7/28/2019 ,mmnbg

1/6

E L S E V I E R Talanta 44 (1997) 2103 2108Talanta

T h e d e t e r m i n a t i o n o f w e a r m e t a l s i n u s e d l u b r i c a t i n g o i l s b yf l a m e a t o m i c a b s o r p t i o n s p e c t r o m e t r y u s i n g s u l p h a n i l i c a c i d a s

a s h i n g a g e n tE . J . E k a n e m * , J . A . L o r i , S . A . T h o m a s

De par tme nt ~[~ Ch em isto, , Ah madu Bel lo Uni t ,ersi ty, Zaria, NigeriaReceived 5 Novem ber 1996: received in revised form 24 February 1997: accepted 27 February 1997

A b s t r a c t

A s imp le and r e l iab le ash ing p rocedu re i s p rop osed fo r the p repara t io n o f u sed lub r ica t ing o il s amp les fo r thed e t e r m i n a t i o n o f c a lc i u m , m a g n e s i u m , z i n c , ir o n , c h r o m i u m a n d n i ck e l b y fl a m e a t o m i c a b s o r p t i o n s p e c t r o m e t r y .Su lphan i l ic ac id was added to o i l s amp les and the mix tu re c oked an d the coke ashed a t 550C . The so lu t ions o f thea s h w e r e a n a l y s e d b y f la m e A A S f o r t h e m e t a ls . T h e r e le a s e o f ca l c i um , z i n c , i r o n a n d c h r o m i u m w a s i m p r o v e d b ythe add i t ion o f su lphan i l ic ac id to samp les . The r e la t ive s tand ard d ev ia t ions o f meta l con cen t r a t ion r esu l ts in thein i t ial o i l sample s we re 1 .5% for C a (1500 m g 1 ~ level) , 0 .3% fo r Mg (100 mg 1 " l level) , 3 .1% for Zn (1500 mg 1level) , 0 .7% fo r Fe (500 mg 1-1 level) , 0 .02% fo r Cr (50 m g I i level) and 0 .002% for Ni (10 mg 1 i level) . Th eop t imum samp le s ize fo r e f f ic ien t meta l r e lease was 20 g wh i le the op t imum su lphan i l ic ac id to o i l r a t io was 0 .05 gper g r am o f o il f o r Zn and C r and 0 .10 g fo r C a and Fe . R esu l t s ob ta ined by th i s p roce du re w ere h igh ly r ep roduc ib leand co m para b le w i th those ob ta ined fo r the same samp les u s ing s tanda rd p roc edu res . 1997 Elsev ie r Sc ience B .V .K e y w o r d s : A i d e d - a s h i n g ; L u b r i c a n t ; M e t a l d e t e r m i n a t i o n

1 . I n t r o d u c t i o n

T h e p e r f o r m a n c e c h a r a c te r i st i c s o f l u b ri c a t i n go il s a re u s u a l l y i m p r o v e d b y t h e a d d i t iv e s i n c o r -p o r a t e d t o i m p r o v e t h e c o l o u r , p o u r p o i n t , v i s c o s -i ty , a n t i w e a r , a n t i f r ic t i o n a l , a n t i f o a m i n g ,o x i d a t i o n a n d c o r r o s i o n i n h i b i t i o n p r o p e r t i e s .T h e s e a d d i t i v e s c o n t a i n v a r i o u s m e t a l s in v a r y i n ga m o u n t s . I n u se , a l o t m o r e o f s o m e o f t h e s em e t a l s a r e a d d e d t o t h e l u b r i c a t i n g o i l s f r o m

* Corresponding author.

