ASTM C1267 - 06 is a standard test method for uranium by iron (II) reduction in phosphoric acid followed by chromium (VI) titration in the presence of vanadium.
The Davies and Gray titration is a destructive analysis method for quantitative determination of uranium in samples taken from virtually any point in the nuclear fuel cycle.
It is widely used in nuclear safeguards material accountability measurements.
A weighed sample is subjected to the titrimetric analysis and the result is the amount of uranium contained in the sample, often reported as a percentage or mass content.
Uranium in solution is first reduced to U(IV) by the addition of Fe(II) ions, the excess Fe(II) ions being destroyed by oxidation.
The U(IV) is titrated against a standard solution of potassium dichromate to U(VI).
The end point of the titration is determined through measurement of the electrode potential of the solution.
The uranium content is calculated from the amount of dichromate used in the titration.
The titration is carried out in phosphoric acid medium in the presence of vanadyl ions, the latter acting as a catalyst for the oxidation reaction.
The uranium in concentrated phosphoric acid medium is first reduced to U(IV) with ferrous sulphate.
Excess ferrous ion is removed by molybdatecatalysed oxidation with nitric acid.
The nitrite formed in the above reaction is removed by sulfamic acid.
Vanadyl solution is then added to sharpen the endpoint and the U(IV) is titrated against a standard potassium dichromate solution using a combined platinum electrode.
The Davis and Gray titration method for the determination of UF6 has long been used in the power industry and can be automated using the Metrohm Titrando and Sample Processor.
The solutions added (concentrated phosphoric acid, reducing agent, oxidising agent and vanadyl sulphate) are dangerous.
Using the 814 Sample Processor and an 809 Titrando titrator means the whole system is automated and the percentage UF6 is determined the same way for every sample.
Prior to the solution additions, the temperature of the sample is measured and is used in the subsequent calculation of the percentage UF6.
The solutions are added via four Dosinos and the operator does not come into contact with any of the solutions.
All the relevant pauses in between additions (reaction times) are set, so too are the increase or decrease of stirrer speeds during these additions.
The solution is then titrated against potassium dichromate and the UF per cent is calculated.
The sample beaker and electrode set-up are sprayed after the analysis with de-ionised water and all the hazardous waste is pumped away safely before moving on to the next sample.
Tiamo titration software controls the whole operation from beginning to end and the results can be exported to a LIMS system should it be required.
The flexibility of the system can be tailored to almost any type of titration that needs to be automated.
The Davis and Gray method, although accurate and reproducible, is time consuming, as the solutions added prior to titration must be added within specified time constraints.
