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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  Dissolved Silca

Dissolved Silca

Silica is an important nutrient for diatoms. Below levels of approximately 0.5mg/L most diatoms loose the ability to reproduce effectively. This method suitable for concentrations below 100mg/L.


Water samples for silica analysis should be collected in clean polythene bottles, and filtered on site with non-glass filters (e.g. cellulose nitrate 0.5µm pore size). The filtrate is relatively stable but should be analysed within 28 days. Samples can be frozen but this converts the silica to an unreactive form; it is therefore necessary to allow thawed samples to stand at room temperature for one or two hours before analysis. It is preferable to keep samples refrigerated.


In solution silica exists as either silicic acid (H4SiO4) or silicate (SiO32-). This reacts with acidic ammonium molybdate to form a yellow silico-molybdate complex, which on reduction with sodium sulphite forms a molybdate blue colour. This is then compared to standards of known concentration using a spectrophotometer at 700nm in a 1cm cell.


  1. Hydrochloric acid, 0.25N: mix 22ml of conc. HCl (sp. gr. 1.18) with water and make up to 1 litre
  2. Ammonium molybdate, 5%: dissolve 5.2g (NH4)6Mo7O24.4H2O in water, dilute to 100ml
  3. Disodium EDTA, 1%: dissolve 1g of disodium EDTA in water and dilute to 100ml
  4. Sodium sulphite, 17%: dissolve 42.5g of sodium sulphite in water and dilute to 250ml (i.e. 170g in 1 litre)
  5. Silica standard: stabilise approximately 30g of Na2SiO3.5H2O by placing it in a desiccator for 3 hours. Dissolve 17.65g of the stable compound in water and dilute to 1 litre. Pipette 10ml of this solution into a volumetric flask and make up to 1 litre with deionised water.

1.00ml = 0.050mg SiO2

General Procedures

All samples (including blanks, but not standards) should be analysed in duplicate.

  1. Pipette 10ml of each sample into 50-100ml Erlenmeyer flasks.
  2. Pipette 10ml of deionised water into a flask to serve as a reagent blank.
  3. Make up a series of standards from the stock solution. Make up each standard to 10ml with deionised water:
  4. 0.5ml = 0.025mg/10ml = 2.5mg/L
  5. 1.0ml = 0.05mg/10ml = 5.0mg/L
  6. 2.0ml = 0.10mg/10ml = 10.0mg/L
  7. 3.0ml = 0.15mg/10ml = 15.0mg/L
  8. 4.0ml = 0.20mg/10ml = 20.0mg/L
  9. 8.0ml = 0.40mg/10ml = 40.0mg/L
  10. Add 5ml of reagent a. to each flask; swirl.
  11. Add 5ml of reagent b. to each flask; swirl.
  12. Add 5ml of reagent c. to each flask; swirl.
  13. 5 minutes after the addition of the molybdate (b) add 10ml of reagent d.; mix and allow to stand for approximately 30 min. The colour is stable for several hours after this time. (NB. Do not use rubber stoppers to close the flasks)
  14. Using a wavelength of 700nm and a 1cm cell measure the absorbance of the samples and standards against a deionised water blank.
  15. Silica concentration can then be determined from a plot of the absorbancies of the standards against their known concentrations.

If refrigerated all samples should be analysed within 28 days of collection.