f r i c t i o n a n d w e a r [ 1 ,2 ]. A s i m p l e , r a p i d a n d a c c u -r a t e a n a l y t i c a l p r o c e d u r e is n e e d e d f o r ro u t i n ed e t e r m i n a t i o n o f t h e s e e l e m e n t s i n o r d e r t op r o v i d e a n i n d i c a t io n o f t he d e g r e e o f w e a r o f t h eo i l w e t t e d p a r t s w i t h i n a n e n g i n e . A s p e c i f i c a p p l i -c a t i o n is in t h e a n a l y s i s o f u s e d l u b r i c a t i n g o i ls t oi n d i c a t e p o t e n t i a l f a i l u re i n i n t e r n a l c o m b u s t i o ne n g i n e s a s w e l l a s t h e m e t a l p o l l u t i o n p o t e n t i a l s o fs u c h o i l s i f t h e y a r e d i s p o s e d d i r e c t l y i n t o s u r f a c ew a t e r o r o n l a n d .

A l t h o u g h w e a r m e t a l p a r t i c l e s a r e n o t u n i -f o r m l y d i s t r i b u t e d i n o il s , t h e y a r e g e n e r a l l y d e t e r -

0039-9140/97/$17.00 {2 1997 Elsevier Science B.V. A ll r igh ts reserved.P I I S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 0 8 6 - 6

7/28/2019 ,mmnbg

2/6

2104 E.J. Ekanem et al ./ Talanta 44 (1997) 2103 2108mined by using techniques traditionally applicableto the determination of metals in homogeneoussolutions [3]. Various methods have been reportedfor the preparation of samples prior to instrumen-tal analysis [3-7]. The possible loss of certainnaturally occurring volatile constituents duringunaided ashing of petroleum or its products hasbeen reported [8-10]. Hence, the determination oftrace metals in petroleum oils has relied on ashingprocedures that aim at preventing metal losses byvolatilization [11,12] during ashing.

Benzenesulphonic acid has been applied as ash-ing agent for the determination of Ni and V inpetroleum distillates [11,13] while p-xylenesul-phonic acid has been used for ashing petroleumdistillates [12] and crude petroleum [14] for thedetermination of trace metals. Unlike thesereagents which are hygroscopic and difficult tohandle and store [15], sulphanilic acid has a verylong shelf life and was investigated as ashingreagent in this work. The purpose of the presentwork was to establish the optimum conditions forthe ashing of used lubricating oils applying sul-phanilic acid to contain any volatilization losses.

2. Experimental

2 . 1. R e a g e n t s , a p p a r a t u s a n d s a m p l e sAll reagents used in this work were of analytical

grade. Sulphanilic acid was obtained from BDH.Doubly distilled water was used in preparing allaqueous solutions. The muffle furnace used was aCarbolite (Sheffield, UK) capable of maintainingconstant temperatures in the range 20-1200C.The Perkin Elmer 290 B atomic absorption spec-trophotometer, equipped with a premix chamberand a single slot burner head operated on air-acetylene flame, with automatic background cor-rection and mono-element hollow cathode lampswas used for solution analysis. The parametersselected on the spectrophotometer for the deter-mination of the test metals conform with thoserecommended by Perkin Elmer for this spectrom-eter. The oil samples analysed were collected fromprivate vehicles and automechanic workshops inZaria, Nigeria.

2 . 2. W o r k i n g s t a n d a r d s o lu t i o n sCaCO3, MgC126H20, ZnO, (NH4)2SO .FeSO4"6H20, Ni(NO3)2"6H20 and chromiumpowder were dissolved appropriately and diluted

with water to obtain 1000 mg 1-J stock solutionsof Ca, Mg, Zn, Fe, Ni [16] and Cr [17]. Workingstandard solutions were obtained from the respec-tive stock solutions by appropriately dilutingthem with water introducing 5% (m/v) lanthanumas the nitrate in the case of Ca and Mg to controlthe interferences of ions like phosphate, aluminateand silicate.2 .3 . R e c o m m e n d e d p r o c ed u r e

