V O L U M E 22, NO. 12, D E C E M B E R 1 9 5 0 obtained. Zinc dust gave :I well coagulated precipitate, but the recovery of platinum was always low, for undetermined reasons. .%nother obvious line of attack is to extract the unignited sample with hot acid, thus dissolving the metallic impurities, filter, and extract tht, platinum from the ignited residue. This expedient was tried, using 1 to 1 by volume hydrochloric acid, but again the recovery of platinum was IOT. The loss was found to be due to solution of some of the platinum because of either its extremely fiue state of subdivision or an oxide coating. )Then titanous chloride w:ts added to the dilute hydrochloric acid, this loss was avoided. T o c - h r ~ kthe performance of the method, a standard ratalyst \vas prq):trcd from accurately wigheti quantities of powdered c4h:ucmiI : i r l t i platinum wire dismlved in aqua regia. This catalyst 1 .OOO% platinurn. Three :~nalysesof it yielded 0.992, c~)iitaiiir~tI 1.000, :ind 0.085%. T h e addition of ro1;~tivelylarge amounts (up to 10 mg. each to a 0.30- to 0.35-gram assay) of the ahovrinentioiied vonimon metals a9 various salts did not affect the rec'overy of platinum. 13ecause platinum is below nierc8ui.y in the electromotive series, it i.: possiblr that an apprwiable propoi,tion of it precipitates on the. n i e ~ ~ c wdrops y during the in;rking of a po1:wograni. To test this, zsuc*c~rs.sivr runs were made upon the same cell charge of platinuni .wlutiori, step heights of 166.5, 167.0, 166.0, and 167.0 nini. Ileirig o b t a i n d . The :mount of precipitation during a run is t hc,rt,fore iiegligitile. On t h e other hand, if the plutinum solution is ~ 1 1 : i k c ~ i vigorously i ivitli mercwy hefore electrolyzing, a Ionr r c o v r r ~ 'ensues, due either to precipitation or rduc,tion to the divalent state. .As a find test of the procedure imples of spent catalyst were :iii:ilyzed (Table 11). S:implw 1 and 2 contairied a high proportion of a siliceous filter : i i ( l . 1cB:tving a large residue after ignition. T o extract the platiiiuin conij)lcBtrly froin this, it IVW found necessary to use 2.5 ml.
1503 eavh of hydrochloric and nitric acids instead of the 1 ml. called for in the standard procedure. This larger amount of arid increases the concentration of the supporting electrolyte, which affects the step height. A calibration under these conditions yielded an average value of 60.6 mm. per mg. of platinum. The values for samples 3 and 4, which contained little silica, were calculated on the basis of the calibration figures given in Table I.
Table 11.
Analysis of Spent Catalysts
3.00 2.50 3.00 2.50 2.00 1.75 1.50 2 2.5 2.00 1.75
171.5 145.5 216.5 177.0 141.0 229.5
2.83 2.41 3.55 2.92 2.33 3.36
1,D.O
1.89
"''3.5 201.6 172 0
3.57 3.24 2.78
0,094 0.096
( I . 118
0.117
0 11;
0.192 0 . 193 0.159 0 . 162 0 159
LITER.41'LIRE CITED
( I ) English, F. L., A s . 4 ~ C'HEM., . 20, 489 (1948). (2) Lingane and Laitinen. ISij. 1,:si:. ('H'EM., .\N.~L. h., 11, 504 (1939).
( 3 ) NacIlvaine, J . Biol. Chem., 49, 1S3 (1921). (4) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. 1 6 . p . 316, Xew York. Longmans, Green and Co., 1938. (91 ITillis, ,J. B., J . .4m. Chcnt. .Sot.. 67, 547 ( 1 9 4 5 ) . RECEIVED h l a y 12, 1950.
Determination of Copper and Iron in Oils by Amperometric Titration T€IO>I.iS D. PARKS' A N D LOUIS LYKKEK Shell Dereloprnent Company, Emeryiille, Calif, .i method is described for the determination of copper and iron in inorganic residues, such as that obtained from oils, by reduction in a silver reductor and amperometric titration with dichromate ion at a rotating-platinum electrode. When copper is determined i n the absence of iron, the reduced solution is receiyed under ferric alum solution and the equivalent amount of ferrous iron produced is titrated. When both iron and copper are being determined, the reducing value of the mixture is determined on
T
HE importance of metal arialysis in new arid used lubricating oils in the evaluation of engine tests for corrosion and wear has been pointed out by Lgkken, Fitxsimnions, Tibbets, and Wyld ( 7 ) . They gave detailed procedures for the separation and determination of the metals normally encountered in these materials, including colorimetric methods for iron and copper. The latter methods ( 1 ) have proved to be reliable and useful but somewhat exacting in use and somewhat lacking in precision and accuracy. The advent of new amperomet>ric techniques suggested titration procedures for copper and iron ivhich are more rapid and precise than the common colorimetric methods. 1
Present address, Stanford Research Institute, Stanford, Calif.
