HK1087177B - Method of calibrating the zero point of an apparatus used to determine a quantity of silica using a colorimetric method - Google Patents

Description

Method for zero calibration of a device for determining the amount of silica by colorimetry

The invention relates to a method for zero-point calibration of a device for colorimetrically determining the amount of silica.

It is known that the measurement of silica dissolved at low concentrations is an extremely important measurement, especially in the field of power generation and in the semiconductor industry.

In fact, the silica present in water may precipitate when the water vapor expands on the turbine of a power plant, or in some stages of processing disks or "wafers" when preparing semiconductors.

In addition, the presence of silica in the demineralized water can also serve as an indicator of the termination of the action of the deionizing resin. This presence of silica always precedes the salting out of monovalent ions, in particular sodium ions, chloride ions.

The determination of the amount of silica in solution can be achieved by colorimetric methods: for this purpose, complexes are produced which are specific for silica and which can be measured and exhibit their own color.

The light emitted into the solution is absorbed by the resulting complex in proportion to its concentration according to Beer-Lambert's law:

I＝I₀exp(-kLC) (1)

in the formula:

-I is a measurement of the amount of light received through the solution;

-I₀is the amount of light emitted into the solution;

-L is the length of the optical path traversed by the light;

-C is the concentration of the complex produced;

-K is a constant related to the molar extinction coefficient of the measuring device and the solution to be analyzed.

The Beer-Lambert law expresses the correlation between the concentration and the measured value of the light amount absorption. In other words, the law states that the concentration is linked to the light quantity absorption measurement value by a linear law.

C＝C₀+K.log(I₀/I) (2)

In the formula:

-C₀referred to as the measurement zero;

-K is the measurement slope;

-I₀is an emitted light intensity measurement;

-I is a measurement of the intensity of light received through the solution.

As can be seen from the above relation (2), in order to determine the coefficients of a linear equation that relates concentration to optical measurements of light absorption, two methods can be used:

the slope is determined by measuring the light absorption of a solution containing the reagents required for the measurement but without silica to determine the zero point and then measuring the light absorption of a highly concentrated silica solution within the measurement range of the apparatus.

-measuring the light absorption of two solutions of known silica concentration.

Both methods require knowledge of the initial silica concentration of the water used to prepare the calibration solution in order to calculate the zero coefficient and slope. And the concentration is unknown. However, the following two linear equations (3) and (4) should be able to calibrate the silica measurements:

C₁+X＝C₀+K.log(I₀/I₁) (3)

C₂+X＝C₀+K.log(I₀/I₂) (4)

wherein:

-I₀is a measure of the amount of light that varies with the solution present in the measurement cell;

-C₁is the known concentration of calibration solution 1;

-C₂is the known concentration of calibration solution 2;

-I₁is along with the cell in the calibration processThe amount of light measurement as a function of the presence of calibration solution 1;

-I₂is a measure of the amount of light that varies with the presence of calibration solution 2 in the cell during calibration;

x is the unknown concentration of the preparation water for solutions 1 and 2 of low silica concentration.

The two linear equations (3) and (4) comprise three unknowns C₀K and X, and thus cannot be solved.

It is not possible to easily know the actual slope and zero point of the measurement by simple preparation of the solution prepared as described above.

Moreover, other known simple methods of determining low concentration silica solutions do not provide an absolute measure of silica concentration and have the exact same disadvantages.

Among the methods for carrying out the calibration of the silica determination, the following two methods may be mentioned in particular.

According to the first method, the residual silica in the diluted water can be measured by mass spectrometry (ICP-MS: inductively coupled plasma mass spectrometry): this method is difficult to implement in industrial on-line measurements and seems economically unusable.

According to a second method, the slope is determined by measuring a highly concentrated solution for which the errors relating to the preparation of the solution and to the zero point of measurement are negligible before its concentration. To this end two calibration solutions are prepared, wherein the more concentrated solutions are obtained by pre-concentrating (pre-concentration ratio N of at least 20) the original solution by means of controlled microwave heating and evaporation. Thus, the concentrations of the two solutions are X and N.X, where N is the pre-concentration ratio, which is equal to the volume ratio before and after evaporation. Two linear equations (5) and (6) are thus obtained:

X＝C₀+K.log (I₀/I₁) (5)

N.X＝C₀+K.log (I₀/I₂) (6)

the parameters have the same values as the relations (3) and (4).

This method allows solving linear equations (5) and (6) and deriving therefrom the zero point of the device, K being considered known. However, this method has the disadvantage of having an extremely long response time (the pre-concentration process should be slow and controlled) and has not been applicable in industrial media and process control.

It is therefore an object of the present invention to provide a method for zero calibration of an apparatus for colorimetrically determining the amount of silica, which enables such calibration to be carried out within an acceptable period of time.

It is a further object of the invention to provide such a method which is simple to implement.

These objects, as well as others which will be apparent from the following, are achieved by a method for calibrating the zero point of an apparatus for colorimetrically determining the amount of silica contained in a sample of a silica solution to be analyzed, comprising the successive addition to the sample of a molybdate solution, a developer and a reagent, said method being characterized in that, according to the invention, the successive addition to the sample of a silica solution to be analyzed is a developer, then a molybdate solution and finally a reducing agent.

