Metal Caprates as Analytical Standards for Spectrometric Oils Analysis William E. Hearn, Reginald A. Mostyn, and Brian Bedford Quality Assurance Directorate (Materials), Royal Arsenal, London, S.E.18,U.K. Metal salts of capric acid are proposed as standard materials for the spectrometric determination of metals in oils and other organic samples. Detailed procedures are given for the preparation of metal caprates and for the accurate determination of their metal content. These salts possess certain advantages over alternative organometallic standards. Experimental results are presented of the comparative spectrometric performance of metal caprates and the widely-used metal cyclohexanebutyrates; methods of solubilization and the stability of standard solutions in various media are discussed.
ATOMICABSORPTION spectrophotometry and direct-reading emission spectrometry are widely used for the determination of metals in organic materials such as oils, fuels, and lubricants. For the analytical procedures involved, organometallic compounds of known metal content form the primary standards and from these the required range of working standards is prepared by dilution with either the pure organic material or a convenient organic solvent. Ideally, the organometallic compounds should be stable stoichiometric substances, miscible with a wide variety of organic liquids to give stable standard solutions ; they should be readily obtainable or readily prepared in the laboratory and, since multielement standards are normally required, there should be compatibility with other materials which may be present. An important application of the above type, now being operated by many military and commercial organizations, is the Spectrometric Oils Analysis Program (SOAP), designed to monitor trace levels of metals in used engine oils, t o provide a n engine-wear early-warning system, particularly for aircraft. A comprehensive survey of possible organometallic analytical standards for this type of program, conducted by the National Bureau of Standards ( I ) , resulted in the recommendation that the metal salts of cyclohexanebutyric acid were generally suitable and these compounds have been available to SOAP users for some years. However, the authors’ experience of SOAP operation has shown that, while the cyclohexanebutyrate standards have proved satisfactory analytically, there are disadvantages in other respects. Since considerable quantities of standards are needed and there must be no failure in supply, we have sought to maintain laboratory-prepared stocks but find that cyclohexanebutyric acid and the solubilizing agent 6methyl-2,4-heptane dione can be difficult and expensive to obtain. In addition, some metal cyclohexanebutyrates from certain commercial sources carry a n unreliable certification of metal content. An alternative system of analytical standards has therefore been investigated and we find that the metal salts of capric acid, CH3(CH&COOH, (decanoic acid or n-decylic acid) merit consideration ( 2 ) . Metal caprates may be solubilized “Analytical Standards for Trace Elements in Petroleum Products,” H. S . Isbell and coworkers, Nut. Bur. Statid. (U.S.) Moiiogr., 54, 1962. (2) U.K. Patent Application No. 39048/70, August 13, 1970. (1)
directly with naphthenic acid for many applications or, alternatively, they may be used in place of cyclohexanebutyrates in the preparation of SOAP standards .by the NBS solubilization procedures, without the need t o recalibrate the spectrometric method. PRELIMINARY STUDY
The monobasic fatty acids of from 6 to 12 carbon atoms chain-length constitute a range of organic acids suitable for the preparation of metal salts and, of these, capric acid can be selected as giving a good compromise in the metal content of salts of the light and heavy metals. The molecular weight of capric acid (172.27) is very close t o that of cyclohexanebutyric acid (170.24) and the metal contents of its salts are therefore of the same order. Metal caprates are easily prepared in most cases by addition of a n aqueous solution of a salt of the metal to a n aqueous solution of ammonium caprate ; after separation by suction-filtration they may be vacuum-dried over P205 and analyzed for metal content. The products are not generally ‘amenable to purification by recrystallization but this does not preclude their use as standards. Initially, caprates of Al, Cr, Cu, Fe, Mg, Pb, and Z n were prepared and were found, with one exception, to be readily soluble in both 2-ethyl hexoic acid and n-octoic acid; AI caprate formed a gel which was soluble only on prolonged heating. However, the solubilized salts would not all give stable solutions when diluted with mineral or synthetic oils, and the addition of a further solubilizer, 6-methyl-2,4-heptane dione, was necessary. Two common P-diketones, acetyl acetone and benzoyl acetone, were unsuitable as substitutes for 6-methyl-2,4-heptane dione. Since it was desired to avoid the use of scarce reagents, naphthenic acid (Acid value 180, sp gr 0.98 a t 20 “C, B.D.H., Ltd., Poole, England) was tested as a solubilizing agent. Although naphthenic acid is a nonstandard material and metal naphthenates are unsuitable as primary standards for several reasons, this does not exclude its use as a solubilizer. Experiments showed that the metal caprates were readily soluble in small volumes of naphthenic acid and gave stable solutions, both as single- and multielement standards, when diluted with oils. A1 caprate again was not entirely satisfactory but it was judged that these preliminary results were encouraging and offered a viable alternative scheme for the preparation of organometallic standards for SOAP and other applications. During our work, the use of metal alkaryl sulfonates for the same purpose has been reported (3-5), but no comparison is available at the present time. (3) F. M . Evans, T. G. Cowley, and A. E. Goodwin, 8th Meeting of Society for Applied Spectroscopy, Anaheim, Calif., October 1969. (4) F. M. Evans, T. G. Cowley, and A. E. Goodwin, 9th Meeting of Society for Applied Spectroscopy, New Orleans, La., October 1970. ( 5 ) T. P. Matson, At. Absorptio/i Newsleft.,9 (6), 132 (1970).
