METHOD AND APPARATUS FOR END POINT DETECTION IN POTENTIOMETRIC TITRATION

• US 3,769,178 A
• Filed: 04/27/1972
• Issued: 10/30/1973
• Est. Priority Date: 04/27/1972
• Status: Expired due to Term

• Current Assignee: Charles E. Rothermel Jr
• Original Assignee: Charles E. Rothermel Jr
• Inventors: Rothermel, Charles E. Jr.
• Primary Examiner(s): Tung, T.
• Application Number:
  US05/248,092
• Time in Patent Office: 551 Days
• Field of Search: 23/23R,232E,253R,254E 204/1T,195T 324/3C
• US Class Current: 205/787.5
• CPC Class Codes: G01N 31/164 by electrical or electrochemical means
• CPC Class Codes: Y10T 436/116664 with automated titrator

Abstract

A method of automatically determining the end point of a potentiometric titration and apparatus for carrying out the method are disclosed. A titrant is added to a solution at a measured rate by a motor-driven burette. A counter and a count accumulator are provided to record the amount of titrant added to the sample and means are provided to measure changes in the pH of the sample as the titrant is added. The count representing the volume of titrant is recorded until a preset threshold point is reached, this point being determined by a voltage representing the first derivative of the changing pH potential in the sample. The occurrence of the threshold point changes the counter and count accumulator so that only one-half of the actual count is registered in the accumulator. The titration continues beyond the normal end point until the first derivative value returns to the threshold point, at which time the titration ends. The count in the accumulator at this time provides a direct reading of the volume of titrant added to the end point, thereby providing a fast and accurate determination of the end point. The half count technique is applicable to any titration that can be carried out by standard or developed potentiometric procedures.

Claims

2. The method of claim 1, wherein the measurement of titrant added to said sample includes:

generating electrical signals which represent unit quantities of titrant delivered to said sample; and

accumulating said signals.

3. The method of claim 2, wherein said electrical signals are digital representations of the quantity of titrant delivered, said digital signals being accumulated to provide a continuous record of the quantity of titrant added to said sample.

4. The method of claim 3, further including the step of halting said titration when said first derivative returns to said preset value.

5. The method of claim 4, wherein the measurement of titrant added to said sample further includes:

accumulating all of the said digital signals generated before said first derivative reaches said preset value; and

accumulating one-half of the said digital signals thereafter generated.

6. The method of claim 5, further including the step of displaying a value represented by said accumulated digital signals to obtain the volume of titrant added to said sample to reach the said end point.

7. The method of claim 6, wherein the step of displaYing a value includes the steps of summing the digital signals accumulated before said preset first derivative value is reached with the digital signals thereafter accumulated, the value of said sum representing the total volume of titrant added to reach said end point.

8. A method of determining the end point of potentiometric titration comprising:

adding titrant to a sample at a controlled rate;

measuring the amount of titrant added to said sample;

generating electrical signals representing the measured amount of titrant;

measuring changes in the potential of said sample as titrant is added;

obtaining the first derivative of said potential;

accumulating said electrical signals at a first rate until said first derivative reaches a predetermined value; and

thereafter accumulating said electrical signals at a second rate until said derivative returns to said value.

9. The method of claim 8, further including selecting said predetermined value sufficiently near the maximum value of said first derivative that said derivative is substantially symmetrical about said maximum value above said predetermined value.

10. The method of claim 9, wherein the said second rate of accumulation of said electrical signals is one-half the said first rate of accumulation.

11. The method of claim 10, further including the step of halting said titration when said first derivative returns to said predetermined value, and thereafter obtaining directly from the accumulated electrical signals the amount of titrant added to reach the end point of said titration.

12. A method of determining the amount of titrant added to a sample to reach the end point of potentiometric titration, comprising:

supplying titrant to a sample at a controlled rate;

generating at least one digital electrical pulse for each unit quantity of titrant added to said sample;

measuring changes in the potential of said samples as titrant is added;

obtaining a variable signal representing the first derivative of said potential, said variable signal having a maximum value at the end point of said titration;

determining a threshold value of said first derivative signal;

counting the pulses generated during said titration;

accumulating the full count of said pulses until said first derivative signal reaches said threshold value and thereafter accumulating one-half the count of said pulses; and

halting said titration after said first derivative signal passes through said maximum value and returns to said threshold value.