A mixture of 10.0 g used lubricating oil and 1.0g sulphanilic acid in a 250 cm 3 pyrex beaker washeated in a heating mantle in the fume cupboard.The smoke issuing from the beaker was ignitedand the set-up left until the flame was extin-guished. The coke formed was muffled at 550Cuntil the ash was free of carbon particles; thisrequired 45 min. The ash was allowed to coolbefore it was dissolved in 20 cm 3 of 6M HC1 anddigested in a boiling water bath to obtain a clearsolution. This solution was made up to 50 cm 3with distilled water in a volumetric flask, intro-ducing 5% (m/v) La and analysed by FAAS forCa, Mg, Zn, Fe, Cr and Ni against a blanksolution prepared by digesting 20.0 cm 3 of the 6MHC1 in the boiling water bath in the same way asthe sample digests and making up to 50 cm 3 withdistilled water as done for the sample solution.

3. Results and discussion3 .1 . Opt im iza t ion o f ash ing cond i t ions3 . I . 1 . T e m p e r a t u r e a n d t i m e

Optimization of parameters was based on 10 greplicates [18] of samples. Temperatures were ob-served in the range 400-650C for durations of15-120 min. The progress and completeness ofashing was visually determined.

The lowest muffling temperature that yieldedcarbon-free ash was 550C and this was achieved

7/28/2019 ,mmnbg

3/6

E.J. Ekanem et al ./ Talanta 44 (1997) 2103 2108 2105T a b l e 1Ef fec t of i gni t i onS a m p l e t r e a t m e n t C o k i n g t im e ( m i n ) M e t a l c o n c e n t r a t i o n ( m g 1 ~)

C a M g Z n F e C r N iW i tho ut i gni t i on 20 1470 88 1350 387 32 .5 9 .5W i th igni t i on 18 1470 88 1390 426 35 .6 9 .6

in the shortest time of 45 min. Though a highermuffling temperature could be selected for a morerapid ashing, 550C was regularly applied in thiswork to avoid the disfiguration of the pyrexbeakers that may occur at higher temperatures[19]. The oxides of the metals determined areknown to be stable even above 1000C [20], hencevolatilization of the metals would be prevented byrendering them as oxides in the ash at 550C. Thisashing temperature has been recommended by theInstitute of Petroleum [21] in a procedure thatdoes not specify coking and muffling times andincludes an overnight delay and a filtration step.The present procedure has eliminated the need forash solution filtration and the attendant delay byachieving carbon-free ash in a short time. Thechances of error arising from contamination dur-ing the filtration are removed in this procedure.

3.1.2. Ignition o f samplesCoking time was the duration up to the cessa-

tion of smoke when the smoke issuing from thesamples was not ignited. In the cases where thesmoke was ignited as it issued from the sample,coking time was the time taken for the smoke tofinish burning and the flame to be extinguished.

Satisfactory coking was achieved in 20 minwithout ignition and in 18 min when the smokewas ignited. Ignition did not, however, altermuffling time because the coke derived with andwithout ignition both yielded good ash in 45 minof muffling.

The ashes obtained were completely soluble in20 cm 3 of hydrochloric acid and a clear solutionwas obtained after 2 min of digestion. Therefore,there was no need to filter the solution as recom-mended by the direct ignition method for thedetermination of additive metals in unused lubri-

cating oils [21]. The results presented in Table 1for the determination of the test metals in samplenumber 1 were typical of those obtained for allsamples in this work and show that there wasincreased metal retention in the ignited sampleparticularly for Zn, Fe and Cr. For these metals,the ignition allows metal complexes that wouldhave escaped in the smoke to burn and returntheir metal constituents to the coke as metaloxides. The differences in the determinant levelsof ignited and unignited samples give an indica-tion of which determinants are more prone tovolatilization losses in lubricating oils. Thoughthere was no change in the levels of some of thetest metals, the enhancement in the levels of evena few of the test metals advised the adoption ofignition of lubricating oils for determining tracemetals. A similar advantage of sample ignitionwhile coking has been reported for petroleumcrudes [14] and petroleum distillates [22].

