o f ( ~ r i v rc~ncwuntt~rc~tl by using :in incwrrect blank.
P - 3 , 6 , \ 1 A Y D 21 GRA'4S/iOOml. 0 - 7,7,7 G R A M S e00 ml.
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A EMISSION
1531
-
30
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t
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1
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20 EVISSION
Figure 2.
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30
40
50
Effect o f \ iscosity
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.\ suniniary of thr averagr. crrors rrsulting f r o m sevc~ralan:ily~es \\-:\s p w p : ~ i t d . -1s shown in T;il)Ic,11, A, the averagr devi:ition on individual qumtitative blends o f inet:il naphthenates in base oils was found to be slightly over 1%. Average deviations were also found on determinations made o n metals in typical oils (Table
11, B). In the case of lithium, where mrasurements were made a t extremely low concentrations, less reproducibility \\-as found. In these determinations a deviation of 0.1 in 1 emission causes :In error of tiyo. This was the lobvest conccsntration that could be determined a i t h acceptable accui':ic~y. Oils used in these determinations contained hoth sulfur xntl phouphorus, but only one caniitting cation. \\70i,king in very low concentrations minimized one of thr greatest defrcts i n atomizing org:inic solvents. When several grams of a So. 20 lubricntingoil were dissolved in 100 ml. of solvent, t,he rwulting viscosity of the solution as compared to the solvent was greatly increased ; similarly, the rnt'e of atomization and the subsequent emission wei'r gre;itl)- ~ ~ r t l u c e d .By excessive dilution this effrct \vas minimix(. A I I extreme case of osity effcct is shown in Figurc. 2, A . \\'lien s:miples weighing in 3 to 21 gvains were dissolved in 100 ml. of solvent, the emission i'ailcd to follow a straight line. The amount of variation a i t h sanipk, w i g h t is shown in Figure 2, 13. Hew a straight-line oulibi~ation1 ~ : ~ obtaiiird s only when simi1:ir w i g h t s of three known blends tvere dissolved ill 100 nil. of solvent and analyzed. \\Then widely diffrrent weights of oiie of thest: known blends were dissolvcd in 100 ml. and exaniinecl, the curve was no lonyc r a straight line, shoning the effect of viscosity. n dt,viations over 200/, in samplc From these data it can 1 ) s~ e ~ that weights would cause deviations due to viscosity effects. Thus standai,ds used in the anal were weighed to within 20% of the weight o f the samplo. In thi.? way viscosities of standitrds arid s:iniplps \ V V I V very similai . BLANK CORRECTIONS
l k x u s c ~:ill calibratioii curves consist of :I plot of emissions ag:iinst cwncaentrations, the blank re:itlings 1,emnir~critics1 in
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C - B A S E O!i BLANK
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30 A EMISSION-
Figure 3 .
Table I.
Lit hi 11ni 0.4 26 24
2
Ca Ba I
