ULTRASENSITIVE DETERMINATION OF
SILICATE IN PROCESS AND BOILER WATER USING RAPID PHOTOMETRIC TESTS
Economic effi ciency is becoming an ever more important aspect of everyday life, with the effi ciency of industrial plants and equipment constituting one of the basic preconditions for sustainable economic operations. One avoidable problem that can result in losses of effi ciency is posed by the undesirable build-up of deposits, i.e., scale – in pipes, boilers, and turbines.
A principal cause of scale in such equipment is silicate. Especially at high pressures – as in high-pressure turbines – silicate is deposited on the internal surfaces. This problem occurs mainly as a result of silicate dissolved in the steam.1
The expansion of the steam results in a reduction of the solubility capacity of silicate, which in turns leads to the formation of solid silicon dioxide on the surrounding surfaces, for example the turbine blades, reducing the effi ciency of the plant.2
One measure that can help to minimise the need for time- consuming cleaning operations that interrupt the operation of the machine is to regularly inspect the boiler and boiler feed water for their silicate concentrations.
The guidance values depend on a variety of operating conditions (e.g., steam capacity, heating-surface load, and working pressure) of the boiler. In high-pressure turbines, even the slightest concentration of silicate in the steam can lead to deposits. To avoid such deposits, in most cases it is recommended to prevent steam silicate concentrations from exceeding 20 µg/L SiO2
.3,4 or less.2 Depending
on the operating conditions, the limit for silicate may even be as low as 10 µg/L SiO2
Analytical Methods
The determination of concentrations of silicate in such a low range requires an extremely sensitive detection method. Graphite furnace atomic absorption spectrometry (GF-AAS) is frequently the method of choice here, capable of detecting concentrations of silicate down into the lower ppb range. Besides the element- analytical methods, classic photometry has also proven to be a reliable method.
This method is based on the reaction of silicate ions in acidic solution with molybdate ions to produce yellow silicomolybdic acid. The addition of a suitable reduction agent then produces deep blue silicomolybdenum blue, which is subsequently determined photometrically.5
Silicate Test Kit
The molybdenum blue method is also the principle used in the photometric silicate test (Cat. No. 101813) of our Spectroquant® test kit series.
The advantage of this test is that it is quick and easy to use without major instrument investment needed. All necessary reagents are supplied in the test kit in a ready-to-use format. Compared with classic photometry, the use of the corresponding Spectroquant®
photometers enables the time-consuming
calibration procedure to be dispensed with, since the method is already pre- programmed into the devices. Using the 100-mm
Performance of the Measurement with Spectroquant®
Silicate Test
The silicate content of process-water samples lies within the lower part of the measuring range of the test kit. During the course of the experiments it was found that the precision in the lower part of the measuring range can be enhanced when reagents Si-1 and Si-2 are added with a pipette instead of dropwise.
The procedure for the tests was correspondingly adapted. Moreover, the procedure described in the instruction sheet enclosed with the product has been changed from dropping
Table 1: Recovered content of silicate Sample
Ultrapure water Steam water from power plant Boiler water from power plant DI water Double-distilled water
Addition [µg/L SiO2 1.00
5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 ]
cell, the Prove 600 spectrophotometer is capable of measuring silicate concentrations as low as 0.25 µg/L SiO2
, thus ensuring
the detection of extremely low amounts of dissolved silicate. The overall measuring range of the test kit is 0.25–500.0 µg/L SiO2
.
to pipetting to ensure the highest possible accuracy. Care was also taken to ensure that no glass equipment was used during the entire procedure. In the event of any turbidity of the sample solution, this must be fi ltered beforehand.
The silicate test starts by pipetting 20 mL of sample solution into a plastic test vessel, after which 200 µL of reagent Si-1 is added. The solution is mixed and then left to stand for 5 minutes. After the standing time, 200 µL reagent Si-2 is added and the solution mixed, then 1.00 mL of reagent Si-3 is added. The solution is mixed once again, left to react for 5 minutes, and then measured in the photometer against a reagent blank prepared with Ultrapure water in an analogous manner.
A detailed description of the procedure is described in the application “Ultrasensitive determination of silicate in process and boiler water”. The application can be found online on the product page for Spectroquant®
Silicate Test 101813. Figure 1. Spectroquant® Prove 600 photometer
Standard Addition with Spectroquant®
Silicate Test
In an experiment to gain an expressive statement on the suitability of the Spectroquant®
Silicate Test for the determination of the
silicate content in process water, the standard addition method was applied to fi ve samples. Each sample was spiked with three different concentrations of silicate. In order to determine the recovered silicate concentration, the silicate concentration of the sample, also gained using the Silicate Test, was subtracted from the measured result of the spiked sample. For evaluation, the deviation of the recovered concentration from the target value (spiked concentration) was calculated. The results are shown in Table 1.
Recovered concentration [µg/L SiO2 0.86
6.25
10.60 1.83 6.14
11.01 1.24 6.09
10.20 1.97 5.74
11.31 1.75 7.40
11.53 ]
Deviation [µg/L SiO2 0.14
1.25 0.60 0.83 1.14 1.01 0.24 1.09 0.20 0.97 0.74 1.31 0.75 2.40 1.53
]
OCTOBER / NOVEMBER •
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