UGANDA STANDARD
 
       US ISO 6561-2
First Edition
2009-mm-dd
 
Reference numbe r
     US ISO 6561-2:2005
 © UNBS 2009
  
Fruits, vegetables and derived products — Determination of 
cadmium content — Part 2: Method using flame atomic 
absorption spectrometry 
 US ISO 6561-2: 2005 
 
© UNBS  2009 – All rights reserved ii Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS 2009 
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilised in any 
form or by any means, electronic or mechanical, in cluding photocopying and microfilm, without prior written 
permission from UNBS. 
Requests for permission to reproduce this document should be addressed to 
The Executive Director 
Uganda National Bureau of Standards 
P.O. Box 6329 
Kampala  
Uganda 
Tel: 256 41 505 995 
Fax: 256 41 286 123 
E-mail: unbs@infocom.co.ug
Web: www.unbs.go.ug  
      US ISO 6561-2: 2005  
© UNBS  2009 - All rights rese rved                iii National foreword 
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry  of Tourism, Trade and 
Industry established under Cap 327, of the Laws of U ganda. UNBS is mandated to co-ordinate the elaboration 
of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council. 
This Uganda Standard, US ISO 6561-2, Fruits, vegetables and derived products — Determination
of cadmium content — Part 2: Method using flame atomic absorption spectrometry,  is identical with and has
been reproduced from an International Standard, ISO 6561-2:2005, Fruits, vegetables and derived
products — Determination of cadmium content — Part 2: Method using flame atomic absorption 
spectrometry, and adopted as a Uganda Standard.  
This standard was developed under the subcommittee on fruits and vegetables by the Food and agriculture 
Standards Technical Committee (UNBS/TC 2). 
Wherever the words, "International Standard" a ppear, they should be replaced by "Uganda Standard." 
  
 
 
  
 Reference numbe r
ISO 6561-2:2005(E)
© ISO 2005
  
 INTERNATIONAL 
STANDARD  ISO
6561-2
First edition
2005-02-01
Fruits, vegetables and derived 
products — Determination of cadmium content — 
Part 2: 
Method using flame atomic absorption 
spectrometry 
Fruits, légumes et produits dérivés — Détermination de la teneur en 
cadmium — 
Partie 2: Méthode par spectrométrie d'absorption atomique avec 
flamme 
 ISO 6561-2:2005(E) 
PDF disclaimer 
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed b ut 
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editin g. In 
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Se cretariat 
accepts no liability in this area. 
Adobe is a trademark of Adobe Systems Incorporated. 
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-cr eation 
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bod ies. In 
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below. 
 
©   ISO 2005 
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by an y means, 
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address be low or 
ISO's member body in the country of the requester. 
ISO copyright office 
Case postale 56 • CH-1211 Geneva 20 
Tel.  + 41 22 749 01 11 
Fax  + 41 22 749 09 47 
E-mail  copyright@iso.org Web  www.iso.org 
Published in Switzerland 
 
ii © ISO 2005 – All rights reserved
  ISO 6561-2:2005(E) 
© ISO 2005 – All rights reserved  iii
 Foreword 
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies 
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO 
technical committees. Each member body interested in a subject for which a technical committee has been 
established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. 
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards 
adopted by the technical committees are circulated to the member bodies for voting. Publication as an 
International Standard requires approval by at least 75 % of the member bodies casting a vote. 
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent 
rights. ISO shall not be held responsible for identifying any or all such patent rights. 
ISO 6561-2 was prepared by Technical Committee ISO/TC 34, Food products , Subcommittee SC 3, Fruit and 
vegetable products . 
This first edition of ISO 6561-2, together with ISO 6561-1:2004, cancels and replaces ISO 6561:1983, which 
has been technically revised. 
ISO 6561 consists of the following parts, under the general title Fruits, vegetables and derived products — 
Determination of cadmium content : 
 Part 1: Method using graphite furnace atomic absorption spectrometry 
 Part 2: Method using flame atomic absorption spectrometry INTERNATIONAL STANDARD ISO 6561-2:2005(E)
 
