UGANDA STANDARD
 
          US ISO 6636-1
First Edition
2009-mm-dd
 
Reference numbe r  
     US ISO 6636-1:1986
 © UNBS 2009
  
Fruits, vegetables and derived products — Determination of 
zinc content — Part 1: Polarographic method 
 US ISO 6636-1: 1986 
 
© 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 
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The Executive Director 
Uganda National Bureau of Standards 
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Uganda 
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E-mail: unbs@infocom.co.ug
Web: www.unbs.go.ug  
      US ISO 6636-1: 1986  
© 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 6636-1, Fruits, vegetables and derived products — Determination 
of zinc content — Part 1: Polarographic method,  is identical with and has been reproduced from an
International Standard, ISO 6636-1:1986, Fruits, vegetables and derived products — Determination of zinc 
content — Part 1 : Polarographic 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." 
 G3 
c CD 
8 CD 
2 - international Standard 6636/ 1 
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION*ME)I(AYHAPOAHAR OPTAHUl3AL&lR IlO CTAHAAPTt43AUt4M~ORGANISATION INTERNATIONALE DE N~RMALISATION 
Fruits, vegetables and derived products - Determination 
of zinc content - 
Part 1 I Polarographic method 
Fruits, lhgumes et produits d&iv& - Dktermination de la teneur en zinc - Partie 7 : Mkthode polarographique 
First edition - 1986-04-15 
UDC 635.11.6 : 543.253 : 546.47 Ref. No. IS0 6636/l-1986 (E) 
Descriptors : agricultural products, fruits, vegetables, fruit and vegetable products, chemical analysis, determination of content, zinc, 
polarographic methods. 
Price based on 3 pages Foreword 
IS0 (the International Organization for Standardization) is a worldwide federation of 
national standards bodies (IS0 member bodies). The work of preparing International 
Standards is normally carried out through IS0 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, govern- 
mental 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 IS0 Council. They are approved in accordance with IS0 procedures requiring at 
least 75 % approval by the member bodies voting. 
International Standard IS0 
Agricultural food products. 663611 was prepared bY Technical Committee ISO/TC 34, 
Users should note that all International Standards undergo revision from time to time 
and that any reference made herein to any other International Standard implies its 
latest edition, unless otherwise stated. 
0 International Organization for Standardization, 1986 
Printed in Switzerland INTERNATIONAL STANDARD IS0 6636/l-1986 (E) 
Fruits, vegetables and derived products - Determination 
of zinc content - 
Part 1 : Polarographic method 
1 Scope and field of application 
This part of IS0 6636 specifies a polarographic method for the 
determination of the zinc content of fruits, vegetables and 
derived products. 
NOTE - IS0 6636/2 specifies an atomic absorption spectrometric 
method, and IS0 6636/3 a dithizone spectrometric method. 
Copper, tin, lead and cadmium do not interfere with the deter- 
mination. 
2 Reference 
IS0 874, Fresh fruits and vegetables - Sampling. 
3 Principle 
Ashing of a test portion in a muffle furnace at 525 rfi 25 OC. 
Treatment of the ash obtained with hydrochloric acid. 
Neutralization with 25 % (m/m) ammonia solution and deter- 
mination of the zinc content by polarography using an 
ammonia/ammonium chloride electrolyte. 
4 Reagents 
All reagents shall be of recognized analytical grade and the 
water used shall be distilled water or water of at least equivalent 
purity. 
4.1 Nitric acid (ezO = 1,38 g/ml). 
4.2 Hydrochloric acid, diluted 1 + 1. 
Dilute 1 part by volume of hydrochloric acid (ezO = 1,19 g/ml) 
with 1 part by volume of water. 
4.3 Ammonia, 25 % (m/m) solution. 
4.4 Electrolyte solution. 
Dissolve 53,5 g of ammonium chloride in water in a 1 000 ml 
one-mark volumetric flask, add 155 ml of the ammonia solution 
(4.3), make up to the mark with water and mix. 
4.5 Sodium sulfite (Na2S03), 1 mol/l solution. 4.6 Zinc, standard solution corresponding to 0,Ol to 0,04 mg 
of Zn per millilitre. 
4.6.1 Stock solution 
Dissolve 1 g of metallic zinc [purity at least 99 Y. (m/m)] in 
10 ml of the hydrochloric acid (3.2) in a conical flask. Transfer 
quantitatively to a 1 000 ml one-mark volumetric flask, make up 
to the mark with water and mix. 
4.6.2 Preparation 
Dilute 1 to 4 ml of the zinc stock solution (4.6.1) to the mark 
with water in a 100 ml one-mark volumetric flask and mix. 
5 Apparatus 
Usual laboratory equipment and in particular 
5.1 Polarograph, suitable for determining zinc contents 
greater than 0,05 mg/kg, provided with a mercury-dropping 
electrode as the cathode and an electrolyser with mercury at 
the bottom as the anode. 
5.2 Drying oven, capable of being maintained at 100 to 
150 OC within a tolerance not exceeding + 5 OC. 
5.3 Muffle furnace, capable of being maintained at 100 to 
700 OC within a tolerance not exceeding + 25 OC. 
5.4 Porcelain dishes, of diameter 9 to 11 cm. 
5.5 One-mark volumetric flask, of capacity 50 ml. 
5.6 Graduated pipettes, of capacities 1 and 10 ml. 
5.7 Conical flask, of capacity 25 ml. 
5.8 Analytical balance. 
5.9 Boiling water-bath. 
6 Sampling 
For the sampling of fresh fruits and vegetables, see IS0 874. 
1 Is0 6636/l-1986 El 
Homogenize the laboratory sample thoroughly before taking 
