Analysis of Petroleum for Trace Metals

8

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0156.ch008

Chromium Chromium has been found i n surveys of metals i n crude oils, although its concentration rarely exceeds 1 p p m (μg/g) (1, 2, 3). In its native form, chromium is not particularly volatile and may be concentrated i n the bottom fractions during refining (2). Chromium may also enter petroleum matrices as antioxidants added to jet fuels, as corrosion prod ucts, or as a wear metal i n used lubricating oils. Although its ultimate fate is unknown, chromium may enter the atmosphere during the com bustion of fuels or it may enter natural waterways as part of industrial effluents.

Available Analytical Methods A number of methods exist for the determination of parts-per-billion (ng/g) levels of chromium i n aqueous media (Table 8.I). These are re peatedly reviewed as new techniques are introduced (4, 5, 6). Potentially all these techniques could be applied to petroleum samples after matrix destruction, but i n practice, only a few have been utilized. After wet oxidation of a large sample ( > 1 0 0 g ) , 10 to 50 μg of chromium may be determined by a colorimetric procedure w i t h 1,5-diphenylcarbohydrazide after iron, copper, molybdenum, and vanadium are extracted as the cupferrates ( 3 ) . In survey analyses, C r levels as low as 5 ng/g have been measured by optical emission spectroscopy after ashing (2, 3) or directly by neutron activation with extended irradiation and counting times ( 1 ) . Concentrations of chromium above 100 ng/g i n used lubricating oils have been measured directly by flame atomic absorption ( 8 ) ; for lower con centrations, heated vaporization atomic absorption (HVAA) has been utilized ( 9 ) . In the Trace Metals Project, two procedures using this latter technique were evaluated for the determination of 10 ng Cr/g i n a variety of petroleum matrices.

Special Analytical

Considerations

B o t h ashing a n d a c i d digestion m a y b e used to mineralize organic m a t r i c e s c o n t a i n i n g c h r o m i u m . A l t h o u g h c h r o m i u m itself is n o t v o l a t i l e , p r e t r e a t m e n t w i t h s u l f u r i c a c i d aids i n d e s t r u c t i o n of t h e h y d r o c a r b o n m a t r i x (10).

I n i n i t i a l a s h i n g studies c a r r i e d o u t b y t h e P r o j e c t , l a r g e 95

Hofstader et al.; Analysis of Petroleum for Trace Metals Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

96

ANALYSIS OF P E T R O L E U M

Table 8.1.

METALS

Existing Techniques for Determining n g / g Levels of Chromium in Aqueous Media Instrumental

Chemical Extraction/colorimetry Catalytic kinetics Chemiluminescence


FOR TRACE

Coulometric titrimetry D i f f e r e n t i a l pulse p o l a r o g r a p h y F l a m e atomic absorption (solvent e x t r a c t i o n ) Gas chromatography (electron capture detector) (mass s p e c t r o m e t r y ) Heated vaporization atomic absorption M a s s s p e c t r o m e t r y (direct i n j e c t i o n ) Neutron activation O p t i c a l emission spectroscopy X - r a y fluorescence (ion exchange)

v a r i a b l e b l a n k s w e r e f o u n d (see

Contamination, p. 14).

These blanks

w e r e a t t r i b u t e d to exposed n i c h r o m e h e a t i n g elements, since t h e y c o u l d b e a l m o s t c o m p l e t e l y e l i m i n a t e d b y a s h i n g samples for a m i n i m u m p e r i o d i n a muffle w i t h c o v e r e d elements. Sample

Preparation

T w o p r o c e d u r e s b a s e d o n H V A A h a v e b e e n e v a l u a t e d b y the P r o j e c t . I n one, the s a m p l e is a n a l y z e d d i r e c t l y after d i l u t i o n w i t h t e t r a h y d r o f u r a n . I n t h e other, a 0.5-g s a m p l e is c h a r r e d w i t h 10 d r o p s of s u l f u r i c a c i d i n a 3 0 - m l V y c o r c r u c i b l e a n d t r e a t e d at 5 4 0 ° C for t h e m i n i m u m t i m e r e q u i r e d to r e m o v e carbonaceous

deposits.

s u l f u r i c a c i d p r i o r to measurement.