13. The method of claim 12, further including supplying said titrant to said sample at a first rate until a predetermined quantity has been dispensed, and thereafter supplying titrant at a second rate.

14. The method of claim 13, further including the step of reading the accumulated pulse count to provide a direct indication of the volume of titrant added to said sample to reach said end point.

15. The method of claim 14, including the step of selecting said predetermined threshold value sufficiently near the maximum value of said first derivative that said derivative is substantially symmetrical about said maximum value and above said threshold.

16. The method of determing the amount of titrant added between the first and second equivalence points of a complex sample having two potentiometric breaks in the titration curve, including:

supplying titrant to said sample at a controlled rate;

generating at least one digital electrical pulse for each unit quantity of titrant added to said sample;

measuring changes in the potential of said sample as titrant is added;

obtaining a variable signal representing the first derivative of said potential, said variable signal having a maximum value for each equivalence point of said titration;

determining a threshold value of said first derivative signal, said threshold value being selected to be sufficiently near each of said maximum values of said first derivative that said derivative is substantially symmetrical about each maximum value and above said threshold;

counting the pulses generated during said titration;

accumulating one-half said count while said variable signal exceeds said threshold value and accumulating the full value of said count between said first and second equivalence points while said variable signal is below said threshold value, the total count so accumulated representing the amount of titrant added to carry said sample from said first to said second equivalence points.

17. In a potentiometric titrator for determining the equivalence point of a sample, apparatus for providing a direct and accurate measure of the volume of titrant added to said sample in reaching said equivalence point, comprising:

a sample cell for receiving a sample to be titrated;

means for supplying titrant to said sample cell;

means for generating electrical signals corresponding to the volume of titrant supplied;

means for measuring the electrical potential of said sample and producing an output corresponding to the first derivative of said potential;

means for storing all of said electrical signals until said first derivative reaches a predetermined value and for thereafter storing only a predetermined portion of said electrical signals; and

means responsive to said first derivative output for halting said titration when said first derivative returns to said predetermined value.

18. The apparatus of claim 17, further including means responsive to said first derivative output for producing a threshold signal when said output reaches said predetermined value, said means for storing electrical signals responding to the occurrence of said threshold signal to store only said predetermined portion of said electrical signals.

19. The apparatus of claim 18, further including means for reading out the total value of said stored electrical signals to obtain the total volume of titrant supplied to reach said equivalence point.

20. The apparatus of claim 18, wherein said means for reading includes display means.

21. The apparatus of claim 17, wherein said means for generating electrical signals includes pulse means for producing at least one pulse per unit volume of titrant delivered.

22. The apparatus of claim 21, wherein said means for storing electrical signals comprises counter means for counting said pulses.

23. The apparatus of claim 22, wherein said means for storing electrical signals includes gate means responsive to said first derivative output, said gate means operating in a full count mode to deliver all of said pulses to said counter means until said first derivative reaches said predetermined value, said gate means thereafter operating in a half-count mode to deliver only alternate pulses to said counter means.

24. The apparatus of claim 23, wherein said gate means includes means responsive to said first derivative output for producing a threshold signal when said output reaches said predetermined value, and a count gate responsive to said threshold signal, said threshold signal shifting said count gate from a full count mode to a half-count mode.

25. The apparatus of claim 24, further including readout means for obtaining a direct indication of the accumulated count in said counter, and thus of the total volume of titrant added to said sample.

26. The apparatus of claim 17, wherein said means for supplying titrant includes burette means for delivering metered quantities of titrant to said cell, and electric motor means for driving said burette at a controlled speed.

27. The apparatus of claim 26, wherein said means for generating electric signals comprises means responsive to the operation of said burette to generate at least one electrical pulse for each unit volume of titrant delivered by said burette.

28. The apparatus of claim 27, wherein said burette is operated by a rotary, motor-driven shaft, and said pulse generating means comprises photoeleCtric means reponsive to the rotation of said shaft for producing a plurality of pulses for each rotation of said shaft.

29. The apparatus of claim 27, further including control means for said electric motor means for selectively driving said burette at one of two speeds.

