unfortunately the accuracy with which an impurity dependent physical or chemical property of sodium can be measured decreases with decreasing impurity concentration . to get over this difficulty Alcock has suggested that instead of measuring directly the concentration of oxygen in the flowing sodium its thermodynamic potential should be measured by a suitable galvanic cell incorporated in the circuit . the principal advantages of this should be continuous monitoring of the sodium and an accuracy of monitoring which , if the sodium-oxygen system obeys Henry &apos;s law , should increase with decreasing concentration of the impurity . 2 . theoretical . ( a ) . the cell . the use of solid electrolytes in galvanic cells has been described in detail by Kiukkola and Wagner . in a reversible cell consisting of two metal-metal oxide electrodes and a solid oxide electrolyte through which current is transported solely by 0= ions , the change in free energy dg accompanying the passage of one mole of oxygen is given by : - 2EF where E is the voltage developed across the cell and F is the Faraday . if the electrodes are sodium saturated with its own oxide and unsaturated sodium the change of free energy accompanying the transfer of one mole of 0= from the saturated to the unsaturated metal will be given by : - &amp;formula; where &amp;formula; , &amp;formula; are the activities of oxygen in saturated sodium ( concentration c0 ) and in the unsaturated sodium ( concentration c &lt; c0 ) , T the absolute temperature and R the gas-constant . if the activity of oxygen dissolved in sodium is proportional to its concentration as is required by Henry &apos;s law then the free energy change per mole 0= ion may be written &amp;formula; . thus &amp;formula; . the solubility of oxygen as Na20 in sodium has been determined and is given by the relationship &amp;formula; . substitution of equation ( 3 ) in equation ( 2 ) with appropriate values for the various constants gives &amp;formula; . values of this function between 400 &amp;deg; and 800 &amp;deg; C at 100 &amp;deg; intervals and for oxygen concentrations between 0.1 and 100 p.p.m are presented in fig 1 . at the present time maximum sodium coolant temperatures are around 500 &amp;deg; C and oxygen concentrations are usually intended to be maintained in the range 1-10 p.p.m according to the above this cell under these conditions should give voltages ranging from 224-147 mv . ( b ) . the effect of small changes of oxygen concentration and temperature on the cell E.M.F . the E.M.F of such a cell placed in a sodium circuit will be affected by fluctuations in oxygen content and temperature . these may be estimated from equation ( 4 ) or the following derived equations : - &amp;formula; &amp;formula; . equation ( 5 ) indicates that any voltage fluctuation arising from a sudden small concentration change will be controlled principally by the original concentration . thus changes from 0.1 to 1 p.p.m 1-10 p.p.m 10-100 p.p.m would result in the same change in voltage ( &amp;symbol;76 mv ) . for relevant reactor conditions ( 500 &amp;deg; C , C = 1-10 p.p.m ) the finite change of voltage de accompanying finite concentration changes dc is plotted in fig 3 . the latter as might be expected vary considerably . a rise of oxygen concentration from 1-2 p.p.m is accompanied by a voltage drop of &amp;symbol;23 mv while , a rise from 9-10 p.p.m would produce a change of only &amp;symbol;3 mv . changes in voltage accompanying fluctuations of coolant temperature according to equation ( 6 ) vary only slightly with concentration and are proportional to the temperature change . values at various oxygen concentrations of &amp;formula; together with apparent changes in oxygen level for temperature fluctuations of &amp;plusmn; 10 &amp;deg; C at 500 &amp;deg; C are presented in table 1 . the above figures show that a &amp;plusmn; 10 &amp;deg; C temperature fluctuation at oxygen levels in the range 1-10 p.p.m would indicate an apparent change of &amp;symbol;12 % in oxygen concentration . providing a cell of the above type works satisfactorily the above arguments suggest that it will be sufficiently accurate as an oxygen monitor in a hot trapped sodium coolant circuit . ( c ) . contamination of the sodium circuit by oxygen from the cell . experiments with solid oxide electrolyte galvanic cells have indicated that it is difficult to obtain reproducible voltages using normal potentiometric methods at temperatures below 750 &amp;deg; C . the author has obtained reproducible results with such cells at 400 &amp;deg; C and above by using vibrating reed voltmeters that draw current from the cell only as a result of leakage through insulation resistance of &amp;formula; . thus if voltmeters of this type were used with the Na &amp;sol; Na20 cell it is possible to estimate the contamination of the circuit sodium from oxygen continuously diffusing through the electrolyte . if it is assumed that in practise the maximum voltage developed by the cell at 500 &amp;deg; C will be around 300 mv ( see fig 1 ) then in the case of the instrument with the lower resistance the current will be : - 3 x 10-14 coulombs &amp;sol; sec . the charge on 0= ion &amp;symbol;3.2 x 10-19 coulombs . thus the number of 0= ions travelling through the electrolyte per second &amp;symbol;105 . the mass of oxygen per year at this rate would be approximately 8 x 10-1 g &amp;sol; year which is a quite insignificant quantity . ( d ) . the use of the cell as a corrosion meter . with the cell electrodes consisting of sodium with oxygen at different activities a voltage will be developed that is a function of the difference in the oxygen potential at the two electrodes . unless it is known at what oxygen potential a given material in the sodium coolant circuit will start to oxidise the cell can only be used as has been suggested above , as an oxygen concentration monitor . however , if a material oxidizes in sodium at a given oxygen potential the reference electrode could be held at that potential and oxidizing or reducing conditions in the coolant circuit for that material would be indicated by a negative or positive potential at the reference electrode . thus for the specific case of niobium in a sodium circuit a corrosion indicator could be a reference electrode of sodium saturated and equilibrated with niobium separated from the coolant by a solid anionic electrolyte . a negative voltage from the reference electrode would mean oxidizing conditions for niobium and positive voltage , non-oxidizing conditions . 