T h e a s h is t h e n s o l u b i l i z e d i n I N A reagent b l a n k is c a r r i e d t h r o u g h

t h e p r o c e d u r e f o r e a c h set of samples.

T h e analyses are c a r r i e d o u t

p r o m p t l y after p r e p a r a t i o n so that loss of c h r o m i u m f r o m aqueous s o l u t i o n b y a d s o r p t i o n o n the w a l l s , w h i c h has b e e n r e p o r t e d i n some cases

(II),

can be avoided. Measurement B e f o r e analyses w e r e c a r r i e d out b y t h e t w o p r o c e d u r e s , the o p t i m u m H V A A parameters were established e m p i r i c a l l y using 2 0 - n g / m l aqueous standards. H V A A measurements f o r c h r o m i u m w e r e m a d e w i t h a V a r i a n T e c h t r o n C R A - 6 3 a t o m i z e r ; other t u b e f u r n a c e a t o m i z e r s w h i c h p o s s i b l y c o u l d h a v e b e e n u s e d , w e r e n o t i n v e s t i g a t e d . A l t h o u g h the H V A A r e sponse w a s l i n e a r b e t w e e n 0 - 4 0 0 n g / m l , a w o r k i n g r a n g e o f 0 - 5 0 n g / m l was utilized. T h e detection limit ( S / N =

2 ) w a s c a l c u l a t e d to b e 1 p g .

T h e a b s o r b a n c e of the n e o n 359.4-nm l i n e i n t h e c h r o m i u m h o l l o w c a t h o d e l a m p w a s u s e d to m a k e b a c k g r o u n d corrections.


8.

HOFSTADER E T A L .

97

Chromium

C h e m i c a l m a t r i x effects w e r e e n c o u n t e r e d i n the H V A A d e t e r m i n a tion.

W h e n a constant a m o u n t of c h r o m i u m w a s a d d e d t o

different

p e t r o l e u m samples, the a t o m i z a t i o n response v a r i e d c o n s i d e r a b l y that o b t a i n e d i n T H F alone ( T a b l e 8 . I I ) .

from

S i n c e a l m o s t p a r a l l e l effects

w e r e n o t e d after a s h i n g , t h e effect of F e , N i , a n d V w a s s t u d i e d . Excesses of v a n a d i u m h a d a significant effect at levels w h i c h often o c c u r i n c r u d e oils (10 to > 100 p p m ) .

O n c e v a n a d i u m was introduced i n the atomizer,

the c h r o m i u m s i g n a l was consistently s u p p r e s s e d u n t i l the a t o m i z a t i o n f u r n a c e w a s b a k e d at m a x i m u m t e m p e r a t u r e to r e m o v e r e s i d u a l v a n a d i u m . F r o m the d a t a i n T a b l e 8.II i t is a p p a r e n t that c o m p o n e n t s other t h a n v a n a d i u m affect the c h r o m i u m s i g n a l . N o e x p l a n a t i o n has b e e n f o u n d f o r Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0156.ch008

the e n h a n c e m e n t

observed

i n s o m e cases.

S i n c e a l i n e a r response

is

o b t a i n e d after t h e i n i t i a l i n j e c t i o n of a g i v e n s a m p l e s o l u t i o n , t h e m e t h o d of s t a n d a r d a d d i t i o n s c a n b e a p p l i e d to c o m p e n s a t e f o r these c h e m i c a l m a t r i x effects. Recommended

Method

I n the d i r e c t H V A A p r o c e d u r e the s a m p l e is d i l u t e d w i t h t e t r a h y d r o furan.

T h e c o n c e n t r a t i o n of c h r o m i u m is d e t e r m i n e d b y t h e m e t h o d of

s t a n d a r d a d d i t i o n s c o u p l e d w i t h b a c k g r o u n d corrections i f necessary. T h i s p r o c e d u r e has b e e n a p p l i e d to s e v e r a l c r u d e o i l samples, a n d t h e results w e r e c o m p a r e d to those o b t a i n e d b y H V A A ashing (Table 8.III).

after s u l f a t e d

T h e g o o d a g r e e m e n t b e t w e e n t e c h n i q u e s verifies

t h a t n o c h r o m i u m is lost p r i o r to a t o m i z a t i o n i n t h e d i r e c t t e c h n i q u e . F u r t h e r m o r e , i t demonstrates t h a t the c o m b i n a t i o n of s t a n d a r d a d d i t i o n s a n d c o r r e c t i o n for reagent b l a n k a n d b a c k g r o u n d o v e r c o m e s t h e interTable 8.II.