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PREPARATION OF METAL CAPRATES
Reagents. Metal salts were of analytical reagent grade wherever possible; all contained negligible amounts of other metal impurities as shown by emission spectrographic analysis. Capric acid (B.D.H., Poole, England) was also checked for metal impurities by the same method. All other chemicals were of analytical reagent grade. Ammonium Caprate Stock Solution. Dilute 200 ml of ammonia solution (sp gr 0.90) to 1 liter with distilled water. Titrate a small aliquot with 1N HC1 or 1N Hi304 and calculate the molarity. Weigh 344.5 grams (2 moles) capric acid into a 4-liter beaker, add 500 ml of water and warm until the acid liquefies. Add, with stirring, the calculated volume of ammonia solution and continue warming until a clear solution is obtained. Cool, transfer to a 2-liter graduated flask, and dilute to volume with water. Copper, Lead, Magnesium, Silver, and Zinc Caprates. The preparation of these five salts follows the same general procedure. Add 250 ml of ammonium caprate solution to 2.5 liters of water in a 4-liter beaker. Dissolve, in 300 ml of water, the required metal salt as follows: 26 grams of cupric chloride, CuC12.2H20; 50 grams of lead acetate, (CH3C00)2Pb.3 H z 0 ; 28 grams of magnesium acetate, (CH3C00)2Mg.4Hp0; 43 grams of silver nitrate, AgN03, or 30 grams of zinc acetate, (CH3COO)zZn.2Hz0. Add the metal solution, with stirring, to the ammonium caprate solution and separate the precipitated metal caprate under gentle suction into a 1-liter Buchner-type glass funnel (No. 3 porosity sinter). Wash the product with 5 successive 500-ml portions of water and 3 successive 150-ml portions of acetone. Allow to air-dry, powder, sieve, and finally dry for 24 hours under vacuum over P206. Iron Caprate. Proceed as above, using 375 ml of ammonium caprate solution and 35 grams of ferric chloride, FeC13. 6H20, but do not wash with acetone. Dry the water-washed precipitate as far as possible by continued suction, transfer to a shallow dish, and allow to air-dry before powdering, sieving, and vacuum-drying over P205. Chromium Caprate. Add 300 ml of ammonium caprate solution to 1 liter of water in a 2-liter beaker. Dissolve 32 grams of chromic chloride, CrCl3.6H20, in 300 ml of water and add, with vigorous stirring, until reaction is complete and a sticky coherent mass of precipitate is formed. Wash by decantation with 4 successive 500-ml portions of water, finally draining thoroughly. Add 100 ml of acetone, macerate the precipitate, decant, and repeat the process with successive 100-ml portions of acetone until the product breaks down to a coarse powder. Separate into a Buchner funnel under gentle suction, wash with acetone, and complete the drying process as before. Other Caprates. In addition to the above seven caprates, which were of immediate interest in SOAP, several others have been prepared by the same general method in order to study their solubility, compatibility, and other characteristics. These include the caprates of lanthanum (6), which has been suggested as a releasing agent in atomic absorption analysis, and those of Al, Ba, Ca, Cd, Co, Mn, Ni, and Sr. Of these, A1 caprate forms as a difficultly-soluble gel when heated with naphthenic acid and it is not recommended for this process, although it is readily solubilized by 6-methyl-2,4heptane dione. OTHER STANDARD METAL COMPOUNDS
For several current applications, including SOAP, other metals are of analytical interest but do not give satisfactory caprates. Since the compounds used for these metals are required to be compatible with caprate systems and are some( 6 ) R.A. Mostyn, B. T. N. Newland, and W. E. Hearn, Anal. Chin?. Acta, 51, 520(1970). 1822
