DRAFT UGANDA STANDARD  
  DUS 572  
Second  Edition  
2017 -mm-dd 
 
 
 Reference number  
DUS 572: 2017  
© UNBS 2017   
  
Sodium bicarbonate — Specification  
 DUS 572: 2017  
ii © UNBS 2017  – All rights reserved  
 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 2017   
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, including 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 4 333 2 50/1/2/3  
Fax: +256 41 4 286 123  
E-mail: info@unbs.go.ug  
Web: www.unbs.go.ug  DUS 572: 2017  
© UNBS 2017  – All rights reserved  iii 
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
Introduction  ................................ ................................ ................................ ................................ .........................  v 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Grades  ................................ ................................ ................................ ................................ ....................  1 
5 Requirements  ................................ ................................ ................................ ................................ .........  2 
6 Contaminants  ................................ ................................ ................................ ................................ .........  3 
7 Packaging  ................................ ................................ ................................ ................................ ...............  3 
8 Weights and Measures ................................ ................................ ................................ ..........................  3 
9 Labelling  ................................ ................................ ................................ ................................ .................  3 
10 Sampling  ................................ ................................ ................................ ................................ ................  3 
Annex  A (normative )  Determination of Chlorides  ................................ ................................ ..........................  4 
A-2  Method A (Volumetric Method)  ................................ ................................ ................................ ...........  4 
A-2.1  For Pure and Refined Grades  ................................ ................................ ................................ ..............  4 
A-2.2  For Analytical Reagent Grade  ................................ ................................ ................................ .............  4 
A-3  Method B (Spectrophotometric method)  ................................ ................................ ...........................  5 
ANNEX B (normative)  Test for Nitrates  ................................ ................................ ................................ ...........  7 
ANNEX C (normative)  Test for Silicates  ................................ ................................ ................................ ..........  8 
C-2 Method A (Visual Colour Comparison)  ................................ ................................ ...............................  8 
C-3 Method B (Spectrophotometric Method)  ................................ ................................ .............................  9 
Bibliography  ................................ ................................ ................................ ................................ ......................  12 
 
 
 
 
 
 
 
 
 DUS 572: 2017  
iv © UNBS 2017  – All rights reserved  
 Foreword  
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry of Trade , Industry and 
Cooperatives established under Cap 327 , of the Laws of Uganda , as amended .  UNBS is mandated to co -
ordinate the elaboration of standards and is  
(a) a member of International Organisation for Standardis ation (ISO) and ,  
(b) a contact point for the WHO/FAO Codex Alimentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT  Agreement  of the World Trade Organisation (WTO).  
The work of preparing Uganda Standards is carried out through Technic al Committees. A Technical 
Committee is established to deliberate on standards in a given field or area and consists of key stakeholders 
including government, academia, consumer groups, private sector and other interested parties.  
Draft Uganda Standards a dopted by the Technical Committee 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 Standar ds Council.  
The committee responsible for this document is Technical Committee  UNBS/TC 2 [Technical Committee on 
Food and Agriculture standards], Subcommittee SC 6, [Food Additives & Contaminants Subcomm ittee] . 
This second edition cancels and replaces the first edition (US 572: 2006) which has been technically revised.  DUS 572: 2017  
© UNBS 2017  – All rights reserved  v 
 Introduction  
Sodium bicarbonate (NaHCO 3), also known as bicarbonate of soda or baking soda is used as a leavening 
agent for baked products; an ingredient of baking powder; a reagent in analy tical chemistry, and as an 
ingredient in the manufacture of effervescent salts and beverages. It also finds use in gold and platinum 
plating, the tanning industry, treating wool and silk, fire extinguishers and in the production of ceramics.  
This standard  has been revised to set requirements  for the commonly used grades of sodium bicarbonate and 
to provide  for methods of test that were lacking in the previous edition . This will facilitate  fair trade, customer 
satisfaction and consumer safety .  DRAFT UGANDA STANDARD    DUS 572: 2017  
  