Effect of Matrix on the Determination of Chromium by the Direct H V A A and A s h - H V A A Procedures Δ Α Χ 10 per ng Cr/ml Added 3

A

Direct (THF, 1:1)

Crude/Solvent (Response

"L

Ash (m

5:1)

H SO t

h

Ratio Ratio) Ash

Direct

Reagent B l a n k (THF)

0.99

5.63

—

—

Crude A

1.33

7.85

1.34

1.39

Crude Β

1.20

7.07

1.21

1.25

Crude C

0.83

6.04

0.84

1.07

Crude D

0.42

5.57

0.43

0.98

Crude Ε

0.77

6.57

0.78

1.17


98

ANALYSIS O F P E T R O L E U M

ferences e n c o u n t e r e d

i n the H V A A

measurement.

FOR TRACE

METALS

T h e sulfated a s h

procedure m a y b e used to obtain reliable data, but the direct technique is s i m p l e r , has s m a l l e r reagent b l a n k s , a n d is less subject t o c o n t a m i n a t i o n . C o n s e q u e n t l y , t h e d i r e c t H V A A p r o c e d u r e , w h i c h has b e e n e v a l u a t e d b y t h e cross-check p r o g r a m , is p r e f e r r e d . T h e p r e c i s i o n o f the d i r e c t p r o c e d u r e w a s e v a l u a t e d for a v a r i e t y o f m a t r i c e s i n t h e i n i t i a t i n g l a b o r a t o r y . A base kerosene, w h i c h c o n t a i n e d n o d e t e c t a b l e c h r o m i u m , w a s s p i k e d w i t h 47.9 n g C r / g as c h r o m i u m n a p h t h e n a t e . T h e other samples c o n t a i n e d n a t i v e c h r o m i u m . T h e results are s h o w n i n F i g u r e 8.1, w h e r e t h e average values are r e p r e s e n t e d b y the points a n d t h e total spread of data b y the brackets.

The

percentages


g i v e n are 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 , a n d the degrees o f f r e e d o m (η — 1) are g i v e n i n parentheses.

F r o m t h e d a t a s h o w n the o v e r a l l R S D i n t h e

initiating laboratory was 1 7 % .

KEROSENE (Spiked) SHALE OIL

\-+-\

16% (6)

\-+-\

18% (6)

1

#6 FUEL OIL A

CRUDE A

1 · 1 Η

CRUDE Β

1 40

·

|l7%(6)

26% (5)

6.5% (4)

1 80

1 120

1 160

1

200

ngCr/g (see text for explanation of symbols)

Figure 8.1.

Intralaboratory data for determi nation of chromium

F o u r samples w e r e a n a l y z e d i n the cross-check p r o g r a m . T h e results o b t a i n e d a t t h e i n i t i a t i n g l a b a n d a t t w o c o o p e r a t i n g laboratories a r e s u m m a r i z e d i n F i g u r e 8.2. laboratory study.

T h e kerosene s a m p l e w a s u s e d i n t h e i n t r a -

T h e a r r o w i n d i c a t e s t h e d e t e c t i o n l i m i t i n t h e base

kerosene; o t h e r w i s e the s y m b o l s are the same as i n F i g u r e 8.1. F o r t h e kerosene a n d the shale o i l , the R S D w a s 2 5 % w h i l e f o r the N o . 6 f u e l o i l i t w a s 1 3 % . S o m e scatter w a s e n c o u n t e r e d w i t h t h e N o . 6 f u e l o i l ; t h i s i s a t t r i b u t e d t o d i f f i c u l t y w i t h b a c k g r o u n d corrections, since b o t h a n e g a t i v e b a s e l i n e a n d t h e b a c k g r o u n d p e a k f o r this h e a v y m a t r i x w e r e i n v o l v e d . H o w e v e r , t h e average v a l u e o b t a i n e d b y the d i r e c t p r o c e d u r e w a s 145


8.

HOFSTADER E T A L .

Table 8.III.

