Determination of Silica in Fluorspars by using Boric Acid

Fluorspar or Fluorite is a mineral composed of Calcium fluoride (CaF2), the principal fluorine-bearing mineral. It occurs as cubic, isometric crystals and cleavable masses.


Mineral Fluorspar compositions are as CaF2, CaCO3, SiO2 & other trace elements: Major components are CaF2 (60% to 90%) & SiO2 (2% to 30%).

Silica & Fluorine both are interfering radicals. In acid digestion method, silica in presence of fluorine gives the following reaction: 

F + SiO2 → SiF4↑+H2O

Hence SiO2 could not be estimated by acid digestion method in Fluorspars.

During Silica analysis in Fluorspars, initially, Fluorine has to be removed by using Boric acid (H3BO3) .

Boric acid reacts with fluoride as follows:  

H3BO3 + F   → H[BF4] ↑ + 3H2O 

Fluorine reacts with Boric acid & forms a complex (fluoroborate), it is gaseous in nature & evaporated  

Procedure :

  1. Approximately 1g (M) of prepared sample was taken into a 400 ml beaker. 1 g of boric acid and 10 to 15 ml water were added to it.
  2. The material was warmed to dissolve boric acid, cooled slightly and 20 ml of 70 percent perchloric acid was added. The mixture was heated to light fumes on moderate heat, covered and then heated strongly for 15 minutes. 
  3. 1 ml of hydrochloric acid was added in the cooled mass and 200 ml of water. Boiled for 10 minutes, filtered on a Whatman filter paper (No. 40 ) and washed thoroughly with hot water.
  4. The filter paper with the precipitate was taken in a weighed platinum crucible (M1), dried and ignited, first at a low temperature until the carbon was oxidized and finally at 950oC to 1000oC.
  5. The crucible as cooled in a desiccator and weighed (M2). Then 10 ml of hydrofluoric acid was added to it and evaporated. Evaporation was repeated with hydrofluoric acid to remove silica completely. Finally, the mass was ignited at 950oC – 1000oC for 15 minutes. Kept in a desiccator, cooled and weighed.
  6. Calculation: 
  • Silica (as SiO2), % by mass = (M2 – M1) × 100 / M
  • where, M1= Mass in g of the empty crucible, 
  • M2=Mass in g of the crucible and residue after ignition 
  • and M=Mass in g of the sample taken for analysis.

Results :

The experiment was carried out in triplicate by using 2 numbers of Certified reference Materials & one sample.  In all the cases, the same quantity of sample and the same amount of chemicals were used. The results were also compared with the declared value certificates of the Certified Reference Materials. The findings are tabulating as below and were found close to the certified values.  

Sl No. CRM -1

(NCS DC 28226 )


( JK- C )



1  17.95%  8.28%  15.32%
2  18.08%  8.32% 15.36%
3  18.12  8.14% 15.48%
Declared Value (%) 18.04 % 8.2%


Analysis of Silica in Fluorspar by the gravimetric method is a fast, accurate and less time-consuming method.  The reagents used are also less expensive. Moreover, the repeatability of the method is quite good with a low standard deviation. Consequently, this method is very much applicable to determine silica in Fluorspar.

Contributed by: Sukhenjit Datta