Anodic stripping volammetry and apparatus therefor

• US 3,904,487 A
• Filed: 10/29/1974
• Issued: 09/09/1975
• Est. Priority Date: 10/29/1974
• Status: Expired due to Term

• Current Assignee: Alberto Zirino, Stephen H. Lieberman
• Original Assignee: Alberto Zirino, Stephen H. Lieberman
• Inventors: Zirino, Alberto, Lieberman, Stephen H.
• Primary Examiner(s): Kaplan, G. L.
• Application Number:
  US05/518,539
• Time in Patent Office: 315 Days
• Field of Search: 204/1T,195H,195R 324/29
• US Class Current: 205/789.5
• CPC Class Codes: G01N 27/42 Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte
• CPC Class Codes: G01N 27/48 using polarography, i.e. measuring changes in current under a slowly-varying voltage

Abstract

An apparatus for and a method of determining the presence and concentration of trace metals in seawater relies on anodic stripping voltammetry. A tubular mercury-graphite electrode is suitably coupled to receive a flowing mercury plating solution and a thin film of mercury is deposited on the inner surface of the electrode when a plating potential is coupled to the electrode. Next, a seawater sample is pumped through the electrode and trace metals are reduced onto the active mercury film when the plating potential is reconnected to the electrode. After a predetermined time, the potential is shifted to a scanning potential gradient. At discrete levels within the scanning potential gradient certain ones of the trace metals are stripped from the active metal film and the values of the currents at these levels are monitored and recorded. From the magnitudes of the currents at the discrete potential levels, the concentration of individual ones of the trace metals is determined. Zinc, cadmium, lead and copper concentrations are readily observable when their stripping potentials are embraced within the breadth of the scanning potential gradient. Increasing the scanning potential allows the stripping away of the mercury film. A following analysis merely calls for switching the seawater sample back to a container and reconnecting the mercury plating solution to the electrode. Another active mercury thin film is deposited on the electrode and trace metals are deposited for a following stripping operation. Thus, the disclosed method and apparatus are capable of nearly real-time iteration to enable a more accurate determination of trace metal concentrations. The degree of accuracy is enhanced since the flow rates of the plating solution and the sample are controlled, the magnitudes of the plating and stripping potentials are controlled and the thickness of the active mercury film is precisely regulated to ensure consistent parameters for a number of sample analyses. In addition, because the apparatus operates as a closed system, outside influences which might otherwise degrade reliability, are eliminated.

Claims

1. A METHOD OF MEASURING THE PRESENCE AND CONCENTRATION OF TRACE METALS IN A SAMPLE SOLUTION FLOWING THROUGH A ACTIVE ELECTRODE AND A REFERENCE ELECTRODE COMPRISING:

DEPOSITING A FILM OF ERCURY FROM A PLANTING SOLUTION ON THE INNER SURFACE OF SAID ACTIVE ELECTRODE, FEEDING A SAMPLE SOLUTION HAVING SAID TRACE METALS THROUGH SAID ACTIVE ELECTRODE AND SAID REFERENCE ELECTRODE, APPLYING A FIRST POTENTIAL ACROSS SAID ACTIVE ELECTRODE AND SAID REFERENCE ELECTRODE, REDUCING SAID TRACE METALS ON THE MERCURY FILM, SCANNING A POTENTIAL GRADIENT ACROSS SAID ACTIVE ELECTRODE WITH RESPECT TO SAID REFERENCE ELECTRODE, STRIPPING AWAY VARIOUS ONES OF SAID TRACE METALS AT DISCRETE POTENTIAL LEVELS WITHIN THE SCANNED POTENTIAL GRADIENT, AND MONITORING THE CURRENTS PRODUCED AT THE DISCRETE POTENTIAL LEVELS TO PROVIDE AN INDICATION OF THE PRESENCE AND CONCENTRATION OF THE TRACE METALS

2. A method according to claim 1 further including maintaining said inner surface of said active electrode in a closed circuit during said depositing, feeding, applying, reducing, scanning, stripping and monitoring.