99

Chromium

Comparison of Chromium by T w o Procedures Chromium Direct

Sample

Concentration

(ng/g)

Mini-Ash-HVAA

HVAA

Crude A

44, 7 7 , 7 5 , 90, 77, 49

57, 73, 61

Crude Β

50, 56, 58, 59, 54

44, 61

Crude C

38

42,49

Crude D

5 χ d e t e c t i o n l i m i t levels of the m e t a l s , t h e s a m p l e s h o u l d b e f u r t h e r


8.

HOFSTADER E T A L .

101

Chromium

d i l u t e d b e f o r e s t a n d a r d a d d i t i o n s . T h i s d i l u t i o n t e c h n i q u e is r e c o m m e n d e d r a t h e r t h a n c h a n g i n g o p e r a t i n g parameters. T h e r e s i d u a l h e a t i n t h e a t o m i z a t i o n t u b e c o n t r i b u t e s to t h e r e p e a t a b i l i t y of s a m p l e signals. T h i s effect m a y b e m i n i m i z e d b y i n j e c t i n g samples o n a fixed s c h e d u l e . F o r t h e m e t h o d d e s c r i b e d h e r e , injections m a d e at 90-sec i n t e r v a l s g i v e good repeatability. ( 3 ) A l i q u o t 5 μ\ of the s a m p l e s o l u t i o n i n t o t h e s y r i n g e , i n i t i a t e the C R A - 6 3 p r o g r a m , a n d i m m e d i a t e l y inject the s a m p l e .


( 4 ) R e c o r d the a b s o r b a n c e p e a k h e i g h t ( A ) o b s e r v e d d u r i n g t h e a t o m i z a t i o n step. ( 5 ) R e p e a t Steps 3 a n d 4 t w o m o r e t i m e s ; c a l c u l a t e t h e average p e a k h e i g h t f o r e a c h . I f t h e t h r e e signals are w i t h i n =fc 1 0 % of t h e a v e r a g e , use this v a l u e ( Α) i n the c a l c u l a t i o n ; i f not, r e p e a t the s e q u e n c e of Steps 3 a n d 4 t w o m o r e times a n d use t h e average of t h e five r e a d i n g s . ( 6 ) A d d 5 / J of the c a l i b r a t i o n s t a n d a r d ( e q u i v a l e n t to 25 n g ) t o t h e s a m p l e s o l u t i o n , m i x w e l l , a n d c a r r y o u t Steps 3 - 5 . R e c o r d t h e average p e a k h e i g h t as Αχ. ( 7 ) A d d a s e c o n d 5 μ\ of the c a l i b r a t i o n s t a n d a r d ( 2 5 n g , a t o t a l of 50 n g ) to the s a m p l e s o l u t i o n a n d a g a i n c a r r y o u t Steps 3 - 5 . R e c o r d t h e a v e r a g e p e a k h e i g h t as A . 2

( 8 ) A d d a t h i r d 5 μ\ of the c a l i b r a t i o n s t a n d a r d ( 2 5 n g , a t o t a l of 75 n g ) t o the s a m p l e s o l u t i o n a n d a g a i n c a r r y o u t Steps 3 - 5 . R e c o r d t h e a v e r a g e p e a k h e i g h t as A . 3

( 9 ) Reset w a v e l e n g t h to 359.4 n m a n d inject t h e s a m p l e s o l u t i o n ( S t e p 8 ) to m e a s u r e b a c k g r o u n d . R e c o r d the r e a d i n g as b. Calculation ( 1 ) C a l c u l a t e the c o n c e n t r a t i o n of m e t a l i n the s a m p l e f r o m the first a d d i t i o n as:

n

(2)

g

p

g

C

( 2 5 n g C r ) added -

r

A

- A .

l

w

X

A

-

Q

b

"W

R e p e a t t h e c a l c u l a t i o n for the second a d d i t i o n :

n

(3)

ζ /

g

/

g

C

r

=

A

- A

2

0

-23Γ

X

Repeat the calculation for the third a d d i t i o n : , n

g

/

g

(75 n g C r ) added C

r

JT=T

e

A

0

X

-

b

~2lT

( 4 ) A v e r a g e the t h r e e v a l u e s ( f r o m Steps 1, 2, a n d 3 ) a n d r e p o r t t h a t n u m b e r as n g / g i n the s a m p l e .


Analysis of Petroleum for Trace Metals

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