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Estimation of Residual Nickel and Some Heavy Metals in Vanaspati Ghee

Hizbullah Khan,a

Mohammad Fida, b

Imdad Ullah Mohammadzaic

* and Mumtaz Khand

a Department of Environmental Sciences, University of Peshawar, Pakistan b NCE in Physical Chemistry, University of Peshawar, Pakistan

c Institute of Chemical Sciences, University of Peshawar, 25120, Peshawar, Pakistand PCSIR Laboratories Complex Peshawar, Pakistan

To convert vegetable edible oils into vanaspati ghee, nickel is used as a catalyst in the hydrogenation

process. A simple and fast method for the trace level determination of nickel in ghee is reported. In this

work different methods were applied for the extraction of residual nickel from ghee samples. Using tolu-

ene, benzene and carbon tetrachloride as organic solvents, an acid mixture was used for the extraction of

nickel. Extracted nickel was quantified with atomic absorption and colorimetric methods. Among the or-

ganic solvents, toluene proved to be the best solvent mediating a 95% extraction of nickel from ghee sam-

ples. Nickel was extracted and determined in ten different brands of ghee and in all samples its amount waswell above the permissible limit of WHO (0.2 mg/g). Other metals like lead, zinc, copper, and cadmium

were also determined and their concentrations were found to be much below the WHO permissible limits.

Keywords: Hydrogenated edible oil; Nickel formate catalyst; Solvent extraction; Spectrophoto-

metric quantification.

INTRODUCTION

The human body uses oils and fats in the diet for three

purposes, as an energy source, as a structural componentand to make powerful biological regulators. Oils and fats

also play an important role in metabolic reactions in the hu-

man body. Oils and fats contain fatty acids, which are sus-

ceptible to attack by a number of agents e.g. light, oxygen,

metals, etc.1 Bad flavor known as rancidity is produced by

the oxidation of fats. Catalytic hydrogenation and/or use of

antioxidants can suppress or eliminate rancidity. Fats and

oils consist of three fatty acids chemically combined with a

molecule of glycerol to form triglyceride. All vegetable ed-

ible oils mostly have polyunsaturated and monounsatu-

rated fatty acids, while fats consist mostly of saturated fattyacids. Catalytic hydrogenation of vegetable oils into ghee

increases the level of saturation in the hydrocarbon chain

and a corresponding increase in the melting point of the

product is generally observed. Nickel, supported on an in-

ert carrier, is widely used as catalyst in this process and is

removed from the ghee after the hydrogenation process is

completed. Hydrogenated oils are generally contaminated

with catalyst due to faulty filtration or intentional non-

removal of catalyst to lower the cost of production.

This paper deals with a more rapid, economical and

efficient method for the extraction of nickel from ghee.

Several methods are available for the extraction of nickel

from the hydrogenated product.2,3

These extraction pro-

cesses have not yet produced the desired results, are cum- bersome, time consuming and non-efficient for maximum

extraction. This often leads to the wrong conclusion about

the quality of ghee.

In this work, several organic solvents were tested for

dissolution, and improvements in the existing methods

were checked and applied. Besides nickel, other metals

were also determined using AAS and spectrophotometric

methods.

Environmental Levels and Human Exposure

Nickel levels in terrestrial and aquatic organisms canvary over several orders of magnitude. Typical atmospheric

nickel levels for human exposure range from about 5 to 35

ng/m3

at rural and urban sites, leading to a nickel uptake via

inhalation of 0.1-0.7 mg/day. Nickel in the drinking-water

is generally less than 10 mg/L, but occasionally may be re-

leased from the plumbing fittings, resulting in concentra-

tions of up to 500 mg/litre.4

Nickel concentrations in food are usually below 0.5

mg/kg fresh weight. Daily intake of nickel from food will

vary widely because of different dietary habits and can

Journal of the Chinese Chemical Society, 2007, 54, 737-741 737



range from 100 to 800 mg/day; the mean dietary nickel in-

take in most countries is 100-300mg/day. Release of nickel

from kitchen utensils may contribute significantly to oral

intake. Smoking 40 cigarettes a day can result in pulmo-

nary intake of nickel in the range of 2-23 mg/day.5

Gastrointestinal absorption of nickel is variable and

depends on the composition of the diet. In a recent study on

human volunteers, absorption of nickel was 27% from wa-

ter compared with less than 1% from food. All body secre-

tions are potential routes of excretion including urine, bile,

sweat, tears, milk, and mucociliary fluid. Non-absorbed

nickel is eliminated in the feaces. Transplacental transfer

has been demonstrated in rodents. Following prenatal ad-

ministration of nickel salts, the highest nickel accumula-

tion occurs in the kidney, endocrine glands, lung, and liver:

