UGANDA STANDARD
 
       US ISO 6636-2
First Edition
2009-mm-dd
 
Reference numbe r
    US ISO 6636-2:1981
 © UNBS 2009
  
Fruits, vegetables and derived products — Determination of 
zinc content — Part 2 : Atomic absorption spectrometric 
method 
 US ISO 6636-2: 1981 
 
© 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 6636-2: 1981  
© 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 6636-2, Fruits, vegetables and derived products — Determination
of zinc content — Part 2:  Atomic absorption spectrometric method,  is identical with and has been
reproduced from an International Standard, ISO 6636-2:1981, Fruits, vegetables and derived products —
Determination of zinc content — Part 2:  Atomic absorption spectrometric method, 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." 
 International Standard 
INTERNATIONAL ORGANIZATION FÜR STANDARD~ZAT~ON~MEXLIYHAPOAHAR opr~Ht43~ut4~ no cTAH~APT~~A~MM*~RGANI~ATION INTERNATIONALE DE K~~RMALI~ATION 
Fruits, vegetables and derived products - Determination 
of zinc content - 
Part 2 : Atomic absorption spectrometric methode 
Fruits, lhgumes et produits dkrivhs - Dktermina tion de Ia teneur en zinc - Partie 2 : Mthode par spectrombtrie d’absorption 
a tomique 
First edition - 1981-10-15 
UDC 634/635 : 543.42 : 546.47 Ref. No. ISO 6636/2-1981 (E) 
Descriptors : agricultural products, food products, fruits, vegetables, tests, determination of content, zinc, atomic absorption spectroscopic 
analysis. 
Price based on 4 pages Foreword 
ISO (the International Organitation for Standardization) is a worldwide federation of 
national Standards institutes (ISO member bodies). The work of developing Inter- 
national Standards is carried out through ISO technical committees. Every member 
body interested in a subject for which a technical comm’ittee has been set up 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. 
Draft International Standards adopted by the technical committees are circulated to 
the member bodies for approval before their acceptance as International Standards by 
the ISO Council. 
International Standard ISO 6636/2 was developed by Technical Committee 
ISO/TC 34, Agricultural food products, and was circulated to the member bodies in 
December 1979. 
lt has been approved by the member bodies of the fotlowing countries : 
Australia 
Austria 
Brazil 
Bulgaria 
Canada 
Czechoslovakia 
Ethiopia 
France 
Germany, F. R. Hungary 
India 
Ireland 
Israel 
Italy 
Korea, Rep. of 
Netherlands 
New Zealand 
Poland 
No member body expressed disapproval of the document. 
0 International Organkation for Standardkation, 1981 Portugal 
Romania 
South Africa, Rep. of 
Spain 
Thailand 
Turkey 
USSR 
Y ugoslavia 
Printed in Switzerland INTERNATIONAL STANDARD ISO 6636/2-1981 (EI 
Fruits, vegetables and derived products - Determination 
of zinc content - 
Part 2 : Atomic absorption spectrometric method 
1 Scope and field of application 
This patt of ISO 6636 specifies an atomic absorption spec- 
trometric method for the determination of the zinc content of 
fruits, vegetables and derived products. 
2 References 
ISO 5515, Fruits, vegetables and derived products - Decom- 
Position of organic matter Prior to analysis - Wet method. 
ISO 5516, Fruits, vegetables and derived products - Decom- 
Position of organic matter Prior to analysis - Ashing method. 
3 Principle 
Decomposition of organic matter by the dry or wet method and 
determination of the Zn2+ cation by atomic absorption spec- 
trometry. 
NOTE - In the case of decomposition by the dry method, dissolution 
of the ash in hydrochloric acid allows transformation of all mineral salts 
into easily dissociable chlorides. 
For some liquid samples (such as wines, clear fruit juices free 
from pulp), the determination may be carried out directly, 
without previous decomposition. 
