PHOTOELECTRIC ENDPOINT DETECTION

• US 3,723,062 A
• Filed: 12/21/1970
• Issued: 03/27/1973
• Est. Priority Date: 12/13/1967
• Status: Expired due to Term

• Current Assignee: Harald Dahms
• Original Assignee: Harald Dahms
• Inventors: Dahms, Harald
• Primary Examiner(s): Wolk, Morris O.
• Assistant Examiner(s): Marantz, Sidney
• Application Number:
  US05/100,306
• Time in Patent Office: 827 Days
• Field of Search: 23/23R,253R,253A 356/180,184,186,195,206 250/218
• US Class Current: 436/51
• CPC Class Codes: G01N 31/16 using titration
• CPC Class Codes: G01N 31/221 for investigating pH value
• CPC Class Codes: G01N 33/84 involving inorganic compounds or pH
• CPC Class Codes: Y10T 436/116664 with automated titrator

Abstract

A method and apparatus useful for colorimetric titration utilizes an indicator which changes from one colored form to another at or near an endpoint, and an optical endpoint detection system which generates signals responsive to the relative concentrations of the different colored forms of the indicator. The endpoint is indicated when the ratio of the logarithms of the signals reaches a predetermined value.

Claims

2. In a device for automatic titration of a liquid containing an indicator substance adapted to change between different forms having different light absorption characteristics over a color change interval indicative of a predetermined endpoint, said device comprising means for introducing titrant ions into a liquid to be titrated, means for detecting the endpoint, means for measuring the amount of titrant ions required to achieve the endpoint, the improvement wherein the means for detecting the endpoint comprises:

means for directing light through the liquid;

means for providing first and second light beams, the intensity of each light beam being colorimetrically responsive to one form of the indicator;

light detecting means for generating first and second signals corresponding to the intensities of the first and second light beams;

means for generating a third signal corresponding to the ratio of the logarithms of the first and second signals;

means responsive to the third signal generating means for detecting when the third signal reaches a predetermined value corresponding to the endpoint; and

means responsive to said third signal detecting means for signalling to the measuring means that the endpoint has been reached.

3. The device of claim 2 wherein the means for supplying the titrant ions includes a constant current source for electrolytically generating titrant ions in the liquid, wherein the titrant measuring means includes a timer connected to the current source, and wherein the means for signalling that the endpoint has been reached includes means for deactivating the timer and the constant current source.

4. The device of claim 2 wherein the means for generating the third signal comprises logarithmic amplifier means connected to the light detector means for providing outputs proportional to the logarithms of the first and second signals, and ratiometer means connected to the logarithmic amplifier means for providing an output signal proportional to the ratio of the logarithms of the first and second signals.

5. The device of claim 4 wherein the liquid to be titrated is an acidic solution of an acid-base indicator, wherein the means for supplying the titrant ions includes means for passing an electrolysis current through the liquid to generate hydroxyl ions therein, and wherein the measuring means comprises means for measuring the quantity of electricity carried by said titrant ion supplying means.

6. A method useful for colorimetric titration of a liquid having different light absorption characteristics on different sides of a color change interval indicative of a predetermined endpoint, comprising:

introducing a titrant into the liquid;

passing light through the liquid during the introduction of titrant;

generating a first electrical signal corresponding to the intensity of light passed through the liquid at a first wave length corresponding to the light absorption characteristics of the liquid at one side of the color change interval;

generating a second electrical signal corresponding to the intensity of the light passed through the liquid at a second wavelength corresponding to the light absorption characteristics of the liquid at the other side of the color change interval;

generating a third electrical signal corresponding to the ratio of the logarithms of the first and second signals;

continuing the introduction of titrant until the third signal reaches a predetermined value corresponding to said endpoint; and

measuring the quantity of titrant introduced into said liquid prior to the time said third signal reaches said predetermined value.

7. The method of claim 6 wherein the third signal is generated by generating signals corresponding to the logarithms of the first and second signals, and generating a rAtio signal corresponding to the ratio of the logarithmic signals.

8. A method useful for colorimetric titration of a liquid acidic solution of an acid-base indicator having different light absorption characteristics on different sides of a color change interval indicative of predetermined acid-base endpoint, comprising:

passing an electrolysis current through said liquid to generate hydroxyl ions therein to titrate the acidity;

passing light through the liquid during the titration thereof;

measuring the intensity of light passed through the liquid at a first wavelength corresponding to the light absorption characteristics of the liquid at one side of the color change interval;

measuring the intensity of the light passed through the liquid at a second wavelength corresponding to the light absorption characteristics of the liquid at the other side of the color change interval;

continuing the titration until the ratio of the logarithms of the intensities of said light at the first and second wavelengths reaches a predetermined value corresponding to said endpoint; and

measuring the amount of electricity passed through the liquid in said electrolysis current until the ratio of said logarithms reaches the predetermined value corresponding to the endpoint.

9. The method of claim 8 wherein the liquid to be titrated is prepared by mixing together serum containing an unknown amount of bicarbonate ion, bicarbonate-free acid in excess of an amount required to neutralize the bicarbonate ion in the serum, and an acid-base indicator which changes from an acid form to a base form over a pH range which includes a pH value of 7.4.