Calibration method for water hardness measurement

• US 10,317,385 B2
• Filed: 07/01/2016
• Issued: 06/11/2019
• Est. Priority Date: 07/01/2015
• Status: Active Grant

• Current Assignee: Ecolab USA Incorporated (Ecolab, Inc.)
• Original Assignee: Ecolab USA Incorporated (Ecolab, Inc.)
• Inventors: Li, Hui, Knoth, Alexandra, Schwartz, Joe, Wilson, Christopher Brant, Kahaian, Arthur
• Primary Examiner(s): Wallenhorst, Maureen
• Application Number:
  US15/200,728
• Publication Number:
  US 20170003261A1
• Time in Patent Office: 1,075 Days
• Field of Search: 436 8, 436 19, 436 73, 436 79, 436164, 436166, 436172, 422 8205, 422 8208
• CPC Class Codes: C09B 11/24 Phthaleins containing amino groups ; Phthalanes; Fluoranes; Phthalides; Rhodamine dyes; Phthaleins having heterocyclic aryl rings; Lactone or lactame forms of triarylmethane dyes
• CPC Class Codes: C09B 67/0083 Solutions of dyes
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/278 Constitution of standards
• CPC Class Codes: G01N 21/6428 Measuring fluorescence of fluorescent products of reactions or of fluorochrome labelled reactive substances, e.g. measuring quenching effects, using measuring "optrodes" in vivo A61B5/00; immunoassay G01N33/53
• CPC Class Codes: G01N 21/643 non-biological material
• CPC Class Codes: G01N 21/645 Specially adapted constructive features of fluorimeters
• CPC Class Codes: G01N 2201/127 Calibration; base line adjustment; drift compensation
• CPC Class Codes: G01N 33/1813 Specific cations in water, e.g. heavy metals
• CPC Class Codes: G01N 33/1853 Hardness of water

Abstract

Disclosed are methods that can be used to automatically calibrate a fluorescence-measuring instrument capable of continuously measuring the concentration of hardness in process water. The calibration method is used to compensate for drift of instrument and equipment, changes of operational conditions, and contamination of reagents and calibration standards. Calibration of the fluorescence-measuring instrument using the present calibration method improves both accuracy and repeatability for subsequent hardness concentration measurements.

Claims

1. A method of calibrating a fluorescence-measuring instrument for soluble magnesium, the method comprising:

measuring background fluorescence of a low calibration standard solution comprising soluble magnesium at a known concentration;

combining a magnesium-reactant solution comprising an inert fluorescing agent at a known concentration and a magnesium coordinating fluorescing reagent with the low calibration standard solution, thereby forming a combined low calibration standard solution comprising the inert fluorescing agent and a coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the combined low calibration standard solution;

measuring fluorometric response of an inert fluorescing agent in a high calibration standard solution comprising both soluble magnesium and inert fluorescing agent at known concentrations;

combining the magnesium-reactant solution with the high calibration standard solution, thereby forming a combined high calibration standard solution comprising inert fluorescing agent and a coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the combined high calibration standard solution; and

calibrating the fluorescence-measuring instrument based on the measured fluorometric responses and the known concentrations of soluble magnesium and inert fluorescing agent in the low calibration standard solution and in the high calibration standard solution.

2. The method of claim 1, wherein the fluorescence-measuring instrument is calibrated by plotting a calibration curve for magnesium and inert fluorescing agent based on the measured fluorometric responses and known concentrations of soluble magnesium and inert fluorescing agent in the low calibration standard solution and in the high calibration standard solution and the calibration curve is used to calibrate the fluorescence-measuring instrument.

3. The method of claim 2, wherein an equation of the calibration curve is stored by a controller and the equation is used to automatically calibrate the fluorescence-measuring instrument.

4. The method of claim 1, wherein the fluorescence-measuring instrument is flushed with the low calibration standard solution or the high calibration standard solution prior to each measuring step.

5. The method of claim 1, wherein the low calibration standard solution comprises soluble magnesium at a concentration of from about 50 ppb to about 200 ppb, and the high calibration standard solution comprises soluble magnesium at a concentration of from about 300 ppb to about 900 ppb, and the inert fluorescing agent at a concentration of about 1 ppb to about 20 ppb.