© UNBS 2017  – All rights reserved  1 
 Sodium bicarbonate — Specification  
1 Scope   
This standard specifies  the requirem ents and  methods of  sampling and test for the grades of sodium 
bicarbonate  defined herein . 
2 Normative references  
The following referenced documents are referred to in the text in such a way that some or all of their content 
constitutes requirements of this document. For dated references, only the edition cited applies. For undated 
references, the latest edition of th e referenced document (including any amendments) applies.  
US CAC/GL 50  – General guidelines on sampling  
AOAC 965.18  – Phosphorus in baking powder – Gravimetric method  
AOAC 999.11 – Determination of Lead, Cadmium, Copper, Iron, and Zinc in Foods   
AOAC 920.4 6 – AOAC Official method : Metals and Other Constituents in Baking Powder  
US ISO 10523  – Water quality – Determination of pH 
DUS ISO 2199  – Sodium hydrogen carbonate for industrial use – Determination of Sodium hydrogen 
carbonate content – Titrimetric method  
DUS ISO 2460  – Sodium hydrogen carbonate for industrial use – Determination of Iron content  – 1, 10 -
phenanthroline photometric method  
US ISO 2482  – Sodium  chloride for industrial use - Determination of calcium and magnesium conten ts –  
EDTA complexo metric methods  
The Weights and Measures (Sales and Labeling of goods) Rules, 2007  
3 Terms and definitions  
For the purposes of this document, the following terms and definitions apply.  
Food grade packaging  
packaging material, made of substances which are safe and suitable for their intended use and which will not 
impart any toxic substance or undesirable odour or flavour to the product .  
4 Grades  
There shall be three grades of the product;  
a) Pure grade: suitable for the fine chemical and pharmaceutical industry,  
b) Analytical reagent grade: suitable as a chemical reagent, and  
c) Refined grade: the food grade product, suitable for  the baking powder  industry  and other food 
industries . DUS 572: 2017  
2 © UNBS  2017  – All rights reserved  
 5 Requir ements  
5.1 General requirements  
For all the three grades, the products shall be : 
i) in the form of small, opaque, monoclinic crystals or, as whit e, minutely crystalline powder  
ii) free from  dirt and other foreign matter  
5.2 Specific requirements  
The respective product  grades , when tested  per the specified methods or methods elaborated in the 
respective annexes shall comply with the requirements specified in Table 1.  
Table 1 — Requirements for Sodium bicarbonate  
 
S/N  
Characteristic  Requirements  for  Method of 
Test / 
Reference to 
Annex  Pure Grade  Analytical 
Reagent Grade  Refined 
Grade  
1.  Total alkalinity (as NaHCO 3) 
percent by mass, Min 99.0 99.8 99.0 DUS  ISO 2199  
2.  pH (1 percent aqueous 
solution), Max 8.6 8.4 8.8 US ISO 10523  
3.  Iron (expressed as  Fe2O3), 
percent by mass, Max 0.0015  0.0005  0.004  DUS ISO 2460  
4.  Calcium and magnesium (as 
Ca), percent by mass, Max - 0.015  - US ISO 2482  
5.  Phosphorus  (as PO 4). 
percent by mass, Max - 0.0005  - AOAC 965.18  
6.  Insoluble matter, percent by 
mass, Max 0.05 0.010  0.100  AOAC 920.46 
(C) 
7.  Sulphates (as SO 4), percent 
by mass, Max 0.05 0.003  0.07 AOAC 920.46 
(I) 
8.  Ammon ium compounds  To pass test  To pass test  To pass test  AOAC 920.46 
(A) 
9.  Potassium (as K), percent by 
mass, Max - 0.01 - AOAC 920.46 
(H) 
10.  Substances reduc ing iodine, 
percent by mass, Max - 0.0065  - AOAC 920.44  
11.  Chlorides (as Cl), percent by 
mass, Max 0.01 0.001  0.06 A 
12.  Nitrates (as NO 3), percent by 
mass, Max - 0.001  - B 
13.  Silicates (as SiO 2), percent 
by mass, Max - 0.005  - C DUS  572: 2017  
© UNBS 2017  – All rights reserved  3 
 6 Contaminants  
When tested in accordance with the cited methods , the contaminant  levels shall conform to the requirements 
specified i n Table 2.   
Table 2  — Requirements for Sodium bicarbonate  
 