high concentrations are also observed in the brain follow-

ing administration of nickel carbonyl. Data on nickel ex-

cretion suggest a two-compartment model. Nickel concen-

trations in the serum and urine of healthy non-occupation-

ally exposed adults are; 0.2 mg/L (range: 0.05-1.1 mg/L)

and l.5 mg/g creatinine (range: 0.5-4.0 mg/g creatinine), re-

spectively. Increased concentration of nickel is seen in both

of these fluids following occupational exposure. The body

burden of nickel in a non-exposed 70 kg adult is 5 mg.4-6

EXPERIMENTAL

Chemicals

All chemicals and reagents used in the present studies

were of extra pure grade and were used without any further

purification. Nickel catalyst, as nickel format, was ob-

tained from the local market.

Apparatus

A Perkin Elmer 3100 Model spectrophotometer was

used for atomic absorption (AA) measurements. All hollow

cathode lamps used were manufactured by Perkin Elmer.

Spectral measurements of the nickel-dimethylglyoxamate

complex were recorded on a Shimadzu 1600 UV-Vis spec-

trophotometer at 525 nm.

Sampling

Different brands of ghee samples were collected from

the local market at different time intervals with a gap of at

least two months. As such three different lots were col-

lected in six months.

Extraction Procedure

A modified version of the reported method2 was used

for the extraction of nickel from ghee samples, whereas 30

g of ghee was heated to melt and then dissolved in 15 mL of

toluene followed by the addition of 20 mL of acid mixture

(20% HNO3 +20% H2SO4). The mixture was shaken vigor -

ously forten minutes in a separating funnel. After the equil-

ibration of the mixture, the acid layer was separated and

collected in a china dish. To remove any un-extracted resid-

ual nickel, an acid mixture (10 mL) was added to the or-

ganic layer in the separating funnel, was shaken for 5 min-

utes and the acid layer was transferred and added to the pre-

viously collected extract. The acid extract was directly ana-

lyzed with AAS.

For colorimetric determination of nickel, the acid ex-

tract was slowly evaporated to dryness in a fume hood. The

china dish containing the residue was heated on an oxidiz-

ing flame. After cooling, to the oxidized residue was added

1% ammonium dimethylglyoxime (DMG) solution that

produced a rose colored precipitate of Ni-dimethylglyoxa-

mate complex. After filtration the precipitate was washed

several times with de-ionized water and was dissolved in

chloroform (10 mL) that produced a yellow colored solu-tion.

7The absorbance of the final solution was measured at

525 nm.

RESULTS AND DISCUSSION

Solvent Extraction of Nickel

After the conversion of vegetable edible oil into ghee,

the spongy catalyst is separated from the product. How-

ever, due to overuse of the catalyst and poor filtration

equipment, fine particles of the catalyst are retained in the

product as nickel formate. In the solid state, nickel formate

typically exists in the form of a dihydrate, Ni(HCOO)2‚

2H2O. Thermal gravimetric analysis (TGA) indicates that

the dihydrate loses crystallized water at a temperature of

approx. 160 °C and thermally decomposes between 230 °C

and 260 °C, which is consistent with reports in the litera-

ture.8

On dissolution of melted ghee in organic solvent and

then shaking it with aqueous acidic mixture, nickel formate

is transferred from an organic into an aqueous medium and

is analyzed.

738 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Khan et al.



Extraction Procedure

The extraction of nickel was carried out with several

methods for comparison. The method developed by Price3

was used as a standard method in this regard. 30 g of melted

ghee sample was dissolved in 30 mL of CCl4 followed by

addition of 30 mL HNO3. The mixture was then vigorously

shaken in a separating funnel for 1 hour. Then 50 mLof dis-

tilled water was added to it and it was again shaken for 30

min and was allowed to stand. After equilibration of the

two layers the inorganic layer was separated and analyzed

for nickel (Table 1). Benzene and carbon tetrachloride were

also used for comparison purposes.9

Choice of Organic Solvent

The original method of Price used carbon tetrachlo-

ride as organic solvent,3

while Souliotis substituted ben-

zene.9

In this regard, using different acidic mixtures, the

extraction of nickel was checked using organic solvents

such as toluene, carbon tetrachloride and benzene.2

Keep-

ing in view our past experience, toluene was used as or-

ganic solvent in the present work.