6. The method of claim 1, wherein the inert fluorescing agent is Rhodamine WT.

7. The method of claim 1, wherein the magnesium coordinating fluorescing reagent is 8-hydroxyquinoline-5-sulfonic acid.

8. The method of claim 1, wherein the low and high calibration standard solutions have a soluble magnesium concentration difference of from about 50 ppb to about 850 ppb.

9. The method of claim 1, wherein the low calibration standard solution has a concentration of soluble magnesium of from about 50 ppb to about 200 ppb.

10. A method of calibrating a fluorescence-measuring instrument for soluble magnesium and total hardness, the method comprising:

measuring background fluorescence of a low calibration standard solution comprising soluble magnesium and soluble calcium each at known concentrations;

combining a magnesium-reactant solution comprising an inert fluorescing agent at a known concentration and a magnesium coordinating fluorescing reagent with the low calibration standard solution, thereby forming a combined low calibration standard solution comprising the inert fluorescing agent and a coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the combined low calibration standard solution;

combining the magnesium-reactant solution and a total hardness-reactant solution comprising a magnesium-containing reagent with the low calibration standard solution, thereby forming a calcium-displaced low calibration standard solution comprising the inert fluorescing agent and the coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the calcium-displaced low calibration standard solution in order to measure total hardness;

measuring fluorometric response of an inert fluorescing agent in a high calibration standard solution comprising soluble magnesium, soluble calcium, and the inert fluorescing agent, each at known concentrations;

combining the magnesium-reactant solution with the high calibration standard solution, thereby forming a combined high calibration standard solution comprising the inert fluorescing agent and a coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the combined high calibration standard solution;

combining the magnesium-reactant solution and the total hardness-reactant solution with the high calibration standard solution, thereby forming a calcium-displaced high calibration standard solution comprising the inert fluorescing agent and the coordinated magnesium compound;

measuring fluorometric responses of the inert fluorescing agent and the coordinated magnesium compound in the calcium-displaced high calibration standard solution in order to measure total hardness; and

calibrating the fluorescence-measuring instrument based on the measured fluorometric responses, the known concentrations of soluble magnesium and inert fluorescing agent, and the measurements of total hardness in the low calibration standard solution and in the high calibration standard solution.

11. The method of claim 10, wherein the fluorescence-measuring instrument is calibrated by plotting a calibration curve for soluble magnesium, total hardness, and the inert fluorescing agent based on the measured fluorometric responses and known concentrations of soluble magnesium, soluble calcium, and inert fluorescing agent in each of the low calibration standard solution and in the high calibration standard solution.

12. The method of claim 11, wherein an equation of the calibration curve is stored by a controller and the equation is used to automatically calibrate the fluorescence-measuring instrument.

13. The method of claim 10, wherein the fluorescence-measuring instrument is flushed with the low calibration standard solution or the high calibration standard solution prior to each measuring step.

14. The method of claim 10, wherein the low calibration standard solution comprises soluble magnesium at a concentration of from about 50 ppb to about 200 ppb and soluble calcium at a concentration of from about 50 ppb to about 200 ppb;

and the high calibration standard solution comprises soluble magnesium at a concentration of from about 300 ppb to about 900 ppb, soluble calcium at a concentration of from about 300 ppb to about 900 ppb, and the inert fluorescing agent at a concentration of about 1 ppb to about 20 ppb.

15. The method of claim 10, wherein the inert fluorescing agent is Rhodamine WT.

16. The method of claim 10, wherein the magnesium coordinating fluorescing reagent is 8-hydroxyquinoline-5-sulfonic acid.

17. The method of claim 10, wherein the magnesium-containing reagent is magnesium (1,2-diaminocyclohexanetetraacetic acid disodium salt).

18. The method of claim 10, wherein the low and high calibration standard solutions have a soluble magnesium concentration difference of from about 50 ppb to about 850 ppb.

19. The method of claim 10, wherein the low and high calibration standard solutions have a soluble calcium concentration difference of from about 50 ppb to about 850 ppb.

20. The method of claim 10, wherein the low calibration standard solution has a concentration of soluble magnesium of from about 50 ppb to about 200 ppb.