Contaminant  Requirements for   
Method of Test  Pure Grade  Analytical Reagent 
Grade  Refined Grade  
Heavy metals (as Pb), 
mg/kg , Max 5 5 5 AOAC 999.11  
Arsenic (as As) mg/kg, 
Max 1.5 0.4 1.5 AOAC 920.46  (B) 
7 Pack aging 
The product shall be packaged in containers that maintain  its integrity . The refine grade product shall be 
packaged in food grade containers . 
8 Weights and Measures  
The weight of the packages shall comply with the Weights and Measures regulations of Uganda.  
9 Labelling  
The packages shall be securely closed and legibly marked with the following information:  
a) Name of the product as “Baking soda” / “Sodium hydrogen carbonate”/ “Sodium bicarbonate”/ 
“Bicarbonate of soda”  
b) The product  grade  
c) Name and physical address of the manufacturer/distributor,  
d) Net weight  in metric u nits, 
e) The list  of requirements given in  Table 1 depending on the product grade , 
f) The expiry  date 
g) The b atch number  
10 Sampling  
Representative samples of the product shall be drawn in accordance with US CAC/GL 50 .  DUS 572: 2017  
4 © UNBS  2017  – All rights reserved  
 Annex  A 
(normative)  
 
Determination of Chlorides  
A-l General  
Two methods have been prescribed for determination  of chlorides;  volumetric (Method A) and 
spectrophotometric (Method B). In case of any dispute, Method A shall be the reference method.  
A-2  Method  A (Volumetric Method)  
A-2.1  For Pure and Refined Grades  
A-2.1.1   Reagents  
- Concentrated nitric acid  (A.R. grade: 65 – 70%)  
- Standard silver nitrate solution (0.1 N)  
- Nitrobenzene  
- Standard ammonium thiocyanate solution (0.1 N)  
- Ferric ammonium s ulphate indicator ( 5 percent)  
A-2.1.2  Procedure  
Weigh ac curately  20 g of the product , dissolve it in distilled water and neutralise with concentrated nitric acid . 
Add 5 ml of nitric acid in excess. Boil the solution to expel any dissolved carbon dioxide gas, cool and add 10 
ml of standard silver nitrate solution. Add 3  ml of nitrobenzene, shake vigorously and titrate with standard 
ammonium thiocyanate solution using fer ric ammonium sulphate indicator until  a permanent faint reddish 
brown colouration appears , signalling the end point . 
A-2.1.3  Calculation  
Chlorides (as Cl ) percent by mass  = 
MVNN2 1 ) 10(547.3   -   
Where, N1 = normality of standard silver nitrate solution , 
              V = volume in ml of standard ammonium thiocy anate solution consumed in the titration , 
            N2 = normality of standard ammonium thioc yanate solution , and 
             M = mass of product tested in grams.  
A-2.2  For Analytical Reagent Grade  
A-2.2.1 Apparatus  
- Nessler cylinders  (100 ml capacity ). 
 DUS  572: 2017  
© UNBS 2017  – All rights reserved  5 
 A-2.2.2  Reagents  
- Concentrated nitric acid  (A.R grade: 65 – 70%)  
- Silver nitrate solution (4 percent)  
- Standard chloride solution  
Dry sodium chloride at 250  °C before making the solution. Dissolve 1.649 g of sodium chloride in distilled 
water and make up the volume to 1000 ml. Pipette  10 ml from this solu tion and dilute to 1 000 ml  using distilled 
water . One millilitre of the diluted solution is equivalent to 0.01 mg of chloride (as Cl).  
A-2.2.3  Procedure  
Dissolve 1 g of the product  in 30 ml of distilled water and add slight excess of concentrated nitric acid. Boil the 
solution, coo l and add 1 ml of silver nitrate solution.  Transfer into a Nessler cylinder and make up the volume 
to the 100 ml mark with distilled water. Carry out a control test using 1 ml of standard chloride solution and the 
same quantities of other reagent s in the s ame total volume of the reaction mixture. Compare the turbid ity in 
both cylinders after 5 minutes.  
The limit prescribed in Table 1 s hall be taken as not  exceeded if the turbidity produced in the test with the 
product  is not greater than that produced in th e control test.  
A-3  Method B (Spectrophotometric method)  
A-3.1 Outline of the Method  