Percent Recovery of Nickel

Table 1 represents the percent recovery of nickel con-

tents in animal fat by various solvents. To a 30 g sample of

pure animal fat, various amounts of nickel catalysts were

added for the percent recovery of nickel by different meth-

ods.6,9

Each sample was heated up to 200 °C so that the

nickel catalyst may be thermally decomposed to metallic

nickel. The extraction of nickel was carried out by different

methods. Each percent recovery data is an average sum of

ten different determinations. For different measurements

the percent recovery values lies between 65-95%. From the

comparison of different methods, for the % recovery of

nickel, it is clear that Method-I is more efficient and rapid.

Table 2 presents the data obtained from colorimetric

and atomic absorption analyses in three different brands of

hydrogenated vegetable oil. These results are obtained from

the average of three parallel determinations. It is clear from

the results that maximum extraction of residual nickel was

obtained with method-I, i.e. dissolution in toluene followed

by 20% H2SO4 + 20% HNO3, in a very short time. Further

from the table it is also clear that there is not much differ-

ence in the results of the colorimetric as well as atomic ab-

sorption measurements for the determination of extracted

nickel. Some of the samples were also treated without us-

ing any solvents but the extraction results were not appre-

ciable. In this regard the samples were melted above the

melting point of solid ghee samples and were later on shacked

with 20% H2SO4 + 20% HNO3 in a separating funnel. The

Nickel and Heavy Metals in Hydrogenated Edible Oil J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 739

Table 1. Spectrophotometric data showing the % recovery of nickel from animal fats using different extraction

methods

Extraction

MethodSolvent Acid/Acid mixture

Recovery

time/min% Recovery

Efficiency/

(% recovery/min)

Method-I Toluene 20% H2SO4 + 20% HNO3 15 95.27 ± 0.54 6.35

Method-II Carbon Tetra Chloride 20% HNO3 85 89.94 ± 0.38 1.01

Method-III Carbon Tetra Chloride 20% H2SO4 + 20% HNO3 25 71.18 ± 0.62 2.85

Method-IV Benzene 20% H2SO4 + 20% HNO3 20 65.82 ± 0.98 3.29

Table 2. Nickel (ppb) extracted by various methods in three different ghee samples

Extraction Method Technique Sample 1 Sample 2 Sample 3

AAS 0.279 ± 0.014 0.301 ± 0.011 0.369 ± 0.018Method-I

UV-VIS 0.201 ± 0.020 0.261 ± 0.024 0.341 ± 0.015

AAS 0.194 ± 0.018 0.224 ± 0.013 0.290 ± 0.014Method-II

UV-VIS 0.123 ± 0.021 0.200 ± 0.019 0.250 ± 0.023

AAS 0.108 ± 0.017 0.168 ± 0.015 0.201 ± 0.011Method-III

UV-VIS 0.094 ± 0.014 0.147 ± 0.016 0.191 ± 0.018

AAS 0.082 ± 0.018 0.102 ± 0.014 0.142 ± 0.017Method-IV

UV-VIS 0.072 ± 0.015 0.091 ± 0.019 0.112 ± 0.019



rest of the procedure was followed in the same manner aswas used when toluene is applied as solvent.

Table 3 represents data of other heavy metals like cal-

cium (Ca), magnesium (Mg), copper (Cu), and iron (Fe).