3.2, Sulphanilic acid as ashing reagentVarious levels of sulphanilic acid were added to

10 g replicates of oil analysed by the recom-mended procedure and yielded results as detailedin Table 2. The addition of sulphanilic acid to thesamples did not alter the ashing efficiency of thesamples: it did not alter either the ignition time orthe muffling time and the ash produced in thepresence of it was still completely soluble in 20.0cm 3 of 6M HC1. The muffling of 1.0 g of sul-phanilic acid alone at 550C under similar condi-tions to those applied for ashing oil samples leftno residue at the end of 45 min. This implies thatSA did not contribute to the ash. The digest of 2.0g of sulphanilic acid in 6 M HC1 yielded blanklevels of all the metals determined. The sulphanilic

7/28/2019 ,mmnbg

4/6

2106Table 2Sulphanilic acid (SA ) as a shing reagent

E.J. Ekanem et al. / Talanta 44 (1997) 2103 2108

Mass of SA ad ded (g) M etal concentration (mg 1 ~)Ca Mg Zn Fe Cr Ni

0 14700.I 14720.2 14750.3 14850.4 14980.5 15000.6 15000.8 15011.0 15021.5 15002.0 1501

88 1390 426 35.2 9.688 1390 428 35.7 9.687 1393 428 36.1 9.688 1395 434 36.3 9.788 1397 455 36.7 9.788 1400 456 37.3 9.789 1400 457 37.3 9.789 1400 458 37.0 9.789 1400 468 37.0 9.789 1400 468 37.3 9.789 1400 468 37.3 9.7

a c i d w a s t h e r e f o r e s h o w n t o b e f r e e f r o m t h ed e t e r m i n a n t s .T h e r e s u l t s p r e s e n t e d i n T a b l e 2 f o r 1 0 . 0 g o f o i l

s a m p l e 1 s h o w t h a t t h e r e w a s a n i m p r o v e m e n t i nm e t a l r e l ea s e e sp e c i a l ly f o r C a , Z n , F e a n d C r a sth e su lp h a n i l i c a c id t o o i l r a t i o i n c r e a se d u n t i lp e a k v a l u e s w e r e o b t a i n e d a t 0 .0 5 g o f S A f o r Z na n d C r a n d 0 .1 g f o r C a a n d F e p e r g r a m o f o il .O n c e o b t a i n e d , t h e p e a k d e t e r m i n a n t l e v el s p e r -s i st e d w h e n h i g h e r r a t i o s o f su l p h a n i l i c a c i d w e r ea p p l i e d , u p t o 0 .2 g o f S A p e r g r a m o f o il . T h es a m e t r e n d w a s o b s e r v e d a l s o f o r s a m p l e s o t h e rt h a n s a m p l e n u m b e r 1 . T h a t p e a k m e t a l r e l e a s e i sa c h i e v e d a t d i f f e r e n t r a t i o s o f r e l e a si n g a g e n t f o rd i f f e r e n t d e t e r m i n a n t s r e f l e c t s t h e d i f f e r e n c e s i ns t a b il i ti e s o f th e m e t a l c o m p l e x e s i n w h i c h t h ed e t e r m i n a n t s p e c i e s a r e e n c a g e d i n t h e o i l s a m p l e s[ 2 3 ] . T h e se r e su l t s su g g e s t t h a t t h e z in c a n dc h r o m i u m m e t a l l o c o m p l e x e s a r e l e s s t h e r m o d y -n a m i c a l l y s t a b l e t h a n t h e c a l c i u m a n d i r o n o n e s .S i m i l a r d i f f er e n c e s i n t h e s t a b i li t ie s o f p o r p h y r i n sa n d m e t a l l o c o m p l e x e s o f m e t a l s o c c u r r in g i nc r u d e o i l h a v e b e e n r e p o r t e d [ 1 4 ] . T h e a d d i t i o n o f0 .1 g o f s u l p h a n i l i c a c i d p e r g r a m o f o il t o s a m -p l e s w a s a d o p t e d i n t h i s w o r k a n d i s r e c o m -m e n d e d f o r t h e a s h i n g o f u s ed l u b r i c a t i n g o i ls fo rt h e d e te r m i n a t i o n o f tr a c e m e t a l s b y F A A S . I t ss t a b i l it y a n d l o n g s h e l f l if e m a k e i t p r e f e r a b l e t oo t h e r s i m i l a r r e a g e n t s l i k e b e n z e n e - s u l p h o n i c a n dp - x y l e n e s u l p h o n i c a c i d s , w h i c h a r e h y g r o s c o p i ca n d r e q u i r e sp e c i a l h a n d l in g [ 1 4 ] .