© ISO 2005 – All rights reserved  1
 Fruits, vegetables and derived products — Determination of 
cadmium content — 
Part 2: 
Method using flame atomic absorption spectrometry 
1 Scope 
This part of ISO 6561 specifies an atomic absorption spectrometric method for the determination of the 
cadmium content of fruits, vegetables and derived products. 
NOTE  The method of cadmium determination in fruit, vegetables and derived products is based on AOAC Official 
Methods of Analysis [1]. 
2 Principle 
This method is based on the decomposition of organic matter with HNO3, H2SO4 and H2O2, extraction of 
cadmium by dithizone-CHCl3 at pH 9, and determination of cadmium by flame atomic absorption spectrometry. 
3 Reagents 
Use only reagents of recognized analytical grade, unless otherwise specified, and water which has been 
distilled twice in borosilicate glass apparatus, or water of at least equivalent purity. 
3.1 Nitric acid , concentrated ( ρ20 = 1,38 g/ml). 
3.2 Sulfuric acid , concentrated ( ρ20 = 1,84 g/ml). 
3.3 Hydrochloric acid , dilute, 0,2 mol/l. 
Place 16,5 ml of concentrated hydrochloric acid ( ρ20 = 1,19 g/ml) into a 1 000 ml one-mark volumetric flask 
and dilute to the mark with water. Mix. 
3.4 Hydrogen peroxide , concentrated (50 %). 
3.5 Citric acid monohydrate . 
3.6 Sodium hydroxide , 0,05 mol/l solution. 
3.7 Thymol blue indicator . 
Triturate 0,1 g of thymol blue indicator in an agate mortar with 4,3 ml of sodium hydroxide solution (3.6). Dilute 
to 200 ml in a flask with water. 
3.8 Ammonia solution , 28 % to 30 % solution. ISO 6561-2:2005(E) 
2 © ISO 2005 – All rights reserved
 3.9 Chloroform . 
3.10 Dithizone , 1,0 mg/ml solution. 
Place 0,2 g of dithizone in a 200 ml volumetric flask and add chloroform to the mark. 
3.11 Dithizone , 0,2 mg/ml solution. 
Dilute dithizone solution (3.10) 1 + 4 with chloroform (3.9). Prepare fresh daily. 
3.12 Cadmium standard solution , corresponding to a cadmium concentration of 1,0 mg/ml. 
4 Apparatus 
Before use, wash all glassware with 8 mol/l nitric acid, followed by a thorough rinse with water. 
Cover beakers with watch glasses during all operations. Usual laboratory apparatus and, in particular, the following. 
4.1 Mechanical grinder , the inside and the blades of which are coated with polytetrafluoroethylene (PTFE). 
4.2 Round-bottom flasks , of 1 500 ml capacity. 
4.3 Beaker , of 400 ml capacity. 
4.4 One-mark volumetric flasks , of capacities 50 ml, 100 ml and 1 000 ml. 
4.5 Pipettes , of appropriate capacity. 
4.6 Separators , 125 ml and 250 ml capacities. 
4.7 Atomic absorption spectrometer , provided with an air/acetylene burner (10 cm), suitable for 
measurements at a wavelength of 228,8 nm. 
4.8 Burner or heating mantle . 
4.9 Hot plate . 
4.10 Analytical balance . 
5 Sampling 
A representative sample should have been sent to the laboratory. It should not have been damaged or 
changed during transport or storage. 
Sampling is not part of the method specified in this part of ISO 6561. If there is no specific International 
Standard dealing with the product concerned, it is recommended that the parties concerned come to an 
agreement on the subject. ISO 6561-2:2005(E) 
© ISO 2005 – All rights reserved  3
 6 Procedure 
6.1 Preparation of test sample 
Mix well the laboratory sample, if necessary, first remove any seeds, stalks and hard seed-cavity walls, and 
then grind in the mechanical grinder (4.1). 
Frozen or deep-frozen products shall be previously thawed in a closed vessel, and the liquid formed during 
this process shall be added to the product before mixing. 
6.2 Test portion 
Weigh, to the nearest 0,01 g, 50 g of the test sample (6.1) into a 1 500 ml round-bottom flask (4.2). 
6.3 Decomposition 
If the test portion contains ethanol, first remove the ethanol by evaporation. 