To a neutralised solution of sodium bicarbonate, addition of mercuric thiocyanate and ferric ammonium 
sulphate, produces a highly coloured ferric thiocyanate complex, the  intensity of which is proportional to the 
chloride content. The absorbance of this colour is measured at 450 nm wave length.  
A-3.2 Apparatus  
-   Standard laboratory a pparatus  
-   Spectrophotometer or Photocolorimeter  
-   Platinum evaporating d ish (60 ml capacit y). 
A-3.3 Reagents  
-   Ferric ammonium sulphate s olution (0.25 mol/l)  
Dissolve 49.02  g of Ferric Ammonium Sulphate d odecahydrate [FeNH 4(SO 4)2.12H 2O] in 203 ml of dilute nitric 
acid (about 1.5 mol/l) and dilute to 500 ml with distilled water.  
-   Mercury (II) t hiocyanate solution; Saturated solution in absolute alcohol.  
-   Standard sodium chloride solution  
Accurately weigh 0.165 g of previously dried (at 500°C) and cooled A .R. grade sodium chloride into a beaker. 
Dissolve it in distilled water and quantitatively transfer into a one -litre volumetric flask, dilute to the mark and 
mix well. 1 ml of this solution = 100 µg of Cl.  
-    Dilute standard sodium chloride solution  
Transfer 50 ml of standard sodium chloride solution into a 500 ml volumetric flask, dilute with distilled water to 
the mark and mix well. 1 ml of this solution = 10 µg of Cl.  
- Sulphuric Acid Solution  (Approximately 1 mol/l ) 
Pour 56 ml of concent rated sulphuric acid (A.R. grade ) into 944 ml of distilled water slowly. Mix and cool.  DUS 572: 2017  
6 © UNBS  2017  – All rights reserved  
 A-3.4 Procedure   
A-3.4.1  Calibration  
Into a series of six 50 ml volumetric flasks, transfer standard sodium chloride solution as given below.  
Standard Sodium Chloride Solution, ml Corresponding Mass of Chloride (as Cl) , µg 
0(compensation)  0 
2 20 
4 40 
6 60 
8 80 
10 100 
To each volumetric flask , add 2 ml of ferri c ammonium sulphate solution followed by  2 ml of mercury 
thiocyanate solution . Dilute with distilled water to the mark and mix well.  Allow to stand for 15 minutes, transfer 
to the cell of spectrophotometer and m easure absorbance at  450 nm wave length with compensation solution 
in reference cell.  
Plot chloride conte nt in micrograms as abscissa against the  corresponding absorbance values as ordinates.  
A-3.4.2  Determination   
Weigh suitable mass of sodium bicarbonate sample c ontaining 50 to 150 µ g of chloride (as Cl) correct to 1 mg 
and transfer into the platinum evaporating dish . Wet it  with 15 to 20 ml of dist illed water and slowly neutralis e 
with sulphuric acid solution . Add a few drops of the acid in excess. Using a hot water bath, e vaporate t he 
mixture close to  drynes s. Add 20 to 30 ml of distilled water and quantitatively transfer into a 100 ml volumetric 
flask, cool and make up to the mark with distilled water and mix well.  
Filter through Whatman No. 41 filter pa per and discard a small volume of initial filtrate.  Transfer a suitable 
volume of the filtrate (25 to 40 ml) into a 50 ml volumetric flask . Add 2 ml of ferri c ammonium sulphate solution 
followed by  2 ml of mercury thiocyanate solution.  Dilute to the mark w ith distilled water and mix well.  
Allow the mixture to stand for 15 m inutes, transfer to the spectrophotometer cell and measure absorbance at 
450 nm wave length with distilled water in the reference cell.  
A-3.4.3  Blank Test  
Carry out a blank test using exa ctly the same pro cedure in A-3.4.2  using similar  quan tities of reagents, with 
distilled water serving as the test solution.  
A-3.4.4  Calculations  
The chloride content  (as Cl), percent by mass  = 
 02  1
10000 MM M   -     
where, M1 = mass in micrograms of chloride corresponding to absorbance of test solution  
 M2 = mass in micrograms of chloride corresponding to absorbance of blank solution, and  
 M0 = mass in grams of sodium bicarbonate in the test portion of the aliquot used for co lour development.  DUS  572: 2017  
© UNBS 2017  – All rights reserved  7 
 ANNEX B  
(normative)  
 
