DRAFT UGANDA STANDARD  
  DUS 1583-1  
First  Edition  
2018-mm-dd  
 
 
 Reference number 
DUS 1583-1: 2018 
© UNBS 2018  
  
Sampling and test for sodium hydroxide for industri al use —
Part 1 — Determination of silica content 
 DUS 1583-1: 2018 
ii © UNBS 2018 – All rights reserved
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© UNBS 2018 – All rights reserved  iii
 Foreword 
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 DRAFT UGANDA STANDARD DUS 1583-1: 2018 
 
© UNBS 2018 – All rights reserved  1
 Sampling and test for sodium hydroxide for industri al use —
Part 1— Determination of silica content  
1 Scope  
This Draft Uganda Standard specifies a reduced sili co-molybdic complex photometric method for the 
determination of the silica content of sodium hydro xide for industrial use. The method is applicable t o 
products having silica (SiO 2) contents exceeding 10 mg/kg . 
2 Normative references  
The following referenced documents referred to in t he text in such a way that some or all of their con tent 
constitutes requirements of this document. For date d references, only the edition cited applies. For u ndated 
references, the latest edition of the referenced do cument (including any amendments) applies. 
ISO 3195, Sodium hydroxide for industrial use - Sam pling  -- Test sample - Preparation of the main solution f or 
carrying out certain determinations  
ISO 3696:1987 Water for analytical laboratory use - - Specification and test methods 
3 Principle 
Formation of the yellow oxidized silico-molybdic co mplex  at pH 1.1 ± 0.2, in the presence of boric acid to 
suppress  interference by fluorides.  
Selective reduction of this complex with a mixture of amino-naphthol sulphonic acid (4-amino-3-hydroxy naph-
thalene-1-sulphonic acid), sodium metabisulphite an d  sodium sulphite, in the presence of oxalic acid and  in a  
strongly acid medium so as to suppress interference  by  phosphates. Photometric measurement of the blue-
coloured  complex at a wavelength of about 795 nm.  
4 Reagents 
During the analysis use only reagents of recognized  analytical reagent grade and only demineralized wat er. 
Water complying with the requirements for ISO 3696: 1987 shall be used.  Store all the reagents in 
polyethylene bottles.  
4.1  Sulphuric acid,  
approximately 9 N solution.  
4.2  Hydrochloric acid,  
approximately 2 N solution.  DUS 1583-1: 2018 
2 © UNBS 2018 – All rights reserved
 4.3  Boric acid,  
saturated solution (about 48 g/l).  
4.4  Oxalic acid,  
100 g/l solution.  
4.5  Sodium molybdate dihydrate [Na 2Mo0 4.2H 20] 
140 g/l solution.  
Dissolve 35 g of this reagent in 200 ml of water at  about  50 ºC in a polyethylene beaker. Cool to room  
temperature, transfer to a 250ml one-mark volumetri c  flask, dilute to the mark and mix. Transfer to a  
polyethylene bottle.  
If necessary, filter the solution before use.  
4.6  Reducing solution  
4.6.1   Dissolve 7 g of anhydrous sodium sulphite in 50 m l  of water. Then add 1.5 g of 4-amino-3-
hydroxynaphthalene-1-sulphonic acid and dissolve by  trituration. 
4.6.2  Dissolve 90 g of anhydrous sodium sulphite metabis ulphite in 900 ml of water.  
Mix the two solutions (4.6.1) and (4.6.2) and dilut e to 1000 ml. Filter, if necessary, store the solut ion in a cool 
place away from direct sunlight and renew it every 15 to 20 days.   
4.7 Sodium chloride 
70 g/l solution.  
4.8 Silica standard solution corresponding to 0.500  g of SiO 2 per litre. 
In a platinum crucible, weigh, to the nearest 0.001  g:  
• either 0.500 g of silica (SiO 2) produced from  silicic acid (H 2SiO 3) calcined at 1,000 °C to constant  
mass and cooled in a desiccator;  
• or 0.500 g of pure quartz, finely ground and  previously calcined for 1 h at 1,000 °C and cooled in a  
desiccator.  
Add 5 g of anhydrous sodium carbonate to the crucib le.  Mix well, preferably with a platinum spatula, and f use  
carefully. Allow to cool, add warm water, heat mode rately  until completely dissolved, cool, transfer 
quantitatively to  a 1,000 ml one-mark volumetric flask, dilute to the  mark  and mix. Transfer immediately to a 
polyethylene bottle.  
1 ml of this standard solution contains 0.500 mg of  SiO 2. 
4.9 Silica, standard solution corresponding to 10 m g of SiO2 per litre. 
Take 20,0ml of the standard silica solution (4.8), transfer  into a 1,000 ml one-mark volumetric flask, dilute t o 
the  mark and mix.  
1 ml of this standard solution contains 0.010 mg of  SiO 2. DUS 1583-1: 2018 
© UNBS 2018  – All rights reserved  3
 Prepare this standard solution at the time of use.  
4.10  Phenolphthalein, 
 10 g/l solution in 95 % (V/V) ethanol.  
5  APPARATUS  
Ordinary laboratory apparatus and  
5.1  Spectrophotometar  or  
5.2  Photoelectric absorptiometer  
With 2 cm cells and fitted with suitable filters en suring a maximum transmission at about 795 nm.  
NOTE: If such filters are not available, operate at  about 680 nm, with 4 cm cells. 
6  PROCEDURE  
6.1  Test portion  
6.1.1   Weigh, to the nearest 0.01 g, in a 100 ml  polyethylene beaker, 13 ± 0.1 g of the solution A ( see 
4.3  of ISO 3195) stored in a silica-free vessel and con taining 40 g of the test sample in 1 000 ml. During  this 
operation, prevent the solution from coming into co ntact with glass. 
6.1.2  In addition, determine the mass, to the nearest 0.0 1 g, of 10.0 ml of the solution A, taken with the 
aid of a pipette or a burette, to enable the mass o f the test portion (6.1.1) to be converted to a vol ume when 
the results are calculated . 
6.2  Blank test  
Carry out a blank test at the same time as the  determination, following the same procedure and usi ng the  
same quantities of all the reagents used for the  determination (6.4) but replacing the 10.0ml of the  solution A 
by 10.0ml of the sodium chloride solution  (4.7).  
6.3  Preparation of calibration curve  
6.3.1   Preparation of the standard colorimetric solution , for photometric measurements with 2 cm cells at 
a wavelength of about 795 nm or with 4 cm cells for  photometric measurements at about 680 nm. 
Into a series of four 100ml polyethylene beakers, i ntroduce the volumes of the standard silica solutio n (4.9) 
shown in the following table: 
Standard silica solution(4.9), ml  Corresponding mass of SiO 2, mg 
0* 0 
2.0 0.02 
5.0 0.05 
10.0 0.10 
* Compensation solution 
 DUS 1583-1: 2018 
4 © UNBS 2018 – All rights reserved
  