30. The apparatus of claim 27, wherein said means for storing said electrical signals comprises counter means for receiving said generated electrical pulses and for accumulating a count corresponding to the number of pulses received.

31. The apparatus of claim 30, wherein said means for storing said electrical signals further comprises count gate means having a full count mode and a half-count mode, said count gate shifting from one mode to the other when said first derivative reaches said predetermined value.

32. The apparatus of claim 31, further including readout means for displaying the count accumulated in said counter and to display the volume of titrant added to said sample.

33. The apparatus of claim 17, wherein said means for measuring the electrical potential of said samples includes electrode means for immersion in said sample, said electrodes producing a potential difference which is a measure of the pH of said sample, and circuit means connected to said electrodes for producing the first derivative of said electrode potential difference.

34. The apparatus of claim 33, further including trigger means responsive to a predetermined value of said first derivative, said trigger means producing an output threshold signal as long as said first derivative exceeds said predetermined value.

35. The apparatus of claim 34, wherein said means for storing said electrical signals includes count gate means responsive to said threshold signal, said count gate means operating in a full count mode to store all of said electrical signals in the absence of said threshold signal and operating in a half-count mode to store only alternate ones of said electrical signals in the presence of said threshold signal.

36. The apparatus of claim 35, wherein said means for generating electric signals comprises burette means for delivering metered quantities of titrant to said cell, and means responsive to the operation of said burette to generate at least one electrical count pulse for each unit volume of titrant delivered by said burette.

37. The apparatus of claim 36, wherein said means for generating electric signals further comprises second trigger means for supplying said electrical count pulses to said count gate means.

38. The apparatus of claim 37, wherein said means for storing said electrical signals comprises counter means responsive to count pulses passed by said count gate, said count gate being operative in its full count mode to pass all of said count pulses to said counter means, and being operative in its half-count mode to pass alternate ones of said count pulses.

39. In a potentiometric titrator for automatically determining the volume of titrant added to a sample to reach the end point of a titration;

a sample cell for receiving a sample to be titrated;

burette means for delivering metered quantities of titrant to said cell, and electric motor means for driving said burette;

motor control logic circuit means for regulating the speed of said electric motor means;

means responsive to the operation of said burette to generate at lease one count pulse for each unit volume of titrant delivered by said burette;

counter means for receiving said count pulses and for accumulating a count corresponding to the number of pulses received;

count gate means interposed between said means for generating count pulses and said counter means, said count gate having a full count mode wherein all of said generated count pulses are passed to said counter means and having a half-count mode wherein only one-half of said generated count pulses are passed to said counter means;

means for measuring the potential of said sample during titration thereof, and ciRcuit means for producing the first derivative of said potential;

means responsive to a predetermined value of said first derivative to produce a threshold signal; and

circuit means for applying said threshold signal to said count gate to shift said count gate from a full count mode to a half-count mode and for holding said count gate in the half-count mode as long as the value of said first derivative exceeds said predetermined value.

40. The apparatus of claim 39, wherein said motor control logic circuit means includes a delivery selector network responsive to said counter means, said delivery selector network producing an initial delivery signal upon delivery of a predetermined volume of titrant to said sample, said motor control circuit being responsive to said initial delivery signal to change the speed of said electric motor means, whereby an initial quantity of titrant can be added to said sample at a different rate than the rate at which the remainder of titrant is added.

41. The apparatus of claim 40, further including inhibit circuit means for inhibiting the production of said threshold signal, said delivery selector network including means for producing a permit signal subsequent to said initial delivery signal and upon delivery of an additional predetermined volume of titrant, said permit signal disabling said inhibit circuit means.

42. The apparatus of claim 41, wherein said motor means includes a high speed motor and a low speed motor, and wherein said motor control logic circuit means includes switching means for selectively energizing said motors, said switching means initially energizing said high speed motor and being responsive to said initial delivery signal to de-energize said high speed motor and energize said low speed motor.

43. The apparatus of claim 42, further including means responsive to the termination of said threshold signal for disabling said count gate and for de-energizing said electric motor means.

44. The apparatus of claim 43, further including means for reading out the total count accumulated in said counter means to obtain the total volume of titrant delivered to said sample to reach said end point.