Test for Nitrates  
B-l Reagents  
- Dilute Sulphuric Acid (5 N)  
- Concentrated hydrochloric a cid (A.R. grade: 35 – 37%)  
- Potassium nitrate s olution  (0.001 N)  
- Concentrated sulphuric a cid (A.R. grade: 98%)  
- Standard Indigo carmine s olution  
Dissolve 0.20 g of indigo carmine in 400 ml of dilute sulphuric acid and add 20 ml of concentrated hydrochloric 
acid and sufficient dil ute sulphuric acid to produce 1000 ml. Standardis e the solution so that 10 ml added to 
3.3 ml of potassi um nitrate solution is just decolo urised on adding 13 ml of concentrated  sulphuric acid and 
heating to boiling. One millilitre of this solution is equivale nt to 0.02 mg of nitrate (as NO 2). 
B-2 Procedure  
Dissolve 2.0  g of the sample  in 10 ml of dilute sulp huric acid  and 10 ml of standard indig o carmine solution 
followed by 1 0 ml of concentrated sulphuric acid and heat to boiling.  
The limit prescribed in Table 1 shall be taken as not having been exceeded if the blue colour produc ed in the 
test with the sampl e does not entirely disappear.  DUS 572: 2017  
8 © UNBS  2017  – All rights reserved  
 ANNEX C  
(normative)  
 