The Ca content ranges from 1.1-1.8 ppm and Mg Content

ranges from 0.3-0.8 ppm. The Cu content varies from

0.00-0.02 ppm and its permissible range is below 0.02

ppm.12

The Fe content varies from 0.01-0.04 ppm, whereas

its permissible limit is 0.1 ppm.12

The extraction of nickel by different methods in this

work indicates that toluene is an efficient solvent that pro-

vides better recovery than do benzene and carbon tetrachlo-ride. The better efficiency of toluene can also be correlated

with the polar nature of toluene as compared to benzene

and carbon tetrachloride and also its complete immisci-

bility with water. Benzene and carbon tetrachloride are

slightly miscible, i.e. 0.008 and 0.17 parts/100, respec-

tively.9-11

It is also clear from the results of different extrac-

tion methods that a 20% acid mixture of both H2SO4 and

HNO3 is more effective than concentrated acids for the

same purpose. Residual nickel in hydrogenated vegetable

oils has also been investigated by other workers but all

these methods are tedious, time consuming and are not effi-

cient in extracting the maximum amount of residual nickel,

e.g. method of Price et al.3

requires treatment of sample

with HNO3 and CCl4 in a separating funnel for ninety min-

utes. Similarly the method of the Association of Official

Analytical Chemists10 produces extensive fumes and is dif-

ficult to be used for hydrogenated edible oils. Thus the

method used for the extraction of nickel in this work repre-

sents the best compromise currently available for reliable,

economical and efficient extraction of residual nickel in

ghee samples.

Table 4 presents the data of ten different brands of ghee. For each brand three samples were collected at an in-

terval of two months. The data presented in this table is an

average of three parallel determinations for the same sam-

ple. As is clear from the table in the analysis of lot No. 1,

the residual nickel content is well above the WHO permis-

sible limit for nickel, i.e. 0.2 ppb for ingestible foods.6

Af-

ter completing results for lot No. 1, the respective indus-

tries were informed about the observed level of nickel in

their products and remedies like replacement of catalyst

material and filtration cloth after a proper time, etc. were

suggested. The response of the industries was cooperativein this regard. They took action to rectify the problem and

afterwards there was an appreciable decrease in the resid-

ual contents of nickel in their samples. In each lot the varia-

tions in the concentration of residual nickel is random;

however, the decrease in the level of residual nickel in

some samples is encouraging.

CONCLUSION

The extraction of nickel by different methods in this

work indicates that toluene is an efficient solvent that pro-

vides better recovery than do benzene and carbon tetra

chloride. Atomic absorption spectrophotometric and color-

imetric methods proved to be quite sensitive for nickel de-

termination and both have yielded almost the same results.

This method is recommended for the oil and fat industries

for quality control and better estimation of residual nickel

in their products. The analysis proved to be a better way for

correcting residual nickel content in the products as is clear

from the cumulative results. However, a high level of nickel

740 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Khan et al.

Table 3. Concentration (ppm) of other heavy metals in different

brands of ghee samples applying the AAS method

Sample # Ca Mg Cu Fe

1 1.5 0.5 0.01 0.03

2 1.3 0.3 0.00 0.02

3 1.1 0.4 0.02 0.01

4 1.6 0.5 0.00 0.02

5 1.3 0.8 0.01 0.01

6 1.2 0.3 0.01 0.03

7 1.8 0.2 0.01 0.04

8 1.1 0.6 0.01 0.02

9 1.3 0.5 0.01 0.02

10 1.2 0.4 0.00 0.01

Table 4. AAS data for the concentration of nickel (ppb) in

various lots of the same brands

Sample No. Lot#1 Lot#2 Lot#3

1 0.385 ± 0.016 0.302 ± 0.014 0.213 ± 0.015

2 0.380 ± 0.011 0.293 ± 0.012 0.208 ± 0.017

3 0.489 ± 0.017 0.356 ± 0.016 0.194 ± 0.013

4 0.304 ± 0.015 0.276 ± 0.015 0.204 ± 0.016

5 0.489 ± 0.016 0.410 ± 0.013 0.289 ± 0.019

6 0.523 ± 0.019 0.373 ± 0.017 0.202 ± 0.014

7 0.588 ± 0.015 0.412 ± 0.011 0.410 ± 0.014

8 0.879 ± 0.018 0.522 ± 0.018 0.297 ± 0.016

9 0.998 ± 0.016 0.501 ± 0.014 0.309 ± 0.015

10 1.080 ± 0.021 0.659 ± 0.016 0.248 ± 0.017



present in consumer products is undoubtedly a health haz-

ard and strong efforts are required for the strict implemen-

tation of quality control regulations.

Received July 28, 2006.

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Nickel and Heavy Metals in Hydrogenated Edible Oil J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 741

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