3.3. Evaluation of the procedureW h e n t h e s a m e o i l w a s a s h e d i n s i x r e p l i c a t e s

a n d t h e a s h a n a l y s e d f o r t h e t e s t m e t a l s i n t h er e p l i c a t e s , t h e r e s u l t s s h o w e d t h a t t h e r e c o m -m e n d e d p r o c e d u r e i s h i g h l y r e p r o d u c i b l e w i t h r e l -a t i v e s t a n d a r d d e v i a t i o n s ( % r sd ) o f 1 . 5 , 0 . 3 , 3 . 1 ,0 . 7 , 0 . 0 2 a n d 0 . 0 0 2 f o r C a , M g , Z n , F e , C r a n dN i , r e s p e c t i v e l y a t th e r e s p e c t i v e c o n c e n t r a t i o nlevels 1500, 100, 1500, 500, 50 an d 10 m g 1 J.

A l l f i v e s a m p l e s w e r e a n a l y s e d i n t r i p l i c a t e s b yt h i s p r o c e d u r e , t h e d i r e c t i g n i t i o n p r o c e d u r e [ 2 2 ]a n d t h e U n i v e r s a l O i l P r o d u c t ( U O P ) m e t h o d8 0 0 - 7 9 [ 2 4 ] a n d t h e a v e r a g e r e s u l t s a r e c o m p a r e di n T a b l e 3 . F r o m o b s e r v a t i o n , t h e r e s u l t s i n d i c a t et h a t t h e v a l u e s o b t a i n e d b y t h e d i r e c t i g n i t i o np r o c e d u r e a r e g e n er a l ly l o w e r t h a n t h o s e o b t a i n e db y ei th e r th e r e c o m m e n d e d o r U O P m e t h o d 8 00 -7 9 p r o c e d u r e s ; t h e v a l u e s o b t a i n e d b y th e r e c o m -m e n d e d p r o c e d u r e a r e g e n e r a l l y c l o s e r t o t h o s eo b t a i n e d b y t h e U O P p r o c e d u r e . A r e g r e s s io na n a ly s i s o f t h e r e su l t s r e v e a l s a c lo se r f i t b e tw e e nt h e r e c o m m e n d e d p r o c e d u r e a n d t h e s t a n d a r dU O P p r o c e d u r e t h a n b e t w e e n t h e d i r e c t i g n i t i o na n d t h e U O P p r o c e d u r e s . T h e r e s p e c t i v e r e g r e s -s io n l i n e s a r ey = 1 . 0 0 4 x r - 1 . 8 84 r = 0 . 9 9 9 3a n dy = 1 .017x i + 5 .356 r = 0 .9982

7/28/2019 ,mmnbg

5/6

E.J. Ekanem et al./ Talan ta 44 (1997)2103 2108

Table 3Comparison of sample results obtained by three ashing procedures

2107

Sample No. Determinant Measured levels (mg 1 ~)Recommended proced ure Direct ignition procedure UOP method 800-79