Test for Silicates  
C-l General  
Two methods have been prescribed for determination of silicates ; visual colour comparison (Method A) and 
spectrophotometric method (Method B). In case of a dispute , Method A  shall be reference  method.  
C-2 Method A (Visual Colour Comparison)  
C-2.1 Apparatus  
- Nessler c ylinders  (50 ml)  
C-2.2 Reagents  
- Dilute s ulphuric acid (1 N)  
- Ammonium molybdate s olution  
Dissolve 10 g of ammonium molybdate in 100 ml of dilute sulphuric acid (1 N ). 
- Sodium c itrate solution  
Dissolve 8 g of sodium citrate (Na 3C6H307.2H 2O) in 100 ml of distilled water.  
- Potassium metabisulphite s olution  
Dissolve 20 g of potassium metabisu lphite and 1 g of hydroquinone in 100 ml of distilled water.  
- Anhydrous sodium c arbonate  
- Standard silicate s olution  
Fuse 0.10  g of pure, precipitated silica with anhydrous sodium carbonate in a pla tinum crucible and leach the 
mixture with water. Neutralis e with dilute  sulphuric acid and dilute to 1 000 ml. Dilute 10 ml of this solution  to 
100 ml. One millilitre of the diluted solution con tains 0.01 mg of silica (as SiO 2). Store in a polyethylene bottle.  
C-2.3 Procedure  
Dissolve 2.0  g of the product  in 20 ml of distilled water in a platinum dish, neutrali se with about 10 ml of dilute 
sulphuric acid. Add 2 ml of the acid in excess and dilute to 50 ml  with distilled water . Transfer 10 ml of this 
solution into a Nessler cylinder. Add 1 ml of dilute sulphuric acid and 5 ml of ammonium molybdate solution. 
Allow to stand for 5 minutes and then add 5 ml of sodium citrate solution followed by 5 ml of potassium 
metabisulphite solution. Dilute to 50 ml. Carry out a control test in another Nessler cylinder using 2 ml of 
standard silicate solution in place of the sample and the same quantities of othe r reagents in the same total 
volume of the reaction mixture. Stand both t he cylinders in a boiling water bath for 5  minutes, cool and 
compare the c olour produced in the two cylinders after 5 minutes.  
The limit prescribed for silicates shall be taken as not  having been exceeded if any blue colour produ ced in 
the test with the product  is not deeper than that produced in the control test.  DUS  572: 2017  
© UNBS 2017  – All rights reserved  9 
 C-3 Method B (Spectrophotometric Method)  
C-3.1 Outline of the Test Method  
This method specifies a reduced molybdosilicate  spectrophotometric method for the determination of soluble 
silica.  It is applicable for materials with silica content in presence of phosphate.  
Depolymerisation of any polymeris ed silica by treatment with hydrofluoric acid and boric acid.  Formation of the  
oxidized molybdosilic ate complex (yellow) under well -defin ed conditions of acidity (PH 1 ± 0.05).  
Selective reduction of the complex in a strong sulphuric acid medium in the presence of oxalic acid to 
eliminate interference of phosphate.  
Spectrophotometri c measurement of the coloured complex at wavelength of maximum absorption (about 795 
nm). 
C-3.2 Reagents  
- Distilled w ater 
- Sulphuric a cid (2 mol/l)  
Add 112 ml of concentrated Sulphuric acid (A.R. grade)  in 888 ml of distilled water slowly by constant stirrin g 
cool and mix well.  
- Sulphuric acid (8 mol/l ) 
Add 448 ml of concentrated Sulphuric acid ( A.R grade) in 552 ml of distilled water slowly by constant stirring, 
cool and mix well.  
- Sodium m olybdat e (274 g/l Solution)   
Dissolve 27.4 g of sodium molybdate dihydrate (Na 2MoO 4.2H 2O) in warm distilled water in a beaker of 
material free from silica  of suitable capacity and after cooling , dilute to 100 ml.  Store, if necessary, in a bottle 
of material free from  silica and filter, if necessary, before use.  
- Oxalic a cid (100 g/l  solution ) 
Dissolve 10 g of oxalic acid d ihydrate (COOH. COOH.2H 2O) in 100 ml distilled water.  
- Ascorbic a cid (25 g/l Solution)  
Dissolve 2.5 g of ascorbic acid in water and dilute to 100 ml. Store the solution in a bottle of material free from 
silica and protect from light.  Prepare this solution at the time of use or as an alternative, prepare reduction 
solution.  
- Reduction solution  
Dissolve 7 g of anhydrous sodium sulphite (Na 2SO 3) in 50 ml distilled water. Then add 1.5 g of 4 -amino -3-
hydroxyl naphthalene -1-sulphonic acid (C 10H9NO 4S) and dissolve by grinding.  
Dissolve 90 g of anhydrous metabisulphite (Na 2S2O5) in 900 ml of  distilled  water.  
Mix the two prepared solutions and dilute to 1 000 ml. Filter if turbid and store in a n opaque bottle free f rom 
silica.  
- Sodium fluoride solution  (20 g/l solution)  
- Boric acid s olution  (Saturated solution at ambient temperature)  
- Standard Silica Solution  (0.20  g SiO 2 per litre ) DUS 572: 2017  
10 © UNBS  2017  – All rights reserved  
 Weigh to the nearest 1 mg 0.20 g SiO 2 obtained by heating pure silicic acid (H 2SiO 3) at 1000  °C until constant 
mass (consecutive weighings should not differ by more than 1 mg) and cooling in a desiccator into a platinum 
crucible of convenient capacity. Add 2 .0 g of anhydrous Na 2CO 3 to the crucible, mix well preferably with a 
platinum spatula and carefully fuse the mixture. Add warm water directly to the crucible, heat gently until the 
contents are completely dissolved and transfer quantitatively to a beaker of suitable capacity made  of silica 
free material. Cool, dilute the solution to about 500 ml, transfer quantitatively to a 1000 ml volumetric flask, 
dilute to the mark and mix well. Transfer the solution immediately to a bottle of material free from silica. 1 ml of 
this solution =  0.2 mg SiO 2. 
- Standard silica solution ( 2.0 mg SiO 2 per litre )  
Dilute 20.0  ml of standard silica solution above  to 1000 ml volumetric flask  using distilled water . Prepare this 
solution at the time of us e. 1 ml of this solution = 2.0 µg SiO 2. 
C-3.3 Apparatus  
- Standard laboratory a pparatus  
- pH m eter 
- Spectrophotometer or Photocolorimeter  
- Platinum crucible with lid of suitable s ize 
C-3.4 Procedure  
C-3.4.1  Preparation of the calibration g raph  
C-3.4.1.1  Preliminary test for c ontrol and correction of pH  
Place 10.0 ml of the standard silica solution ( 2.0 mg SiO 2 per litre ) in a beaker. Add the distilled water to bring 
volume to 15 ml and then add 5 ml of sodium molybdate solution,  1 ml of sodium fluoride and 5 ml of boric 
acid solution. Mix and measure pH with a pH meter. Adjust pH to 1 ± 0.05 by adding slowly using a graduated 
pipette or a burette drop by drop, mixing after each addition, the necessary quantity of 2 mol/l sulphuric acid  
solution  (about 4.5 ml would be required).  
Record the actual volume of  sulphuric acid required and discard the solution.  
C-3.4.1.2  Preparatio n of the standard c alorimetric solution  
In to a series of six 50 ml beakers, transfer standard silica solution as given below:  
Standard silica solution , ml Corresponding mass of silica , µg 
0(compensation)  0.0 
2.0 4.0 
4.0 8.0 
6.0 12.0 
8.0 16.0 
10.0 20.0 
 