an aliquot of the reduced solution received under ferric alum, the iron is determined by titrating a separate reduced aliquot after selectike oxidation of the copper by aeration, and the copper is calculated by difference. The titration procedure is rapid, precise, and accurate; it is sensitite to 0.02 mg. of iron or copper. The method is particularly suitable for the analysis of copper and iron in lubricating oils because other elements that are normally present in these oils do not interfere.
Walden, Hammett, and 12dmonds (8) introduced the silver reductor for preparation of ferrous ion solutions for titration with ceric sulfate, and Birnbauiii and Edmonds ( 2 ) extended the use of this reductor to the preparation of cuprous ion solutions. They showed that solutions of those metals could be quickly prepared for analysis even in the presence of various other coinnion ions. Kolthoff and May (4)titrated very dilute solutions of potassium dichromate with ferrous iron at a rotating-platinum electrode and suggested the reverse titration of ferrous iron with dichromate solution. In the procedure given bcloiv for the determination of copper iii lubricating oils, the metals are separated from the oil by air
A N A L Y T I C A L CHEMISTRY
1504 ignition, the iron and copper in the ash are dissolved, and the solution is passed through a silver reductor to give ferrous and cuprous ions. One aliquot of the solution is received under ferric alum and titrated with dichromate solution a t a rotatingplatinum anode to measure the total iron and copper in the sample; another aliquot is purged with air to oxidize the cuprous ion preferentially and the remaining ferrous ion is titrated as hefore; the copper is calculated by difference.
Table 1.
Reduction of Copper and Iron in Walden Silver Reductor
Hydrochloric Acid Normality 2.0
a
_ Copper, hIg._ Taken 27.70
Found" 27.3 2 7 . $56 2.81 2.84 2.89 2.83 2.0-1 1.99
.-.___Iron,
~
Taken
1\19, ~
~
~
solution of 0.5 A* ferric alum and the other in open air; purge the latter with a stream of air for 2 minutes. Titrate the solutions obtained amperometrically with a solution of 0.01 S potassium dichromate in a cell which has a rotatingplatinum anode a t a potential of 1.0 volt in respect to a saturated calomel rrference electrode. AIeasure the current produced at the anode and record a current reading for each increment of dichromate added. (The readings a t the beginning are off scale owing to the large amount of ferrous iron in solution. Toward the end point, the current readings decrease in proportion to the dichromate added.) Plot the volume of dichromnt,e added against the current readings obtained and draw straight lines through the two intersecting series of points. From the end point thus obtained, calculate the amount of iron in the sample recpived in air and the amount of copper and iron in the sample received under ferric alum. Calculate the copper content of the oil by difference.
+
Found"
...
...
EXPERIMENTAL
2.81
2.82
...
...
2.81
2.81 2.80 2.82 2.82 2.81 2.80 2.81 2.81
Although it is stated that 2 S hydrochloric acid is necessary for the reduction of cupric ion to cuprous ion ( 2 , 9 ) ,it was found that quantitative reduction took place equally well when 1 N or 0.5 K hydrochloric acid was used. This finding is supported by the data in Table I, which shows that the cupric ions are completely reduced in acid concentrations above 0.5 S. The reduction of the ferric ions is quantitative in acid concentrations as low as 0,001 S, although t,his reaction is usually conducted in a hydrochloric acid concentration of 1 +V ($, 9 ) . For convenience, all the subsequent results given in this paper were obtained on solutions prepared to contain 0.5 -1-hydrochloric acid.
1.0
2.86
0.8
2.86
0.1
2.86
0.01
2.86
Sone Sone
2 81
0.001
...
...
2.81
B y amperometric titration of reduced solution.