1 Ca 1508.4 1503.4Mg 90.0 87.7Zn 1411.8 1372.6Fe 472.2 346.8Cr 36.0 36.0Ni 9.7 9.32 Ca 1739.9 1706.1Mg 93.6 92.6Zn 1475.5 1 46.1Fe 440.5 428.6Cr 28.0 26.0Ni 9.3 9.33 Ca 1638.5 1621.6Mg 85.0 82.0Zn 1441.2 1446.1Fe 412.7 412.7Cr 22.0 22.0Ni 9.3 9.34 Ca 878.4 810.8Mg 32.1 30.5Zn 1034.3 1029.4Fe 281.8 264.8Cr 64.0 63.0Ni 11.3 10.35 Ca 641.9 608.1Mg 28.5 29.8Zn 872.6 853.0Fe 388.9 388.9Cr 67.0 66.0Ni 11.7 11.3

1513.490.71401.9460.335.010.31756.894.91446.1452.428.09.31706.188.71446.1420.621.09.3844.631.11012.8281.864.011.3641.929.5857.9

388.969.011.7

w h e r e y r e p r e s e n t s U O P v a l u e s , x r r e p r e s e n t sv a l u es f r o m t h e r e c o m m e n d e d p r o c e d u r e , x i r e p -r e s e n t s v a l u e s f r o m t h e d i r e c t ig n i t i o n p r o c e d u r ea n d r is t h e c o r r e l a t i o n c o e f f i c i e n t .

T h e a p p l i c a t i o n o f S t u d e n t ' s t - t e s t t o t h e d a t ao f T a b l e 3 re v e a l s t h a t t h e r e s u lt s o b t a i n e d b y t h ed i r e c t i g n i t i o n p r o c e d u r e a r e s i g n i f i c a n tl y d i f f e r e n tf r o m t h o s e o b t a i n e d b y t h e U O P m e t h o d 8 0 0- 79w h i l e t h e r e i s n o s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h er e su l ts o b t a i n e d b y t h e r e c o m m e n d e d a n d t h eU O P p r o c e d u r e s .

T h e s e s t a t is t i cs s u p p o r t t h e a r g u m e n t t h a t w h i l et h e U O P m e t h o d 8 0 0- 79 a n d t h e r e c o m m e n d e dp r o c e d u r e s r e a l i s e m a x i m u m r e l ea s e o f m e t a l s , t h ed i r e c t i g n i ti o n p r o c e d u r e f a il s t o a c h i e v e m a x i -

m u m m e t a l r e l e a s e, h e n c e th e g e n e r a l l y l o w v a l u e so b t a i n e d b y t h e l a t t e r p r o c e d u r e a n d t h e s ig n i fi -c a n t d i f f e r e n c e b e t w e e n i t a n d t h e f o r m e r t w op r o c e d u r e s. T h e a d v a n t a g e o f s u lp h a n i l i c a c idc a u s i n g e n h a n c e d m e t a l r e l ea s e as i n d i c a t e d i nT a b l e 2 i s r e f l e c t e d i n th e r e s u l t s o f t h e s e s t a t i s t i -c a l a na l y se s . T h e r e c o m m e n d e d p r o c e d u r e h a s a na d d i t i o n a l a d v a n t a g e i n b e i n g f a s t e r t h a n t h eU O P m e t h o d 8 0 0- 79 .

4 . C o nc l us i o n

U s e d l u b r i c a t in g o i l m a y b e c o n v e n i e n t l y c o k e da n d a s h e d a t 5 5 0 C i n 4 5 m i n f o r d e t e r m i n i n g

7/28/2019 ,mmnbg

6/6

2108 E . J . E ka n em e t a l . / T a l a n t a 4 4 ( 19 9 7) 2 1 0 3 - 2 1 0 8we ar an d o t h e r me t a l s b y f lame a t omic ab sor p t ionspectrometry. Sulpha ni l ic ac id app l ied as ash ingage n t imp r ov e s t h e r e le ase o f me t a l s an d t h eaccuracy o f the analytical resu lt s . The pro cedurereported is simple, rapid and precise.

