Alternating current polarographic evidence for the reversible reduction of trivalent gallium from acidified 1.0 M sodium thiocyanate at 60.deg.

Abstract

Clinical research (1-5) has established that neoplastic sarcomas and certain categories of carcinoma (lung, breast, colon, etc.) (6) exhibit a pronounced tendency to sequester serum gallium citrate and other gallium salts. The metal's therapeutic effectiveness in regressing or arresting malignant growth-either as absorbed nonradiogallium ( 6 ) or as (principally) the 67 isotope used for in situ radiotherapy (2)-has prompted exploration of new benchtop analytical procedures ( 7 ) for the measurement of trace Ga which might speed up tissue assay efforts and aid in the eventual identification of the responsible biochemical mechanism.Were it not for the classically severe kinetic complications which are associated with the aqueous gallium electrode reaction (GER) (8)(9)(10)(11)(12), the electroanalysis of Ga(II1) a t trace and ultratrace concentration levels could be readily accomplished using a variety of acidified supporting electrolyte media. As described (10-14) in several earlier studies, the chemical composition of the electrolyte can play a catalytic role in moderating the actual charge transfer rate. For example, it has been found (IO) that a kinetically fast (Le., Nernstian) gallium electrode process is attainable a t the dropping mercury electrode (DME) if the supporting electrolyte is adjusted to satisfy two criteria simultaneously: 1) the presence of an inert salt at a very high ionic strength ( J ) ; and 2) the incorporation of >0.1 M SCN-(12) or NB-(13) (halides are also marginally effective a t extremely high J values of 1 1 3 M ( I d ) ] . A supporting electrolyte comprised of acidified NaSCN/G.OM NaC104 was successfully employed in three recent investigations in which trace levels of gallium, including gallium in ashed tissue, were measured by ac phase-selective anodic stripping (PSAS) procedures (15-177, and Demerie et al. employed essentially the same media as the basis for their pulse polarographic analysis of gallium in samples of gallium arsenide semiconductors (18). However, binary electrolytes of the above types-which to our knowledge are the only ones known so far to produce room temperature GER reversibility-can pose a variety of practical analytical difficulties ( 1 9 ) , due to the densely concentrated salts required, problems that could be substantially alleviated by discovery of milder electrolyte conditions. We wish to report in this note evidence that the degree of kinetic reversibility of the gallium electrode process exhibits a pronounced reciprocal dependence on ionic strength and temperature. Results obtained from single sweep alternating current phase-selective polarograms (PSP) show that a simple 30-OC increment in reaction temperature removes the historical necessity for densely concentrated inert salts and enables one to obtain a Nernstian voltammetric current readout from acidified NaSCN alone a t the 1.0-molar level (J = l), and an analytically useful peak in 0.1M NaSCN. EXPERIMENTALApparatus and Reagents. The apparatus and reagents used to obtain the phase-selective polarogr...

Experimental factors affecting the phase-selective reversible anodic stripping determination of gallium from 1.0 molar ammonium thiocyanate electrolytes at 60.degree.C and collation of results with sodium thiocyanate/sodium perchlorate-based room temperature measurements

“…The results of PSAS peak current vs. bulk Ga(lll) concentration, peak current reproducibility for 67.8 ppb Ga(lll), and standard addition experiments are tabulated, with the first two compared to accompanying differential pulse anodic stripping (DPAS) measurements on the same solutions. While our 1.0 M NH4SCN PSAS results at 60 °C indicated that Ga can be measured rapidly with fair precision down to 20 ppb, it is necessary to conclude from a comparative evaluation of the influences of , , , and pH for the room temperature vs. high temperature cases that the 60 °C procedure is experimentally somewhat less flexible and, at the present time, probably more suitable to specialized applications.Electroanalytical and electrochemical studies of the catalyzed reversible gallium electrode reaction (GER) have achieved the stage (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) where rapid and inexpensive anodic stripping methods [e.g., phase-selective anodic stripping (PSAS), or differential pulse anodic stripping (DPAS)] can be applied under special conditions for the analysis of samples containing trivalent gallium in the 1 to 10 parts-per-billion (ppb, or ng/g) range. Potentially useful areas for clinical application of these methods for rapid tissue…”

“…Electroanalytical and electrochemical studies of the catalyzed reversible gallium electrode reaction (GER) have achieved the stage (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) where rapid and inexpensive anodic stripping methods [e.g., phase-selective anodic stripping (PSAS), or differential pulse anodic stripping (DPAS)] can be applied under special conditions for the analysis of samples containing trivalent gallium in the 1 to 10 parts-per-billion (ppb, or ng/g) range. Potentially useful areas for clinical application of these methods for rapid tissue…”

“…studies of localized radioactive and nonradioactive trace concentrations of gallium in neoplastic tumors and bone have recently been discussed (12)(13)(14). Kinetically reversible gallium voltammetry, upon which the feasibility of such measurements rests, has thus far been realizable at room temperature only by employing specially formulated supporting electrolytes made up of a very soluble pseudohalide [e.g., NaNg (5) or NaSCN (4)] together with a noncomplexing, auxiliary inert salt (conveniently NaC104) present at Í 6.0 molar concentration (3,4).…”

“…Recently we presented results from alternating current phase-selective polarograms (PSP) (14) obtained with the dropping mercury electrode (DME) which showed qualitatively, but unambiguously, that the degree of GER kinetic reversibility in the ac sense is governed, inter alia, by a reciprocal dependence on reaction temperature and solution ionic strength (J). That is, Nernstian polarographic behavior was observable with 1.0 M NaSCN alone as the background electrolyte, provided that "high" electrolyte temperatures (60 to 70 °C) were employed.…”

“…That is, Nernstian polarographic behavior was observable with 1.0 M NaSCN alone as the background electrolyte, provided that "high" electrolyte temperatures (60 to 70 °C) were employed. Commercially available glass-jacketed cells enable temperatures in this range to be managed easily (14), and the associated, rather dramatic drop in the ionic strength requirement to ca. the 1.0 M level or less seemed to offer a practical breakthrough to simplifying some of the electrolyte problems which plague stripping attempts at lower temperatures (9, 12).…”