Analytical Method: Ultrasensitive determination of silicate in process and boiler water

Method: Photometric determination by silicomolybdenum blue analogous to APHA 4500- SiO2 D+E and DIN 38 405-21 and ASTM D859-10

Materials and reagents:

Reagents
Cat. No. 101813 – Spectroquant® Silicate (Silicic Acid) Test
Cat. No. 101262 – Water Ultrapur
Cat. No. 109138 – Sodium hydroxide solution 0.5 mol/L Titripur®
Cat. No. 112310 – Silicium standard solution Certipur®

Instruments
Cat. No. 173018 – Spectroquant® UV/VIS Spectrophotometer Prove 600

Materials
Cat. No. 174011 – Rectangular cell 100 mm
Cat. No. XX1102012 – Plastic Syringes, 20 mL, Luer slip, polypropylene
Cat. No. SLGP033NS – Syringe filter holder Millex-GP Filter, 0.22 μm, PES 33 mm, non-sterile
Preparation vessels Volumetric flasks and sample beakers, plastic
Pipettes Precicion pipettes with plastic tips for 200 μL, 1000 μL, and 20 mL

Measuring range:

0.25 – 250.00 μg/L SiO2

Working instructions:

Sample preparation:
It the solutions shows any signs of turbidity, filter the sample over the above mentioned syringe filter holder into a clean and dry plastic vessel.

Conditioning of the working materials:
If disposable plastic materials are not available for use (e.g. disposable syringes for piston pipettes; 50-mL centrifuge tubes), the materials should first be rinsed free of silicate with sodium hydroxide solution 0.5 mol/L and subsequently washed until pH-neutral with Ultrapur water.

Measurement procedure:
Determine the SiO2 content with the above mentioned test kit.
In deviation from the instructions for use for the test kit, to enhance the precision reagents Si-1 and Si-2 are not added by dropping, but instead using a pipette.
For a sample volume of 20 mL, the corresponding volumes of 200 μL of reagent Si-1 and 200 μL of reagent Si-2 and 1.00 mL of reagent Si-3 are pipetted into the sample.
The measurement is carried out against a blank, prepared from Ultrapur water and the reagents in an analogous manner. The reaction times A and B remain unchanged.
After measurement, the result can be read off from the instrument.

Measurement results:

Comparison of results of the Spectroquant® Silicate Test 101813 with graphite-tube AAS reference analysis:
 

  Concentration [μg/L SiO2]
Sample Spectroquant®
Silicate Test 101813
GF-AAS
Ultrapur water < 0.25 < 1.93
Steam water from power plant 3.26 2.25
Boiler water from power plant 4.85 3.66
DI-water 0.29 < 1.93
Double-distilled water 0.77 < 1.93


Standard addition with the Spectroquant® Silicate Test 101813
 

Sample Addition
[μg/L SiO2]
Recovered concentration
[μg/L SiO2]
Ultrapur water   1.00
5.00
10.00
0.86
6.25
10.60
Steam water from power plant   1.00
5.00
10.00
1.83
6.14
11.01
Boiler water from power plant   1.00
5.00
10.00
1.24
6.09
10.20
DI water 1.00
5.00
10.00  
1.97
5.74
11.31
Double-distilled water   1.00
5.00
10.00
1.75
7.40
11.53


Note:

If the accuracy of the method is to be increased further, we recommend carrying out an own calibration for a limited measuring range. We recommend the performance of the calibration according to the ISO 8446-1 method.
For this purpose a user-defined method can be programmed on the Prove 600. For details see Prove Manual chapter 9.6 “User-defined Concentration Methods.”


By recording a 10-point calibration curve for the measuring range 0.50 – 25.00 μg/L SiO2 the following performance characteristics could be achieved:

 

 

  User-defined calibration
0.50 - 25.00 μg/L SiO2
Method standard deviation [μg/L]  
± 0.185
Method coefficient of variation [%] ± 1.44
Confidence interval (P = 95 %) [μg/L]   
± 0.45

All chemicals used for the calibration are mentioned in the section “Reagents.”

Materials

     
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