Add several boiling chips or beads and carefully add 25 ml of nitric acid (3.1). Cover and warm gently using a 
burner or heating mantle (4.8) to initiate reaction. 
When reaction subsides, add 25 ml of nitric acid (3.1), warm again and continue until 100 ml of nitric acid (3.1) 
has been added. Alternatively, add carefully 100 ml of nitric acid (3.1) to the test portion at once, and allow it to stand at room temperature overnight. 
Heat until most NO fumes have evolved; control excessive frothing by cooling or quenching with water from a 
washbottle. 
Add 20 ml of concentrated sulfuric acid (3.2) to the solution. Dilute to approximately 300 ml with water and 
evaporate using a burner or heating mantle (4.8) until charring begins. When charring becomes extensive, cautiously add hydrogen peroxide (3.4), 1 ml at a time. Let the reaction subside before adding the next portion of oxidant, and never add more than 1 ml at a time. Continue the addition of hydrogen peroxide until the solution is colourless. 
Heat vigorously to SO
3 fumes, adding more hydrogen peroxide (3.4) as required, to remove char. Heat 
vigorously to expel excess of hydrogen peroxide. Cool colourless digest to room temperature. 
6.4 Blank test 
Carry out a blank test, using the same decomposition procedure as in 6.3, but replacing the test portion with a suitable amount of water corresponding to the amount of test portion taken for analysis (6.2). 
6.5 Extraction 
WARNING — The method described in this subclause requires the use of chloroform, a toxic and ozone-depleting substance. Avoid inhalation of and exposure to this solvent. Work in a fume cupboard when handling this solvent and solutions thereof. Dispose of the waste chloroform and solutions properly. 
Add 2 g of citric acid (3.5) to cooled digest of a sample or blank and cautiously dilute to about 25 ml with water. 
Add 1 ml of thymol blue indicator (3.7) and adjust to pH 8,8 by slowly adding ammonia solution (3.8), while cooling in ice bath, until the solution changes from yellowish green to greenish blue. Transfer quantitatively to a 250 ml separator (4.6) with water, and dilute to about 150 ml. 
Cool the solution, and extract with two 5-ml portions of concentrated dithizone solution (3.10), shaking 1 min 
to 2 min each time. Continue extraction with 5-ml portions of dilute dithizone solution (3.11) until the dithizone extract shows no change in colour. Combine the dithizone extracts in the 125 ml separator (4.6); wash with ISO 6561-2:2005(E) 
4 © ISO 2005 – All rights reserved
 50 ml water, and transfer solvent to another 125 ml separator. Extract wash-water with 5 ml of chloroform 
(3.9) and add to the dithizone extracts. 
Add 50 ml of dilute hydrochloric acid (3.3) to combined dithizone extracts, shake vigorously 1 min, and allow 
the layers to separate; discard the dithizone layer. Wash the aqueous solution with 5 ml of chloroform (3.9) and discard the chloroform. 
Quantitatively transfer the aqueous solution to a 400 ml beaker (4.3), add boiling chips, and evaporate 
carefully to dryness on the hot plate (4.9). 
Carefully rinse down sides of beaker with 10 ml to 20 ml of water and again evaporate to dryness. 
Dissolve the dry residue in 5,0 ml of hydrochloric acid solution (3.3). 
6.6 Determination 
6.6.1 Preparation of the calibration graph Dilute the cadmium standard solution (3.12) with hydrochloric acid (3.3) to obtain four solutions with cadmium 
concentrations of 0,1 µg/ml, 0,5 µg/ml, 1,0 µg/ml and 2,0 µg/ml. 
Aspirate each of these solutions, in turn, into the flame of the spectrometer (4.7) using 4 times to 10 times 
expansion scale to obtain the maximum absorbance value for the solution having the cadmium concentration 
of 2,0 µg/ml. Use hydrochloric acid (3.3) as a blank. 