3 . practical . the practical application of the above idea will involve considerable experimentation before it can be realised . the first requirement is for an anionic electrolyte , which can be fabricated into suitable shapes impervious to gases and liquid sodium and which is neither corroded by sodium nor by sodium monoxide . possible materials are zirconia stabilised with lime and thoria doped with rare earth oxides . if such a material can be made with these properties a possible way in which the cell may be incorporated in a sodium circuit is depicted in fig 4 . the electrolyte A is made in the form of a thin walled closed off round end tube or probe fitting vertically into the sodium coolant circuit B . the +ve electrode consisting of a small quantity of sodium saturated with sodium monoxide C is situated at the bottom of the tube . the potential acquired by this pool of sodium is transmitted to the voltmeter V by a nickel conductor D , nickel being resistant to corrosive attack by oxide saturated sodium at 500 &amp;deg; C . the -ve electrode which is the coolant stream , is joined to the voltmeter by an earthed nickel conductor attached to the bottom of a well E in the coolant stream . provided the temperatures at C and E are the same , thermoelectric contributions to the voltage should be zero . the probe extends out of the sodium stream through a close fitting thin walled T-junction F and passes into the open via a water-cooled O ring seal G . the open end of the probe is sealed with a vacuum coupling H which also positions the +ve nickel conductor with respect to the sodium by circlips on either side of the seal I . evaporation of sodium from the pool C is minimised by a close fitting cylindrical block of electrolyte J attached to the +ve nickel conductor by nickel circlips . fixing and positioning of the probe relative to the coolant stream is effected by tie-bars of insulating material K joining the vacuum coupling H to the water cooled flange G . the probe can be evacuated and filled with inert gas via the tube L which must of course be electrically isolated after this has been carried out . 4 . discussion . it is not suggested that the above proposal will be successful but rather that it is worth a trial in the event of the inadequacy of some simpler method of monitoring the oxygen in a sodium circuit . the principal difficulty encountered by the author , in determining partial molal free energies by solid electrolyte cells of very stable oxides such as UO2 , MnO etc was vapour phase transfer of oxygen by carbonaceous impurities in the blanket gas . this resulted in the oxidation of the -ve electrode and reduction of the +ve electrode which of course led to a loss in E.M.F from the cell . in the above design the two electrodes are completely separated from one another so that this major source of trouble should not be present . however , the stability of the system may be adversely affected by the thermal gradient up the probe and this can only be tested by experiment . whether such an apparatus can be incorporated in a reactor circuit in a manner that will satisfy safety requirements will need further study . on the face of it however , there seems to be no reason why the cell should not be double-contained to prevent loss of sodium in the event of the ceramic tube being fractured . such containment however , will be complicated by the necessity of providing suitable insulating seals through its walls . 5 . conclusions . if other monitoring methods for oxygen in sodium in the concentration range 1-10 p.p.m are found to be inadequate then this galvanic cell may be worth investigating . however , it will require development of a suitable electrolyte and even then it will only be useful if the activity of the dissolved oxygen varies sufficiently with changes in its concentration . a . outline of method . to a measured portion of the sample , niobium and zirconium carriers are added together with hydrofluoric acid to ensure complete isotopic interchange . rare earth elements are co-precipitated with lanthanum as fluorides . niobium is precipitated with ammonia , partially separating it from zirconium . the niobium precipitate is dissolved in a mixture of oxalic and nitric acids , and niobic acid precipitated by boiling and adding potassium bromate . the niobic acid is dissolved in acid ammonium fluoride and the cycle from the ammonia precipitation repeated . the niobic acid is washed , ignited to niobium pentoxide , which is mounted on a tared counting tray and weighed . the g-activity is measured through a lead &amp;sol; aluminium sandwich using standard gamma scintillation equipment , which has been calibrated with known amounts of niobium-95 . b . reagents required . all reagents are analytical reagent quality where available . 1 . standard niobium carrier solution ( &amp;formula; ) . fuse 20 g of pure niobium pentoxide with 72 g of potassium carbonate in a platinum dish . cool and dissolve the solidified melt in about 400 ml of hot water . transfer the solution and any undissolved solid to a glass beaker , stir thoroughly and add 16 M nitric acid until the solution is strongly acid to litmus . stand the beaker on a hot plate and keep the solution warm for 30 minutes to coagulate the precipitate . transfer to four 200 ml polythene bottles , centrifuge , decant and discard each supernate . wash each portion of the precipitate three times by stirring with 100 ml of 2 % ammonium nitrate . use a glass rod for stirring . centrifuge and discard the supernates after each wash . dissolve each portion of the precipitate in 25 ml of 30 % ammonium fluoride and 15 ml of 16 M nitric acid . combine the solutions from each of the 200 ml polythene bottles , and dilute to 2 litres with distilled water in a polythene bottle . standardize as follows : - pipette 10 ml of the solution into a 400 ml polythene beaker and add 100 ml of a saturated solution of ammonium chloride . heat the solution nearly to boiling , by placing the polythene beaker in a glass beaker of water , heated on a hot plate , and add to the solution 1 g of tannic acid dissolved in hot water . 