times present in standards discussed in the present report, they are listed briefly below. Aluminum. AS an alternative to the caprate, in systems where naphthenic acid is preferred as the sole solubilizer, the compound tris-ethyl acetoacetonate aluminum, (CH3C0. CH.CO.OC2H&Al, is suggested. It is directly soluble in a variety of solvents and is readily prepared by reacting A1 iso-propoxide with ethyl acetoacetate in petroleum ether. Tin. Either dibutyl-tin-di-2-ethylhexoate, as recommended by NBS (I), or dibutyl tin-diphenyl acetate are satisfactory, Vanadium. Vanadium as the benzoyl acetonate complex, (CeHsC0.CH.C0.CH3)2V0, is compatible with metal caprates solubilized with naphthenic acid. The preparation is given in reference ( I ) . Titanium. Stable solutions containing up to 100 ppm of Ti have been obtained from laboratory-prepared titanyl acetoacetonate. The Ti content of this compound was 0.7 % higher than the theoretical figure of 18.3% but the same compound obtained commercially was 2.5 higher and could not be completely dissolved. A promising alternative, titanyl benzoyl acetonate, is under investigation. Silicon. No better alternative to the NBS recommended compound ( I ) has been found. Although octaphenylcyclotetrasiloxane has the disadvantage of limited solubility, it has been used successfully at a concentration of 100 ppm Si in multielement standards. Sodium and Potassium. In naphthenic acid systems, the anhydrous carbonates are convenient standard compounds. Warm the calculated weight of dry salt with excess naphthenic acid until CO, evolution ceases and a clear solution is formed. The product is miscible with various oils and organic solvents. ANALYSIS OF STANDARD COMPOUNDS
Reagents. All chemicals should be of analytical reagent grade. Analytical Procedures. ALUMINUM.Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 50 ml of ethyl alcohol, warm to dissolve compound, and add, by pipet, 50 ml of 0.01M EDTA followed by 2-3 grams of hexamine and 1 ml of glacial acetic acid. Heat to gentle boiling, cool, add 3-4 drops of freshly-prepared 0.1 dithizone solution in acetone and titrate with 0.01M zinc solution to the end-point color change gray-green to red. CHROMIUM.Weigh accurately 0.075 to 0.1 gram of sample into a 100-ml beaker. Add 5 ml of concentrated H2S04 and warm to dissolve. Cool, add cautiously 50 ml of water and set aside until the separated capric acid has congealed. Filter through No. 40 paper into a 350-ml conical flask, washing with water, and dilute to about 200 ml. Add 1 ml of 0.01N AgN03 solution, about 2 grams of ammonium persulfate, and boil gently for 30 minutes. Cool, add by pipet 50 ml of 0.02N ferrous ammonium sulfate solution, 5 ml of HrPOa, and 2-3 drops of diphenylamine sulfonate indicator. Titrate with 0.01N KZCr207solution to the endpoint color change green to violet. COPPER. Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 5 ml of ethyl alcohol, 5 ml of 1N HCI, and warm to dissolve. Add 100 rnl of water and then pH 10 buffer solution dropwise until the precipitate redissolves to give a clear dark blue solution; add 3 drops excess. Add 0.2-0.3 gram of murexide indicator and titrate with 0.01M EDTA to the end-point color change green to violet-red. IRON. Weigh accurately 0.075 to 0.1 gram of sample into a 100-ml beaker. Add 10 ml of 70% H2S04, warm to dissolve, add 50 ml of water and set aside until the separated capric acid has congealed. Filter through a No. 40 paper into a 250-ml conical flask, washing with water, and dilute to about 150 ml. Add 4-5 pieces of pure granulated zinc, cover with a watch-glass, and boil gently for 30 minutes.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