Re f e r e n c e s[1] O. I . Mi lner , Analys is of Pet ro leum for Trace Elements ,

I n t e r na t iona l Se ri es o f Mo nogr a phs on Ana l y t i c a l Che m-istry, vol. 14, Pergamon Press, Oxford, 1963.

[2] C.R. Hodgkins , J . Hansen, Anal . Chem. 26 (1954) 1759.[3] J . Hernandez-M endez , L. Polo-D iez , A. Bernal -M elchor ,

Ana l . Chim. A cta 108 (1979) 39 .[4] C.S. Saba , W.E. Rhine , K. J . Eisent raut , Anal . Chem. 53

(1981) 1099.[5] C.M . D e la Gau rdia , C.A. S alvalor , N.V. Berenguer ,Analus is 8 (1980) 448.

[6] J .L. Fabec , M.L. Ruschak, Anal . Chem. 57 (1985) 1853.[7] C.S. Saba, K.J. Eisentraut, Anal. Chem. 49 (1977) 454.[8] J .H. K archm er , E.L. Gun n, A nal . Chem. 24 (1952) 1733.[9] O. I . Mi lner , J .R. Glass J r . , J .P . Kirchner , A.N. Yur ich ,

Anal . Chem. 24 (1952) 1728.[10] L.W. Gamble , W.H. Jones , Anal . Chem. 27 (1955) 1456.[ l l ] J .E. S hot t J r . , T . J . Gar lan d, R.O. C lark , Anal . Chem . 33

(1961) 506.[12] W.A. Ro w, K .P. Yates , Anal . Ch em. 36 (1963) 368.

[13] Ins t i tu te of Pet ro leum, Method IP 286/77(a) , Standardsfor Pet ro leum and i t s Products , Par t 1 , Methods forAn alysis and T esting, vol. 2, 1985, pp. 286.

[14] A.P. U doh , S .A. Thoma s, E. J . Ekane m, Ta lanta 39(1992) 1591.

[15] A.P. U doh , Ph.D. Thes is , Ah ma du Bel lo Univers i ty ,Zar ia , 1989.

[ 16 ] P .J . W hi t e he a d , Pye Un i c a m At om i c Abs o r p t i on Da t aBook , Pye Un i c a m, Ca mbr i dge , 1975 .

[ 17 ] A . I . Vogel , A Te x t book o f Qua n t i t a t i ve I no r ga n i c Ana l y -s is , 3rd ed . , ELBS-Longmans , London, 1961.

[ 18 ] AS TM me t hod D- 811 -82 , Annu a l Book o f AS TM S t a n -dard s, vol. 5.0 1, 1984 , pp. 341.

[19] A.P. Udoh, S .A. Thomas, E. J . Ekanem, Bul l . Chem. Soc .Ethiop. 4 (1990) 13 .

[20] R.C. W east (Ed.) , H and boo k o f Che mis t ry and Physics,55th Edi t ion , C RC Press , Ohio , 1974, pp . B. 63 .

[21] Ins t i tu te of Pet ro leum, Methods IP 110/82 (a) , IP 111/82(b) and IP 117/82 (c), Standards for Pet ro leu m and i tsProducts , Par t 1 , Methods for Analys is and Tes t ing , 45thed., vol. 1, 198 5, pp. 110.1 (a), 111.1 (b) and 117.1 (c).

[22] A.P. U doh, S .A. Thom as, E. J . Ekane m, A JST (B) 5 (2)(1991) 8.

[23] R.H. F i lby , in : T.F. Yen (Ed. ), Th e Ro le of Trace Meta lsin Pet ro leum, Ann Arbor Science Publ ishers , Ann Arbor ,Michigan, 1975, pp . 31 .

[24] Universa l Oi l Products , UOP method 800-79, Labora toryTest Methods for Pet ro leum and i t s Products , vol . 2 ,UO P Process D ivis ion , 1979, pp . 1 -12.