The amperometric titration of ferrous ion with dichromate ion is sensitive and accurate. B s little as 20 micrograms can be titrated with an accuracy of 170, and lower amounts can be readily estimated. When copper and iron are to be determined on the same sample, best accuracy is obtained when the ratio of one to the other is within the limits of 1 t o 10. The method is directly applicable without difficulty t o lubricating oils which contain barium, cadmium, zinc, calcium, chlorine, lead, phosphorus, silicon, magnesium, tin, aluminum, sodium. cobalt, and nickel. 4PPAR 4TUS
The small silver reductor used is similar to that described by Willard and Diehl (9). Finely divided silver is prepared by suspending a strip of electrolytic copper in 125 ml. of hot solution containing 12.5 grams of silver nitrate. After the reaction is complete, the deposited silver is washed free of copper ion with dilute sulfuric acid. The silver is placed in a glass tube 12 cm. high (inside diameter 0.4 cm.) which has a stopcock a t the bottom. A roll of 40-mesh silver gauze is placed in the bottom of the column to support the silver. When not in use, the silver in the column is covered with 0.5 &V hydrochloric acid. The amperometric titration apparatus consists of a rotatingplatinum electrode and saturated calomel reference electrode, essentially as described by Kolthoff and Harris (S), but n Fisher Elecdropode (Fisher Scientific Company, Fittsburgh, Pa.) is used to apply a 1-volt difference of potential across the platinum (anode) and reference electrodes and to measure the anodic current produced. A 100-ml. lipless beaker is used to receive the solution from thp silver reductor and to serve as a titration vessel.
The amperometric titration of ferrous ion with dichromate solution was investigated using a titration cell similar to that ; a rotating-platinum electrode described by Kokhoff and May (4) was made the anode at a potential of +1.0 volt against a saturated calomel electrode connected to the cell through an agar-agar bridge with a plug of sintered borosilicate glass in the manner described by Laitinen (i).A Fisher Elecdropode was used to supply the necessary potential difference and to measure the current produced in the cell as previously described by Laitinen, Jennings, and Parks (6). (Under t,he conditions of the titration, ferric ions, dichromate ions, and chromous ions are not affected a t the rotating electrode, but ferrous ions are oxidized to produce a current. As dichromate is added, a diminution of current is noted which yields a typical amperometric end point when plot,ted against the volunie of titrant added.) ,4 series of ali. hydroquots, containing known amounts of ferrous ion in 0.5 V chloric acid solution, was titrated with standard dichromate solutions of different strengths, yielding the results given in Table 11. Accurate and precise results were found with as little as 20 micrograms of iron. Correct order of magnitude was found with 5 micrograms of iron, hut the results were erratic and low.
Table 11. Titration of Standard Ferrous Ion with Dichromate Solution at Rotating Platinum Electrode Dichromate Solution Normality 0.0102
Taken 2.01
0.0102
0,502
0.0010
0.201
0.0010
0.030
0.0001
0.020
0.0001
0.005
Ferrous Ion, Mg.
PROCEDUHE
Into a platinum dish, weigh sufficient oil to contain preferablv from 1 to 10 mg. each of iron and copper. Heat the dish with Bunsen burner until the contents ignite and burn readily, then move the dish and flaming contents to a hot plate. Maintain the hot plate a t such a temperature that the sample continues to burn a t a uniform and moderate rate, leaving only the ash and carbon when burning cease5. Place the dish in a cool fur2ace (below 300" C.), gradually increase the temperature to 550 i: 50" C., and continue the ignition until the oxidation of carbon is practically complete. Add 2 to 3 ml. of dilute hydrochloric acid, warm on a hot plate, and transfer with distilled water to a suitable volumetric flask. Pass duplicate 5-ml. poitions of the solution through a silver reductor and wash with four 5-ml. portions of 0.5 S hydrochloric arid. Receive one of the portions and its washings under a
a
Found 1.99 1.99 2.01 0.502 0.502 0.201 0.200 0,201 0.0495 0.0485 0.0495 0.020 0,020 0,0045 0.0038
Low results were obtained when reduced copper was received from the silver reductor in the open air and it was concluded that this was caused by the rapid oxidation of cuprous ion in air. This suggested the determination of both metals in a mixture by determining the dichromate consumed when a sample was re-
V O L U M E 2 2 , NO. 1 2 , D E C E M B E R 1 9 5 0
1505
Table 111, Amperometric Determination of Copper and Iron after Reduction with Silver Reductor in 0.5 .V Hydrochloric Acid Solution Expt.
1
Copper, Mg. Taken Found 3.03 2.99
Iron, XIg: Taken Found 2.72 ‘,23
2 I‘ “ Contained also 3 mg. each of cobalt, nickel. zinc, aluminum, magnesium, and tin.
Table IV. Determination of Copper and Iron in Standard Lubricating Oil Samples Containing Variety of Elements Sainple s-20u s-23
Iron, %Present Found 0.179 0.173 0.177 0.178 0.033 0,028 0.030
Copper, 70 , Present Found 0.011 0.013 0.009 0.009
0.005
2 minutes, and titrated for the iron present, To test possible interference by various metals, an equivalent amount of cobalt, nickel, zinc, aluminum, magnesium, and tin wm added to known mixtures of iron and copper and mother series of experiments made.