Cool, filter through a No. 40 paper into a clean 250-ml conical flask, washing with water, and add 3 ml of concentrated HzS04, 5 ml of H3P04, and 2-3 drops of diphenylamine sulfonate indicator. Titrate with 0.01N K2CrzO7solution to the end-point color change green to violet. LEAD. Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 10 ml of ethyl alcohol, 2-3 ml of triethanolamine, and warm to dissolve. Add 100 ml of water, 5 ml of p H 10 buffer solution, 1 ml of 0.05M Zn-EDTA solution, and 0.2-0.3 gram of Eriochrome T indicator. Titrate with 0.01M EDTA to the end-point color change red-purple to blue. MAGNESIUM.Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 5 ml of ethyl alcohol, 5 ml of 1N HCI, and warm until dissolved. Add 5 ml of pH 10 buffer solution, 100 ml of water, and warm to 60-65 "C. Add 0.2-0.3 gram of Eriochrome T indicator and titrate the hot solution with 0.01M EDTA to the end-point color change red-purple to blue. NICKEL.Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 5 ml of ethyl alcohol, 5 ml of 1N HCI, and warm until dissolved. Add 100 ml of water, 5 ml of pH 10 buffer solution, and 0.2-0.3 gram of murexide indicator. Titrate with 0.01M EDTA to the endpoint color change yellow to violet-red. SILVER. Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 10 ml of ethyl alcohol, 10 ml of 1 N H N 0 3 , and warm to dissolve. Add 100 ml of 1 N H N 0 3 , 1 ml of ferric alum indicator, and titrate with 0.01N NH lCNS solution to the end-point color change to pale red-brown. (Dilution of the dissolved salt with 1N H N 0 3 causes capric acid to form as a milky dispersion, which does not interfere with the titration.) SILICON.Weigh accurately 0.4 to 0.5 gram of sample into a weighed 30-ml porcelain crucible. Add 1 ml of concentrated H2S04,3-4 drops of concentrated H N 0 3 and warm on a water bath to promote the initial breakdown of the compound. Gently fume off H2S04 o n a hotplate, ignite the charred residue over a low flame, and ash a t 800 "C in a muffle furnace for 30 minutes. Cool, reweigh, and determine weight of SiO, residue. TIN. Weigh accurately 0.4 to 0.5 gram of sample into a weighed 100-ml porcelain crucible. Add 10 ml of concentrated HNO?, 1 ml of concentrated H2S04, and heat gently on a hotplate until H N O d is removed and charring occurs. Increase heating to remove H2S04, ignite over a low flame, and ash a t 800 "C in a muffle furnace for 30 minutes. Cool, reweigh, and determine the weight of S n 0 2 residue. ZINC. Weigh accurately 0.075 to 0.1 gram of sample into a 250-ml conical flask. Add 5 ml of ethyl alcohol, 5 ml of 1N HCI, and warm to dissolve. Add 100 ml of water, 5 ml of p H 10 buffer solution, 0.2 to 0.3 gram of Erioctirome T indicator, and titrate with 0.01M EDTA to the end-point color change red-purple to blue. Results. Typical analytical results on laboratory-prepared batches of the metal caprates and other compounds are given in Table I. CHARACTERISTICS OF METAL CAPRATES
Humidity Test. Samples of 15 laboratory-prepared metal caprates were dried under vacuum over P z O for ~ 24 hours. The salts were then exposed to the atmosphere for 7 days at an average relative humidity of 6 0 z and a n average temperature of 20 "C and the percentage weight increase was measured, as shown in Table 11. It is not thought that the high increase in weight of the Mg caprate is due to water of crystallization since the original weight is readily restored by drying. The zinc salt of cyclohexanebutyric acid, for example, will form a monohydrate, and the water of crystallization cannot be removed by vacuum drying.
Table I. Analysis of Standard Compounds Metal, Compound Theoretical Found A I tris-ethyl acetoacetonate (CH3CO .CH 'CO .OCZH;)BAI 6.51 6.49 Ba caprate (CsHlsC00)2Ba 28.63 28. 67a Ca caprate (CoHluCOO)nCa 10.47 10.506 Cr caprate (nonstoichiometric) ... 13.75 Cu caprate (CqHloC00)2Cu 15.65 15.76 ... 9,82c Fe caprate (nonstoichiometric) La caprate (C!,HlsCOO)3La 21.23 20.80d Ni caprate (C,H,KOO)aNi 14.63 14.62 Pb caprate (C,,HlsC00)2Pb 37.69 37.84 M g caprate (C9Hl:iC00)2Mg 6.63 6.56 38,67 38,30 Ag caprate C,,HlnCOOAg Zn caprate (C9H,KOO)2Zn 16.03 16.09 Dibutyl-tin-di-2-ethylhexoate
(C4H8)ISn(0.C0.CiH1-,)2 22.86 22.88 Octaphen ylcyclotetrasiloxane (C6Ha)&Oc 14.17 14.16 Gravimetric analysis. * EDTA titration. The result of 9.82% Fe is close to the theoretical figure of 9.80% for (CC,H,!,COO),Febut the substance has an acetone-soluble component, removal of which leaves a material containing 11.45% Fe. EDTA titration.
Table 11. Moisture Pick-Up of Metal Capratesa Caprate Increase, % Barium