4 Reagents 
All reagents shall be of recognized analytical purity, and shall 
be especially free of zinc. The water used shall have been dis- 
tilled twice in an apparatus of borosilicate glass, or shall be 
water of at least equivalent purity. 
4.1 Nitrit acid, = 1,38 g/ml. ,920 
4.2 Sulphuric acid, = 1,84 g/ml. e20 
4.3 Hydrochlorit acid, 1 + 1 (by volume) solution. 
Mix one volume of concentrated hydrochloric acid 
(@20 = IJ9 g/ml) with one volume of water. 4.4 Hydrochlorit acid, approximately 3,7 g/l Solution. 
In a 1 000 ml one-mark volumetric flask, dilute 8,3 ml of con- 
centrated hydrochloric acid (~20 = 1,19 g/ml) to the mark with 
water and mix. 
4.5 Zinc, Standard Solution corresponding to 1 g of zinc per 
litre. 
In a conical flask, dissolve 1 g of pure zinc in 10 ml of the 
hydrochloric acid Solution (4.3). Transfer quantitatively to a 
1 000 ml one-mark volumetric flask, dilute to the mark with 
water, and mix. 
Store the Solution in a bottle of borosilicate glass fitted with a 
ground glass stopper. 
5 Apparatus 
Usual laboratory equipment not otherwise specified, and the 
following : 
5.1 Mechanical grinder, the inside and blades of which are 
coated with polyethylene. 
5.2 Platinum or quartz dishes, of diameter 70 mm, or 
Kjeldahl flasks, of capacity 250 ml. 
5.3 One-mark volumetric flasks, of capacity 50 ml. 
5.4 Volumetric pipettes, of appropriate capacities. 
5.5 Centrifuge. 
5.6 Boiling water bath. 
5.7 Heating device. 
5.8 Electrically heated muffle furnace, capable of being 
controlled at 525 Zl: 25 OC. 
5.9 Electrically heated oven, capable of being controlled at 
less than 100 OC and at 525 + 25 OC, preferably programmable 
between 20 and 550 OC. 
1 ISO 6636/2-1981 (El 
5.10 Atomic absorption spectrometer, fitted with a to 2 ml of the hydrochloric acid Solution (4.3). Transfer the con- 
burner for an air-acetylene mixture fed at a pre-determined flow tents of the dish quantitatively to a centrifuge tube (5.5), rins- 
ing the dish with about 20 ml of the hydrochloric acid solution rate (in general 4 ml/min) corresponding to the specified op- 
timum aspiration rate, suitable for measurements at a (4.4), centrifuge, and transfer the supernatant liquid to a 50 ml 
wavelength of 213,8 nm. volumetric flask (5.3). Add a further 10 ml of the hydrochloric 
acid Solution (4.4) to the contents of the centrifuge tube, cen- 
5. ‘ll Analytical balance. trifuge, and transfer the supernatant liquid to the same flask. 
Repeat this procedure using 10 ml of water and make up the 
volume in the volumetric flask,to the mark with water. Mix the 
6 Procedure 
6.3.2 Decomposition by the wet method 
6.1 Preparation of the test Sample Introduce the test Portion (6.2) into a Kjeldahl flask (5.2). If the 
Mix the laboratory Sample weil. If necessary, first remove 
seeds and hard seed-cavity Walls and pass through the 
mechanical grinder (5.1). 
Allow frozen or deep-frozen products to thaw in a closed 
vessel, and add the liquid formed during this process to the 
product before homogenization. test Portion (6.2.1) contains ethanol, eliminate it beforehand by 
boiling, and allow to cool. Add 10 ml of the nitric acid (4.11, 
heat, then carefully add 5 ml of the sulphuric acid (4.2). 
In some cases it may be useful to effect a preliminary digestion, 
by leaving the mixture in contact in the flask for a period (over- 
night for example). 