3. A method according to claim 2 in which said depositing includes, flowing a mercury plating solution through said active electrode, coupling a plating potential having the same magnitude as said first potential across said active electrode and said reference electrode, and reducing a mercury film from said active metal plating solution on the inner surface of said reference electrode.

4. A method according to claim 3 further including:

removing said mercury plating solution from said active electrode to a reservoir prior to said feeding of said sample solution through said active electrode and said reference electrode.

5. A method according to claim 4 in which said scanning is extended to the stripping potential of said mercury film to oxidize said mercury film on said active electrode.

6. A method according to claim 5 in which the steps of depositing said mercury film and of the extended scanning of said stripping potential are repeated a plurality of times to bring the current attributed to the stripping of said mercury film to the steady state.

7. A method according to claim 6 further including:

purging a portion of said sample solution and a portion of said plating solution which were admixed during the transition between the steps of said depositing and said feeding to avoid contaminating said plating solution.

8. A method according to claim 7 in which said removing, said feeding, said applying, said reducing, said scanning, said stripping and said monitoring are repeated to give accurate repeated indications of the presence and concentration of trace metals in said sample solution.

9. A method according to claim 7 in which said feeding is the sequential feeding of a second sample solution, said applying is the sequential applying of said first potential, said reducing is the sequential reducing of trace metals from said second sample solution, said scanning is the sequential scanning of said potential gradient, said stripping is the sequential stripping away of various trace metals, and said monitoring is the sequential monitoring of currents to provide an accurate iNdication of the presence and concentration of trace metals in said second sample solution without any further modification of said active electrode.

10. AN APPARATUS FOR MEASURING THE PRESENCE AND CONCENTRATION OF TRACE METALS IN A SAMPLE SOLUTION COMPRISING:

A SOURCE OF A MERCURY PLATING SOLUTION, MEANS FOR SENSING THE PRESENCE AND CONCENTRATION OF TRACE METALS, MEANS CONNECTED TO THE SENSING MEANS FOR FEEDING SAID SAMPLE SOLUTION AND SAID PLATING SOLUTION THROUGH THE SENSING MEANS, FIRST MEANS CONNECTED TO A CONTAINER OF SAID SAMPLE SOLUTION AND THE PLATING SOLUTION SOURCE FOR ALTERNATELY COUPLING EACH TO SAID SENSING MEANS, SECOND MEANS CONNECTED TO THE FEEDING MEANS FOR ALTERNATELY COUPLING SAID FEEDING MEANS TO THE SAMPLE SOLUTION CONTAINER AND SAID PLATING SOLUTION SOURCE,

11. An apparatus according to claim 10 in which the indicating means includes, means for stripping away various ones of said trace metals and said mercury film at discrete potential levels and, means for monitoring the currents produced at said discrete potential levels thereby providing said indicating of the presence and concentration of the trace metals.

12. An apparatus according to claim 11 in which said first and second coupling means are interconnected for recirculating said sample solution between said sample solution container and said sensing means during the stripping and monitoring thereof by the stripping means and the monitoring means.

13. An apparatus according to claim 12 further including:

means connected to said coupling means for purging any said admixture of said plating solution and said sample solution.

14. An apparatus according to claim 13 further including:

means connected to said first and second coupling means for ensuring a sequential said recycling and recirculation with a minimal admixture of said plating solution and said sample solution.

15. An apparatus according to claim 14 further including:

means for interconnecting the mercury plating source, the sensing means, the feeding means, the first and second coupling means, the depositing means and the purging means in a closed system relationship to reduce the possibility of contaminating the mercury plating solution and the sample solution.

16. An apparatus according to claim 15 in which said sensing means is a tubular graphite electrode and said mercury film is deposited uniformly on the cylindrical inner surface.

17. An apparatus according to claim 16 in which said feeding means is a pump drawing said plating solution and said sample solution through the tubular graphite electrode at a variable, selectable flow rate.

18. An apparatus according to claim 17 in which said first coupling means and said second coupling means are a pair of stopcocks, the stripping means is polarography system, and the monitoring means is a strip chart recorder.

19. An apparatus according to claim 10 in which said first and second coupling means are interconnected for recycling said plating solution between said plating solution source and said sensing means during the depositing of said mercury film.