Results given in Table I11 show that copper and iron can be determined satisfactorily in mixtures by this method even in the presence of a variety of common metals. Two lubricating oil samples containing known amounts of a variety of met,als, in the concentration ranges normally found in used, additive-containing inotor oils, were analyzed by the method outlined above. The results, give in Table IV, compare favorably with both the theoretical composition and t,he values obtained by longer colorimetric procedures ( 1 , 7 j. These results are significant in that the amount of copper was very low in relation to the iron present. making a good test of the method.
0.004
0.005 a Major inorganic constituents (0.1 t o 1%): barium, calcium. chlorine, iron, lead, phosphorus, and sulfur; minor constituents (less t h a n 0 . 1 % ) : copper and zinc. b Major inorganic constituents: barium, calcium, chlorine, lead, magnesium, phosphorus. sodium, sulfur,,tin, and zinc; minor constituents: aluminum, cadmium, copper, iron, and silicon.
ceived in air and when the sample was received under ferric alum as in the method of Birnbaum and Ednionds ( 2 ) . I n this scheme, the first titration represents only the iron in the sample, and the difference in the two titrations is a measure of the copper. Mixtures of known amounts of copper and iron were reduced in a silver reductor, received under ferric alum, and titrated amperometrically with dichromate for the total iron and ropper present. An aliquot of the same solution was then passed through the reductor, received in open air, purged with a stream of air for
LITER.ATURE CITED
Am. SOC.Testing Xlatevials, “A.S.T.M. Standards on Petroleuni Products and Lubricants,” A.S.T.M. Designation D 810-48, Kovember 1949. Birnbnum, N., and Ednionds, S . &I., ISD. ENQ.CHEM.,. 4 ~ . 4 ~ED., . 12, 155-7 (1940). Kolthoff, I. hl., and Harris, IT-. E., Ibid., 18, 161-2 (1946). Kolthoff, I. bl., and May, D. R., I h i d . . 18, 208-9 (1946). Laitinen, H. .1.,Ibid., 13, 393 (1941). Laitinen, H. d.,Jennings. TI-. P.. and Parks, T. D., Ihid., 18, 355-8 (1946).
Lykken, L., Fitzsiinmons. K. R.. Tibbeta. 5 . A,, and Wyld, G., Petroleum Refiner, 24, S o . 10, 405-14 (1945). Walden, G. H., Hammett, L. P., and Edmonds, S. M., J . Ana. C‘hena. Soc., 56, 350-3 (1934). Willard, H. H., and Diehl. H., “Advanced Quantitative A4rialysis,” p. 98, New York, D. Tan Sostrand Co., 1943. RECEIVED February 6 , 1050.
Determination of Small Amounts of Silver in lubricating Oils By Amperometric and Gravimetric Methods THO>\I.iS D. PARKS’ AND LOUIS LYKKEN Shell Deueloprnent Company, Emerycille, Calif. Methods are described for the determination of silver in new and used lubricating oils which may also contain aluminum, barium, cadmium, calcium, copper, iron, lead, magnesium, tin, zinc, and alkali metals as well as sulfur, phosphorus, chlorine, and bromine. After ignition of the oil and solution of the silver in the inorganic residue, the silver ion is determined with potassium iodide, by either amperometric titration or semimicro gravimetric precipitation. Both methods give results which are accurate to 1% in the concentrations encountered, but the amperometric method is more sensitive and more rapid than the gravimetric method.
T
H E determination of small amounts of silver in lubricating oils has again become important because of the increased use of silver-alloy bearings in automotive engines. However, there exist no published methods directly related to the determination of silver in new or used oils. This paper describes tn o methods which give accurate results for silvel in oils containing common metallic contaminants, such as a l u m i ~ ~ u m barium, , calcium, copper, lead, magnesium, tin, and zinc, as well as chlorine, bromine, sulfur, and phosphorus. The oil is burned off, the residue is gently ignited to remove 1
Present address, Stanford Research Institute, Stanford, Calif.
the carbon, the silver is extracted with ammonium hydroxide solution, and the silver ion is determined gravimetrically as silver iodide ( 1 ) or amperometrically by titration with potassium iodide solution. The amperometric titration of iodide ion in ammoniacal solution with silver nitrate, in the presence of chloride and bromide ions, x a s shown by Laitinen, Jcnnings, and Parks (3) to be accurate and sensitive. They detected the titration end point graphically by plotting the current produced at a rotating-platinum cathode before and after the end point. The amperometric method described below depends on the reverse titration