Take care to keep the aspiration rate constant throughout the aspiration of each calibration solution. Flush burner with water after each measurement. 
Record the corresponding values of absorbance and plot the calibration graph (absorbance against cadmium 
concentration in micrograms per millilitre). 
6.6.2 Spectrometric measurement 
Set the instrument to the previously established optimum conditions, using air-acetylene oxidizing flame and 
228,8 nm resonant wavelength. 
Aspirate into the flame of the spectrometer (4.7) the test solution (6.5) and the blank solution (6.4), at the 
same aspiration rate as used for preparation of the calibration graph (6.6.1). Record the corresponding 
absorbances. 
If the absorbance of the test solution is greater than for the highest standard solution used for preparation of 
the calibration graph (6.6.1), dilute the test solution with the hydrochloric acid (3.3) and measure the 
absorbance. 
7 Calculation 
The cadmium content of the sample, expressed in milligrams per kilogram, is equal to 
Blank () 5ccFm−×× (1) 
where 
c is the cadmium concentration, expressed in micrograms per millilitre, of the test solution, as 
read from the calibration graph; 
cBlank is the cadmium concentration, expressed in micrograms per millilitre, of the blank test solution, 
as read from the calibration graph; ISO 6561-2:2005(E) 
© ISO 2005 – All rights reserved  5
 m is the mass, in grams, of the test portion; 
F is the dilution factor, if necessary. 
8 Precision 
The precision of the method has been checked by collaborative studies of cadmium determinations in lettuce 
and potatoes; see Reference [2]. 
Statistical parameters have been calculated in accordance with ISO 5725; see Reference [3]. 
Table 1 — Repeatability results 
Type of sample Cadmium content  
mg/kg r a 
mg/kg sr b 
mg/kg 
0,1 0,04 0,01 
0,5 0,17 0,06 Lettuce 
1,5 0,26 0,09 
0,05 0,009 0,003 
0,2 0,05 0,02 Potatoes 
1,0 0,09 0,03 
a r is the repeatability limit. 
b sr is the standard deviation of repeatability. 
Table 2 — Reproducibility results 
Type of sample Cadmium content 
mg/kg  R a 
mg/kg  sR b 
mg/kg  
0,1 0,05 0,02 
0,5 0,17 0,06 Lettuce 
1,5 0,29 0,10 
0,05 0,025 0,09 
0,2 0,07 0,02 Potatoes 
1,0 0,19 0,07 
a R is the reproducibility limit. 
b sR is the standard deviation of reproducibility. 
9 Test report 
The test report shall specify: 
a) all information necessary for the complete identification of the sample; b) the sampling method used, if known; 
c) the test method used, with reference to this part of ISO 6561; 
d) all operating details not specified in this part of ISO 6561, or regarded as optional, together with details of 
any incidents which may have influenced the test result(s); 
e) the test result(s) obtained; or, if the repeatability has been checked, the final quoted result obtained. ISO 6561-2:2005(E) 
6 © ISO 2005 – All rights reserved
  
Bibliography 
[1] AOAC Official Methods of Analysis (1995). AOAC 973.34, Cadmium in Food  
[2] Gajan R.J., Gould J.H., Watts J.O., Fiorino J.A. Collaborative study of a method for the atomic 
absorption spectrophotometric and polarographic determination of cadmium in food. J. Assoc. Off. 
Anal. Chem. , 56, No.  4, 1973, pp. 876-881 
[3] ISO 5725:1986, Precision of test methods — Determination of repeatability and reproducibility for a 
standard test method by interlaboratory tests [now withdrawn] 
[4] ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: 
General principles and definitions  
[5] ISO 5725-2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic 
method for the determination of repeatability and reproducibility of a standard measurement method  ISO 6561-2:2005(E) 
ICS  67.080.01 
Price based on 6 pages 
© ISO 2005 – All rights reserved