Actinide Facility

Actinide Spectroelectrochemistry

Experimenters

Beamline Personnel

Outline of Project

Dates

Beamline

History

Over several years, we have designed and modified a purpose built electrochemical cell that will permit the collection of XAFS data from solutions that are maintained under electrochemical control during data acquisition. The cell has been modified over time to optimize the safety and the data quality while minimizing the equilibrating time.

The cell design has been published [ DESIGN OF SPECTROELECTROCHEMICAL CELL FOR IN SITU X-RAY ABSORPTION FINE STRUCTURE MEASUREMENTS OF BULK SOLUTION SPECIES, Mark R. Antonio, L. Soderholm, and Inho Song, J. Appl. Electrochem. 27 (1997) 784-792.]

The experimenters have had extensive experience in conducting spectroelectrochemical studies at NSLS, at SSRL on the Wiggler beamline, and recently at the APS - 12BM. Most of the work has been done on transition-metal and rare-earth ions, and more recently, with Th and U. This extensive experience allows us to describe the cells routine use and behaviour, and to predict potential problems with a high degree of confidence.

Isotopes

Table I. Isotopes that will be used in spectroelectrochemistry experiments.

Description of cell design

Figure 1. Assembly drawing of the spectroelectrochemical system for in situ XAFS measurements. A: Working electrode; B: Electrolyte feed tube and gas vent port; C: Purge gas feed tube; D: Auxiliary electrode; E: Reference electrode; F: Cell box; G: Bracket for mounting a fluorescence detector (not shown); H: Spectroelectrochemical cell; I: O-ring seal; J: Cell window; K: Window frame; L: X-ray windows; M: Auxiliary electrode compartment; N: Cell positioner; O: Cell box purge ports. In this view, the spectroelectrochemical cell is shown at a 45º angle, as it would be used for fluorescence XAFS measurements with the incident X-ray beam entering the cell box from the right. For clarity, the eight screws (#4-40 x 1 in., allen head, and nuts) for mounting the cell window and the back side window disassembly are not shown.

An actual sample cell and cell box will be available for inspection at the time of the safety review.

The windows on the sample cell will be Kapton, Mylar, Polycarbonate, Polypropylene or Prolene. They are sealed in place with a Kalrez O-ring.

The sample cell will be filled with the electrolyte and left to sit for at least one week to assure that there is no significant interaction of the electrolyte with the sample-cell materials.

The sample cell will be mounted into the cell box in CHM.

Metal joints on the cell box are welded. The box has been demonstrated to hold a volume of liquid in excess of the maximum solution volume without leaking.

The opening in the cell box that will accommodate the detector will have a protective layer of Kapton. The cell box will be shipped (in a plastic bag) without the detector in place. The cell box will be mounted onto the detector at the APS.

Samples will be loaded into the sample cell in CHM. The outside of this primary containment will be smeared to assure that it is NCD abg before it is loaded into the cell box. The sample cell will be loaded into the cell box, also in CHM. The cell box will be further enclosed in a plastic bag, for transport. The bag will be opened at the APS after the sample is in the hutch, during beam alignment and data collection.

The cell box will NOT be opened at the APS.

Sample/Experimental Details

Electrolytes will be less than 2 M solutions of SO₄=, ClO₄=, Cl-, CO₃=, HCO₃^-, CH₃COO^-. Solutions will have a pH ranging from <1 to 14. Some solutions may be buffered.

Sample volumes will be 12 ml of solution or less, depending on electrodes.

Solutions may also contain non-radioactive cations, including Fe, W, Mo, Si, etc.

Electrodes will be Pt or carbon. They will exit the cell box as shown in the Figure. The Pt electrodes will be sealed into thick tubing with Super Sculpey^TM ceramic-like sculptering compound.

Sample will be stirred by small magnetic stirring bar in the soulution.

Sample may be sparged. Flow of the input gas will be limited by a small volume flow gauge. Small diameter tubing will be used to direct the outflow of gas into a small flask within the cell box that is filled with an aqueous solution of a complexant, such as EDTA. The gas can pass through this solution and out a small opening in a rubber septum at the top of the flask. This system is designed to assure that no airborne contamination is carried out into the cell box.

Transportation

All movement of the sample on the floor of the APS, outside of the designated Controlled Area, will be monitored by Health Physics, under the direction and discretion of the floor coordinator.

Movement of the sample(s) within the hutch may be done with the assistance of Health Physics, at the discretion of the senior experimenter.

Description of safety protocol

Sample tracking: A single experimenter, designated the "responsible person" will be assigned to each sample at all times when they are not within the BESSRC storage safe. A log book with a written record of the responsible person will be posted at the beamline at all times. Changes in this person will require the signature of the newly-responsible person. The samples will not be left unattended at the beamline at any time during the run.

Controlled Area: A temporary Controlled Area will be set up for the duration of the experiment. This Controlled Area will include the Hutch itself, and the small area outside the hutch so that there can be direct access to the Hand and Foot Monitor without leaving the Controlled Area. The area will be clearly marked off, and access will be restricted. Signage designating the Controlled Area will be posted in a clearly visible location, as will Restricted Access signage. The list of the persons allowed into this restricted area will also be clearly posted. Personnel permitted to enter this area will be restricted to Experimenters, and Beamline Personnel as listed above. In addition, the floor coordinator, Heath Physics, and other personnel authorized by APS are permitted free access.

No air monitoring will be required, but may be employed at the discretion of the experimenters.

Efforts will be made to limit the quantity of isotope to the minimum amount required to meet the required data quality. The exact quantity required will be dependent on the specifics of the required signal-to-noise, as well as safety requirements imposed by the details of the set-up, such as the thickness of windows and encapsulation.

Sample will be visually inspected, through the windows of the cell box, at least twice every 24 hours.

Possible accident scenarios

Gas evolution during reduction. This scenario may occur if the potential is too reducing, and water is electrolyzed. Electrode potentials that are required will be determined at the time of the trial-run experiments. The potentials necessary to perform the experiment will be chosen within the redox window of water. This will minimize the electrochemical breakdown of water, which could cause a gas build-up in the cell. In addition to this safety precaution against gas build-up, the sample cell will be equipped with thin tubing from the top of the cell over to a Erlenmeyer flask in the cell box. The end of the tubing will be immersed in a solution containing a metal complexant. Any overpressure in the sample cell will escape through the tubing, into the solution, and out through the septum covering the solution. The cell design, with Kapton windows, has been pressure tested, and is leak tight to greater than 3 psi overpressure.

Unforeseen circumstances cause the sample cell to leak. Precautions have been taken to assure that the available volume of solution is entirely contained within the cell box. In the unlikely event that a breach of the sample cell were to occur, the experiment would be immediately terminated and Health Physics personnel would be called to survey the outer containment to assure that no activity was released. The cell box would be bagged, and shipped back to CHM.

Stability of perchloric acid (HClO₄). Perchloric acid is known to be reactive, even explosive in highly concentrated solutions (>72 %) or solid form [see attached]. Hence, it is transported as a solution containing no more than 72% HClO₄. Dilute solutions are not explosive. For the purposes of these experiments, concentrations will not exceed 2 M (18%). In addition, the electrolytic solution will be free of any organic matter.

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