the test portion. Allow frozen and deep-frozen products to 
thaw in a closed vessel and mix the liquid formed during this 
process with the product during homogenization. 7.4.3 Carry out the polarographic measurement by scanning 
between -I,0 and - I,6 V, following the manufacturer’s in- 
structions. Set the sensitivity according to the expected zinc 
content. The setting will vary from instrument to instrument. 
The half-wave potential, EI12, for zinc is approximately 
- I,2 V. The mercury dropping rate shall be IO drops per 25 to 
7 Procedure 
7.1 Test portion 
Weigh, to the nearest 0,Ol g, 10 to 25 g of the test sample, 
according to the expected zinc content, into one of the 
porcelain dishes (5.4). 
7.2 Decomposition 
7.2.1 Place the dish containing the test portion (7.1) in the 
oven (5.2) and dry at a temperature of 110 to 120 OC. Then 
transfer to the muffle furnace (5.3) maintained at 250 OC. 
Slowly raise the temperature to 350 OC and maintain this 
temperature until there is no more foam. Raise the temperature 
gradually to 525 OC (the test portion shall not ignite) and ash for 
6 h. Remove the dish from the muffle furnace and allow it to 
cool. If the ash contains a large amount of carbon particles, 
proceed as follows : 7.4.4 After recording the first polarogram, empty the elec- 
trolyser and before using it again, wash it with small amounts 
of the next test solution. 
7.4.5 To the contents of the second conical flask add 1 ml of 
the sodium sulfite solution (4.5) and a known volume of the 
zinc standard solution (4.61, not exceeding 1 ml. Make up to 
25 ml with the electrolyte solution (4.4). Proceed as specified in 
7.4.2. Repeat this procedure twice more, with similar additions 
of the zinc standard solution, and obtain the zinc content by 
extrapolation. Correct this value for the result of the blank test. 
7.5 Number of determinations 
Carry 
same out two 
sample. determinations on test portions from 
Wet the ash with 0,5 ml of water and then with 0,5 ml of nitric 
acid (4.1) 
8 Expression of results 
Evaporate to dryness on the boiling water-bath (5.9). Transfer 
to the muffle furnace, maintained at 250 OC, increase the 
temperature to 525 OC and continue heating for 1 to 2 h. 
Repeat this procedure if necessary, until the ash is free from 
carbon particles. 8.1 Calculation 
The zinc content, expressed in milligrams per kilogram of pro- 
duct, is equal to 
7.2.2 Add to the ash 10 ml of hydrochloric acid (4.2) and 
place on the boiling water-bath to aid dissolution. Allow to 
cool. ez, V4hl Vl KJ x 1 000 
t&v* - h, V,) mV3 
7.2.3 Quantitatively transfer the solution to a 50 ml volumetric 
flask (5.5), and wash the sides of the dish with 5 to 10 ml of 
water, collecting the washings in the same volumetric flask. where 
I+ is the height, in millimetres, of the polarographic wave 
obtained in the first measurement (see 7.4.3); 
7.2.4 Add ammonia solution (4.3) to the solution until the 
smell of ammonia appears (pH 8). Then add an excess until the 
pH is 10. Make up to the mark with water, mix and filter. h2 is the height, in millimetres, of the polarographic wave 
obtained in the second measurement (see 7.4.5); 
7.3 Blank test lvyl is the mass, in grams, of the test portion (7.1); 
Carry out a blank test in parallel with the determination, by the 
same procedure, using the same quantities of all the reagents, 
but replacing the test portion with an equal mass of water. V. is the total volume, in millilitres, of solution prepared 
from the decomposed test portion (see 7.2.4); 
is the volume, in millilitres, of solution prepared for the 
7.4 Determination first measurement (see 7.4.2); 
7.4.1 Transfer, by means of a pipette, two 8 ml aliquot por- V’ is the volume, in millilitres, of solution prepared for the 
tions of the filtered solution (7.2.4) to two conical flasks (5.7). second measurement (see 7.4.5); 
7.4.2 Add 1 ml of the sodium sulfite solution (4.5) and 1 ml of 
water to the contents of one of the conical flasks and make up 
to 25 ml with the electrolyte solution (4.4). Mix thoroughly. 
Transfer the solution to the electrolyser of the polarograph, rins- 
ing this beforehand with a small amount of the test solution. V3 is the volume, in millilitres, of the aliquot portion taken 
for the preparation of the solution for analysis (see 7.4.1); 
V4 is the volume, in millilitres, of the zinc standard sol- 
ution added (see 7.4.5); 
2 IS0 6636/l-1986 (E) 
@Zn is the concentration, in milligrams per 
zinc standard solution (4.6). millilitre, 
Take as the result the arithmetic mean of the values obtained in 
the two determinations (7.51, provided that the requirement for 
repeatability (see 8.2) is satisfied. 
Report the result to one decimal place. 
8.2 Repeatability 
The difference between the results of the two determinations 
(7.51, carried out simultaneously or in rapid succession under the same conditions (same operator, same apparatus, same 
laboratory), shall not exceed 5 % of the mea n. 
9 Test report 
The test report shall show the method used and the results 
obtained. It shall also mention any operating conditions not 
specified in this International Standard, or regarded as op- 
tional, together with details of any incidents likely to have in- 
fluenced the results. 
The test report shall include all the information 
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