Place the flask containing the mixture on the heating device 
6.2 Test Portion (5.7) and heat cautiously to avoid excessive frothing. 
Liquid products and begin again only 
6.2.4 If necessary, interrupt heating 
vigorous frothing has ceased. when 
Take 10 ml of the test Sample (6.1) by means of a pipette (5.4). 
In the case of viscous liquids or liquids containing suspended 
solid particles, weigh the test Portion (see 6.2.2). As soon as possible, bring the liquid to the boil and continue 
boiling until it begins to turn brown. Then add, drop by drop, 1 
to 2 ml portions of the nitric acid (4.1). 
6.2.2 Pasty, solid or dried products 
Weigh, to the nearest 0,Ol g, 5 to 10 g of the test Sample (6.11, 
expressed as fresh product, according to .the nature of the pro- 
duct. 
6.3 Decomposition 
Decomposition may be carried out by the dry or wet method. 
6.3.1 Decomposition by the dry method 
Introduce the test Portion (6.2) into one of the dishes (5.2) 
and place it on the boiling water bath (5.61, regulating the 
temperature’ of the bath so as to minimize the risk of loss of 
material by spattering. Evaporate to dryness. Continue the 
decomposition in the muffle furnace (5.81, controlled 
at 525 + 25 OC. 
NOTE - If possible, it is preferable to avoid evaporation on a boiling 
water bath and to place the dish directly in the electric oven (5.9) which 
is temperature programmable from 20 to 525 31 25 OC, in progressive 
Steps, to avoid spattering of the test Portion during drying. 
Dissolve the ashes in a few drops of the nitric acid (4.1), 
evaporate on the boiling water bath (5.6), then transfer to the 
oven (5.9) controlled at 525 + 25 OC (or controlled at first at a 
temperature less than 100 OC and then at 525 + 25 “CL 
Transfer to the muffle furnace (5.8), controlled at 525 * 25 OC, 
and leave until white ashes are obtained. Dissolve the ashes in 1 Bring to the boil after every addition, but avoid vigorous 
heating. A small amount of nitric acid shall always remain in the 
mixture, as indicated by the presence of nitrous vapours. 
Cease addition of portions of nitric acid when the Solution no 
longer turns brown on addition of the acid. Continue heating 
until white fumes appear, indicating a high concentration of 
sulphuric acid and a reduction in nitric acid. If the Solution turns 
brown again, continue the addition of nitric acid and repeat the 
operations described above until browning ceases. 
Allow the solution to cool. The absence of colour or the 
presence of a light green or yellow colour indicates that the 
digestion is complete. 
When decomposition is terminated, dilute the sulphuric solu- 
tion with a few millilitres of water. Transfer the contents of 
flask quantitatively to a centrifuge tube (5.5), rinsing the flask 
with about 10 ml of water and collecting the rinsing water in 
the centrifuge tube. Centrifuge and transfer the supernatant 
liquid to a 50 ml volumetric flask (5.3). Add a further 10 ml of 
water to the contents of the centrifuge tube, centrifuge and 
transfer the supernatant liquid to the same flask. Repeat this 
procedure with another 10 ml of water and make up the volume 
in the volumetric flask to the mark with water. Mix the solution. 
6.3.3 Blank test 
Carry out a blank test, using the same conditions for decom- 
Position (6.3.1 or 6.3.2 as appropriate), but replacing the test 
Portion (6.2) by 10 ml of water. 
2 ISO 6636/2-1981 (E). 
6.4 Determinationl) 6.4.2.2, Spectrometric .measurements ’ 
Aspirate the test Solution (6.3.2) and the blank test Solution 
(6.3.3) into the flame of the spectrometer (5.10) at the same 
rate as in 6.4.2.1. Record the corresponding absorbances.3) 6.4.1 Sample decomposed by the dry method 
6.4.1 .l Preparation of the calibration graph 
The absorbance of the blank test Solution shall be less than or 
equal to 0,002. Dilute the Standard zinc Solution (4.5) with the hydrochloric 
acid Solution (4.4) to obtain four solutions containing 0,25 - 
0,5 - 1 and 1,5 mg of zinc per litre. 