Treat the contents of each beaker as follows.  
Add 10,0ml of the sodium chloride solution (4.7), d ilute to  25 ml and add, swirling after each addition, 7,5 ml  of 
the  hydrochloric acid solution (4.2), 20 ml of the bori c acid  solution (4.3) and 10 ml of the sodium molybdate 
solution  (4.5).  
The pH of the solution should now be about 1.1. Wai t  10 min and then add, swirling after each addition, 5 ml 
of  the oxalic acid solution (4.4) and 20 ml of the sul phuric  acid solution (4.1). Allow to stand for 2 min, then  
add 2 ml  of the reducing solution (4.6), transfer quantitati vely to a  100 ml one-mark volumetric flask, dilute to 
the mark and  mix.  
6.3.2  Photometric measurements 
After at least 10 min, but not more than 40 min, ca rry out  the photometric measurements using the Spectro-
photometer (5.1), at a wavelength of about 795 nm,  or the photoelectric absorptiometer (5.2), fitted w ith  
suitable filters, after having adjusted the instrum ent to  zero absorbance against the compensation solution.  
6.3.3  Preparation of the calibration chart  
Plot a graph having, for example, the numbers of mi lligrams  of silica (SiO 2) contained in 100 ml of standard 
colorimetric  solutions on the abscissa and the corresponding val ues of  absorbance on the ordinate. 
Note:   For some grades of sodium hydroxide, it may be ne cessary to adjust the range of calibration graph to  cover 
the expected silica content.  
6.4  Determination  
6.4.1  Colour development  
Add 1 drop of the phenolphthalein solution (4.10) t o the  test portion (6.1), already contained in a 100ml  
polyethylene beaker, and neutralize with the hydroc hloric  acid solution (4.2). Add 10 ml of water and, swirli ng 
after  each addition, 7.5ml of the hydrochloric acid solut ion  (4.2), 20 ml of the boric acid solution (4.3) and 1 0 
ml of  the sodium molybdate solution (4.5). Wait for 10 mi n then  add, swirling after each addition, 5 ml of the 
oxalic acid  solution (4.4) and 20 ml of the sulphuric acid solu tion  (4.1). Allow to stand for 2 min, then add 2ml 
of the  reducing solution (4.6), transfer quantitatively to  a 100ml  one-mark volumetric flask, dilute to the mark 
and mix.  
6.4.2  Photometric measurement  
After at least 10 min, but not more than 40 min, ca rry out  the photometric measurement according to the 
procedure  specified in 6.3.2, after having adjusted the instr ument to  zero absorbance against the blank test 
solution (6.2).  
7  EXPRESSION OF RESULTS  
By means of the calibration curve (6.3.3), determin e the  quantity of SiO 2 corresponding to the value of the  
photometric measurement.  
The silica content, expressed as milligrams of sili ca (Si0 2) per kilogram, is given by the formula 
5
2 03 1
0
321 10 1000 
10 1000 xmmmm
mx
mxmxm =  
where  DUS 1583-1: 2018 
© UNBS 2018  – All rights reserved  5
 m0 is the mass, in grams, of the test sample used to  prepare solution A;  
m1 is the mass, in milligrams, of SiO 2 found by the  determination;  
m2 is the mass, in grams, of the test portion (6.1.1) ;  
m3 is the mass, in grams, of 10.0 ml of solution A (s ee  4.3 of ISO 3195), determined by the procedure 
specified  in 6.1.2.  
8  TEST REPORT 
The test report shall include the following particu lars:  
a) the reference of the method used; 
b) the results and the method of expression used; 
c) any unusual features noted during the determinat ion; DUS 1583-1: 2018 
6 © UNBS 2018 – All rights reserved
 Bibliography 
[1] BS 6075: Part 4: 1981 Sampling and test for sod ium hydroxide for industrial use Part 4: Determinat ion 
of silica content 
. DUS 1583-1: 2018 
© UNBS 2018 – All rights reserved  7
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