Determination of Silica in Alumino Silicate Refractory Materials

Silica in Alumino Silicate Refractory Materials

Determination of Silica in Alumino Silicate Refractory Materials


This SOPs covers a method for determination of silica in alumino silicate refractory materials.


The sample is decomposed by fusion mixture and extracted with acids. Silica is determined by dehydration and baking followed by hydrofluorization. The residual silica is determined photometrically and combined.


  • Platinum crucibles
  • Muffle Furnace (950- 1000 °C)
  • Balance
  • UV-Vis Spectrometer
  • porcelain dish
  • Water bath
  • Hot oven
  • Desiccator


  • Fusion mixture – Mix equal amount of sodium carbonate and potassium carbonate.
  • Potassium iodide – 1 percent (m/v). 
  • Dilute hydrochloric acid – 1:1, 1:3 and 1:20 (v/v). 
  • Dilute sulphuric acid – 1:1 (v/v). 
  • Methyl alcohol 
  • Concentrated hydrochloric acid (R.D. = 1.16) 
  • Hydrofluoric acid – 40 percent, 
  • Potassium bisulphate – Solid. 
  • EDTA solution (0.05 M) – Dissolve 18.611 g of EDTA (disodium salt) in 1 liter of water.
  • Phenolphthalein indicator solution – 0.1 percent (m/v) in ethanol.
  • Dilute ammonia solution – 1:1 (v/v). 
  • Ammonium molybdate solution – Dissolve 10 g of ammonium molybdate [ (NH4)6 /10,024.4Hs0] in 100 ml of water, filter if necessary. 
  • Oxalic acid solution -10% (m/v). 
  • Ascorbic acid solution – 10% (m/v) stored in an amber color bottle (stable up to 5 days). 
  • Standard silica solution 
  • a) Fuse 0.20 g of pure silica with 5.0 g of fusion mixture in a platinum crucible. Cool and dissolve the melt in 300 ml of water containing 20 g of sodium hydroxide in a polythene beaker. Warm the solution on a water bath and dilute to 500 ml in a volumetric flask. Store in a polythene bottle. 
  • b) Prepare immediately before use, a standard working solution containing 0.04 mg/ml of silica (SiO2) by diluting the above stock solution. 


  • Weight 1 gm approx. of finely ground test sample platinum crucible.
  • Add 6gm of fusion mixture followed by 2-3 drops of potassium iodide solution and mix well.
  • Heat the platinum crucible over a gas burner. Increase the temperature and keep it for 20 mins. Then cool it and observe that a transparent glass is obtained.
  • Transfer into a porcelain dish. Add Dil. HCL 1:1 then add 2 ml Dil. Sulphuric acid.
  • Add 10 ml of methanol and placed the dish on a Water bath. To remove boron trioxide by treating 2-3 times with 10 ml of methanol and evaporate the solution to dryness until the smell of HCL is no longer detected.
  • Finally, place the dish in a hot oven, dry it at 110°C. Cool the dish then add 10 ml of conc. HCL and 40 ml of hot water, stir it.
Determination of major silica by Gravimetric method- 
  • Cover the dish and allow the mixture to digest on a water bath for 10 mins. Use Whatman paper no. 42 for filtration then transfer the mixture to filter paper along with dil. HCL 1:20. Wash the precipitate 5 times with hot dil. HCL 1:20 then wash again with hot water.
  • Reserve the filtrate (Solution A).
  • Transfer the precipitated filtrate paper to platinum crucible which is ignited and weighted.
  • Ignite at low temperate until the precipitate is free from carbonaceous matter. Then heat in muffle furnace at 950- 1000 °C to constant mass. Cool the crucible in desiccator and weight it. 
  • Moisten the content with water. Add 5 drops of Dil. Sulphuric acid and 10 ml of hydrofluoric acid (HF). Evaporate to dryness on hot plate.

             NOTE: – Do not use Glass equipment for HF. 

  • Heat the crucible and residue cautiously first over gas flame and finally for 2 min at 950 – 1000 °C in the furnace. Cool in desiccator and weight to constant mass.
Preparation of main solution
  • Remaining residue (HF). Add 1 gm of potassium bisulphate in platinum crucible. Head the crucible over a gas burner for 5 mins. Dissolve the melt in water containing a few drops of concentrated hydrochloric acid and add the solution to the main filtrate (solution A). Cool and dilute to 500 ml in a volumetric flask.
Determination of Residual Silica by Photometric method-
Preparation of calibration solution
Prepare calibration solution from standard solution (1ml= 40µg SiO2)
Vol. from Std. Sol. Conc. (in 100ml)   Conc. In sample
1ml                         0.4 µg                    40mg/l
2ml                         0.8 µg                    80mg/l
3ml                         1.2 µg                   120mg/l
4ml                         1.6 µg                   160mg/l
6ml                         2.4 µg                   240mg/l
  • Take 0 ml ,1ml,2ml,3ml,4ml and 6ml of standard solution in six 100ml volumetric flask and take 10 ml of main solution in 100ml of volumetric flask.
  • Add 5ml EDTA solution followed by a drop of phenolphthalein indicator and then add dil. Ammonium solution until the color changes to red.
  • Add 5ml of ammonium molybdate sol. And allow to stand for 10 mins.
  • Add 10ml of oxalic acid followed by 5ml of ascorbic acid and make up100 ml with D/W.
  • Allow the solution to stand for half an hour until proper color is reached. Measure the absorbance at 810nm against a blank solution.
Silica (in g) = Abs. × final vol. × GCF / vol. of sample× Slope×1000


  Total silica (as SiO2), % – {(M1 – M2) + M3 / M} × 100
M1, = mass in g of the crucible and the impure silica before hydrofluoric acid treatment, 
M2, = mass in g of the crucible and residue after hydrofluoric acid treatment, 
M3 = mass in g of residual silica present in 500 ml of the main sample solution A, and 
M = mass in g of the sample taken.
Reference Standard: IS 12107: 1987 part -2

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