7 Expression of results Aspirate each of these solutions, in turn, into the flame of the 
spectrometer (5.10), at the rate such that the maximum absor- 
bance is obtained for the Solution having a zinc content of 
1,5 mg/l. Record the corresponding values of absorbance and 
draw the calibration graph. 7.1 Method of calculation and formulae 
7.1 .l Liquid products 
The zinc content, expressed 
given by the formula in milligrams per litre of product , is 
6.4.1.2 Spectrometric measurements 
(C 1- c2) x 5 Aspirate the test Solution obtained (6.3.1) and the blank test 
Solution (6.3.3) into the flame of the spectrometer (5.10) at the 
same rate as in 6.4.1.1. Record the corresponding absor- 
bances.2) where 
cl is the zinc content of the Sample, 
read from the calibration graph;b) in milligrams per litre, 
The absorbance of the blank test Solution shall be less than or 
equal to 0,002. c2 is the zinc content of the blank test Solution 
milligrams per litre, read from the calibration graph. , in 
7.1.2 Viscous or 
or dried products mogeneous liquids, pasty, solid 6.4.2 Sample decomposed by the wet method 
6.4.2.1 Preparation of the calibration graph The zinc content, expressed 
duct, is given by the formula in milligrams Per kilogram sf pro- 
Dilute the Standard zinc Solution (4.51 with water to obtain four 
solutions containing 2,5 - 5 - 10 and 15 mg of zinc per litre. 
(Cl - 9) x 50 
r-n Into a series of four 50 ml volumetric flasks (5.31, place 5 ml of 
each of these solutions. Add 30 to 35 ml of water, and then 
5 ml of the sulphuric acid (4.2). Mix, allow to cool and dilute to 
the mark with water. Mix. These solutions contain respectively 
0,25 - 0,5 - 1 and 1,5 mg of zinc per litre. c1 is the zinc content of the Sample 
read from the calibration graph;a) , in milligrams per litre, 
Aspirate each of these solutions, in turn, into the flame of the 
spectrometer (5.101, at the rate such that the maximum absor- 
bance is obtained for the Solution having a zinc content of 
1,5 mg/l. Record the corresponding values of absorbance and 
draw the calibration graph. c2 is the zinc content of the blank test Solution, 
milligrams per litre, read from the calibration graph; in 
m is the mass, in grams, of the test Portion. 
1) lt is also possible to use solutions 
the presence of zinc in the reagents. obtained using the procedures specified in ISO 5515 or ISO 5516, provided that precautions are to avoid 
2) If the absorbance of the test Solution exceeds that of the 
suitably diluted with the hYd rochloric acid Solution (4.4). most concentrated calibration Solution, measure the absorbance of the test solution 
3) If the absorbance of the test Solution exceeds 
diluted with 10 % ( V/ V sulphuric acid Solution. most concentrated calibration Solution, measure the absorbance of the test Solution suitably 
4) If the test Solution was diluted, use the appropriate dilution factor in the calculation. 
3 ISO 6636/2-1981 (EI 
If it is desired to express the zinc content relative to the dry pro- 
duct, take the moisture content of the Sample into account in 
the calculation. 
7.2 Repeatability 
The differente between the results of two determinations 
carried out simultaneously or in rapid succession by the same 
analyst, on the same Sample, shall not exceed 10 % (relative). 8 Test report 
The test report shall show the method used and the result ob- 
tained, indicating clearly the method of expression used. It shall 
also mention all operating details not specified in this Interna- 
tional Standard or regarded as optional, as well as any incidents 
likely to have affected the results. 
The test report shall give all the information necessary for the 
complete identification of the Sample. This page intentionally lef? blank This page intentionally left blank