In the apparatus for analyzing silica contained in a solution, a sample to be analyzed is circulated in a rapid loop so that the sample can be quickly renewed. The regulation of the flow is achieved by means of a needle valve. At the start of the analysis, the sample was added to the cell using an electrically operated valve. Then a molybdate solution is added which reacts with the silica contained in the sample: thus, a silicomolybdic acid complex was obtained. The reaction time was longer (about 300 s).

A color developer such as oxalic acid is then added to avoid interference from phosphates and to develop and enhance the color of the silicomolybdic acid complex.

This silicomolybdic acid complex is finally reduced to a blue molybdenum complex using ferrous ions.

Photometric measurement of light absorption is carried out at the end of the reaction.

The dissolved silica in the solution is present in the form of salicylic acid or different types of silicates. Thus, the acidic molybdate reacts with the silica in solution to form a yellow silicomolybdic acid complex, which can be detected by measuring the light absorption to detect a few mg/l [ ppm: parts per million. After reduction, the yellow compound turned into a blue complex, which allowed a more sensitive detection at μ g/l [ ppb: silica concentration on the parts per billion level.

To calibrate the zero point of the apparatus for carrying out the above-described analytical method, oxalic acid (developer), then the molybdate solution and finally the reducing agent are added to the sample of the solution to be analyzed.

Thus, the method of the invention may:

compensation of the light absorption due to the coloration and possible turbidity of the reagent,

compensation of the light absorption due to the blue silicomolybdic acid complex formed by the silica contained in the molybdate solution,

avoiding the silica contained in the water from reacting and forming blue silicomolybdic acid complexes when prepared for calibrating the zero point.

The tests carried out show that the light absorption measurement I is invariably about 1/1000 for solutions containing 0.5. mu.g/1-200. mu.g/l of dissolved silica. These experiments indicate that the reaction to produce silicomolybdic acid compounds is not likely to occur.

This reference measurement allows to calculate the zero point of the device and independently of the solution concentration of dissolved silica at which the measurement is made, which is then linked to the above pre-concentration method: the measured values obtained by both methods are identical.

The following examples of zero determination are provided to allow those skilled in the art to better understand the practice of the method of the present invention.

Examples

The sample present in the overflow container (r. Servorira. d. Bordetement) is added to a measuring cell having a volume of approximately 8.5 ml. Then 250. mu.l of a solution of 40g of oxalic acid 2H2O was added.

The mixture thus obtained is kept under stirring at all times to homogenize it as quickly as possible: the measuring cell, made of a heat-conducting material, is kept at a constant temperature of 25C.

A first light absorption measurement is made in the cell approximately two minutes after the addition of oxalic acid: the measured value I₀Is a measure of the intensity of the emitted light in the Beer-Lambert equation described above.

Subsequently 250. mu.l of a molybdate solution are added, the composition of 1 liter being:

sodium molybdate, 4H2O 35g

Sodium hydrogen sulfate, 1H2O 80g

Sodium hydrogen sulfate, anhydrous 70g

25g of concentrated sulfuric acid

Then, after one minute, a reducing agent was added, the composition of 1 liter of which was:

12.5g of concentrated sulfuric acid

20g of a molarity salt (ferrous ammonium sulfate, 6H 2O).

A second measurement I of the light absorption is carried out one minute after the addition of the reducing agent.

The zero point of the measuring apparatus can be determined by applying Beer-Lambert's formula indicating the relationship between the silica concentration in the sample and the measured value of the light amount absorption.

Calibration of the zero point can be performed according to the same method but using other reagents used when colorimetrically measuring silica. Thus, in a colorimetric method using a molybdate salt, a mixture of citrate salts, followed by a reducing agent such as aminonaphthol sulfonic acid, the determination of the zero point according to the method of the invention is carried out by adding the mixture of citrate salts, molybdate salt and then reducing agent to the sample.

Claims (7)

1. Method for calibrating the zero point of an apparatus for colorimetrically determining the amount of silica contained in a sample solution, comprising the successive steps of adding a molybdate solution, a developer solution and a reducing agent to the sample, followed by a light absorption measurement, the zero point calibration method comprising the steps of:

a) obtaining a sample to be analyzed;

b) introducing the developer solution into the sample;

c) obtaining a first absorption measurement of the sample;

d) introducing the molybdate solution into the sample;

e) introducing a reducing agent into the sample;

f) obtaining a second absorption measurement of the sample; and

g) the zero point of the device is calculated using the first and second absorption measurements.

2. The method of claim 1, wherein the developer solution comprises a solution comprising an organic acid.

3. The method of claim 2, wherein the organic acid is oxalic acid.

4. The method of claim 2, wherein the organic acid comprises citric acid.

5. The method of claim 1, wherein the molybdate solution comprises a mixture of sodium molybdate, sodium bisulfate and sulfuric acid.

6. The method of claim 1, wherein the reducing agent comprises sulfuric acid and ferrous ammonium sulfate.

7. The method of claim 1, wherein the reducing agent comprises aminonaphthol sulfonic acid.