To each beaker, add  distilled water to make up volume to 15.0 ml. Then add the volume of sulphuric acid 
solution as calculated above ( C-3.4.1.1 ) for pH  correction, mix well. Add 1 ml of sodium fluoride solution, mix 
and allow to stand for 5 min.  
 DUS  572: 2017  
© UNBS 2017  – All rights reserved  11 
 C-3.4.1.3  Colour Development  
To each beaker, a dd 5 ml of boric acid solution and 5 ml of sodium molybdate solution, mix and allow to stand 
for 10 minutes. Add 5  ml of oxalic acid solution and 8 ml of sulphuric acid solution (8 mol/l ). Allow to Stand for 
2 minutes, add 2 ml of ascorbic ac id solution or  2 ml of reduction solution and allow to stand for 10 minutes. 
Transfer the solutions quantitatively to a series o f 50 ml volumetric flasks, dilute to marks and mix.  
C-3.4.1.4  Measurements  
Carry out the spectrophotometric measurements with a spectrophotometer (or photocolorimeter) at 795 nm 
wavelength, after having adjusted the instrument zero absorbance with the compensation solution, use cells of 
4 to 5 cm optical path length.  
Plot a graph with µ g of SiO 2 contained in 50 ml as abscissae and corresponding absorbance values as 
ordinates.  
C-3.4.2  Determination  
C-3.4.2.1  Test solution  
Weigh accurately ( to the nearest  1 mg) an amount of Sodium bicarbonate containing between 4 to 20 µg of 
SiO 2 and transfer into a 100 ml beaker, dissolve in minimum amount of distilled water and neutralis e with 1 
mol/l sulp huric acid (about 6 ml) and further  add 0.5 ml acid in excess. Hea t on a water bath to expect all CO 2 
liberated and cool. Ensure that the total volume is around 15 ml. Then add the volume of sulphuric acid 
solution (2 mol/l ) used for pH correction in the preliminary test and 1 ml of sodium fluoride solution, mix and 
allow to stand for 5 minutes. Develop colour by the procedure given in (C-3.4.1.2) . 
C-3.4.2.2  Blank test  
Carry out a blank test following the  same procedure and using the same quantities of all reagent s used for the 
determination,  replacing the test solution b y the same volume of distilled water.  
C-3.4.2.3  Spectrophotom etric measurements  
Carry out the spectrophotometric measurements by the procedure specified in (C-3.4.1.4)  at 795 nm wave 
length after adjusting the instrument to zero absorbance against water, use the  same size of the cell used for 
calibration.  
C-3.4.3  Calculation  
By means of the calibration graph , determine the mass of SiO 2 content corresponding to the aliqu ot used for 
colour development.  
The silica content is expre ssed as : Silica (as SiO 2), percent by mass  =
10000×  02 1
MM M  -   
where, 
           M1 = mass in micrograms  of SiO 2 in the test portion of the aliquot,  
           M2 = mass in micrograms  of SiO 2 in the blank, and ,  
           M0 = mass of product sample  in grams  in the aliquot portion used for colour development.  
 DUS 572: 2017  
12 © UNBS  2017  – All rights reserved  
 Bibliography  
[1] IS 2124 (2000): Sodium Bicarbon ate (Second Revision ) 
[2] AOAC (2000 ): Official methods of analysis of AOAC International (17th ed.), Gaithersburg , Md.  
 US 572: 2017  
© UNBS 2017  – All rights reserved  13 
 Certification marking  
Products that conform to Uganda standards may be marked with Uganda National Bureau of Standards 
(UNBS) Certification Mark shown in the figure below.  
The use of the UNBS Certification Mark is governed by the Standards Act, and the Regulations made 
thereun der. This mark can be used only by those licensed under the certification mark scheme operated by 
the Uganda National Bureau of Standards and in conjunction with the relevant Uganda Standard. The 
presence of this mark on a product or in relation to a produ ct is an assurance that the goods comply with the 
requirements of that standard under a system of supervision, control and testing in accordance with the 
certification mark scheme of the Uganda National Bureau of Standards. UNBS marked products are 
continu ally checked by UNBS for conformity to that standard.  
Further particulars of the terms and conditions of licensing may be obtained from the Director, Uganda 
National Bureau of Standards.  
 US 572: 2017    
 
ICS 71.060.50  
Price based on nn pages  
© UNBS 2017  – All rights reserved