Calcium ion-selective electrodes based on calcium bis[di(4-octylphenyl)phosphate] sensor and dioctyl phenylphosphonate solvent mediator have been used for monitoring free calcium-ion levels (from below lo-' to above 10-3 M) in the presence of citrate, malate, malonate, oxalate, EDTA, NTA, sulphate, orthophosphate, tripolyphosphate and pyrophosphate anion ligand systems under conditions of constant ionic strength maintained by sodium chloride. Log data fall in the range of those previously measured for the various equilibria by alternative methods, thus demonstrating that calcium ion-selective electrodes of the type used here can be employed for free calciumion measurements to below the detection limits characteristic of calibrations with serial dilution standards and without disturbing the equilibria of complexation.Equilibria existing in the tripolyphosphate and pyrophosphate systems were discerned by application of the MINIQUAD program for computing formation constants and species distribution. Predictions concerning the existence of [CaP30,O,,) 2]8-in tripolyphosphate and of [CaP20,I2-and [Ca(P,O,),]Gin pyrophosphate systems are briefly discussed.These calcium ion-selective electrodes are not affected by added phosphate except insofar as free calcium-ion levels are lowered by complexation.
Sensors of calcium bis[di(4-alkylphenyl)phosphates] (alkyl = hexyl, octyl and 1,1,3,3-tetramethylbutyl) in conjunction with dioctyl phenylphosphonate solvent mediator in PVC matrices give good calcium ion-selective electrodes with near-Nernstian slopes and detection limits of 1.9 x lod6 to 2.7 x M calcium ions in the absence of ion buffers. Electrodes with the octyl-based sensor, when calibrated a t 25 "C with calcium-ion buffers, gave calcium-ion detection limits of 10-8 M.The e,m.f. response of the electrodes in 10-3 M calcium chloride in 10-1 M sodium chloride was stable within the pH range from below 5 to about 9. All of the above electrodes showed good selectivity towards calcium ions over sodium, potassium and magnesium ions.A range of di-n-alkyl phenylphosphonate solvent mediators (alkyl = pentyl, hexyl, heptyl, octyl, nonyl, decyl or undecyl) can be used with calcium bis[di(4-octylphenyl)phosphate] sensor but electrodes from the pentyl and undecyl compounds degrade slightly more quickly than the others. Decan-1-01 solvent mediator leads to characteristic interference from magnesium ions.Electrodes with membranes containing the neutral carrier calcium-ion sensor NN'-di[ (1 l-ethoxycarbonyl)undecyl]-NN'-4,5-tetramethyl-3,6-dioxaoctane diamide with 2-nitrophenyl octyl ether as solvent mediator were capable of near-Nernstian calibrations and calcium-ion detection limits of less than 10-6 M but quickly deteriorated.They also responded well a t 50 "C.
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