A caffeine-sensitive membrane electrode: Previous misleading report and present approach

Abstract

Although a previous study [S.S.M. Hassan, M.A. Ahmed, M.M. Saoudi, Anal. Chem. 57 (1985) 1126] had shown that a caffeine-sensitive electrode made with picrylsulfonate and 1-octanol as a cation-exchanger and a solvent mediator, respectively, had a wide working pH range (5.5-9.5) and exhibited a Nernstian response, we could not find such response in this electrode. The present result was reasonable, because the pK(a) value of caffeinium ion was reported to be around 0.7 and the neutral form of caffeine was predominant in the pH range examined. Thus, we reinvestigated the response characteristics of a caffeine electrode, taking into consideration the pK(a) value, and constructed a new electrode with a combination of the lipophilic cation-exchanger, tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate (HFPB), and the solvent mediator with high degree of dielectric constant, 2-fluoro-2'-nitrodiphenyl ether (FNDPE). This electrode showed a pH-dependent response to caffeinium ion and gave a detection limit of 50muM with a slope of 55mV per concentration decade at pH 2. The use of other solvent mediators was less effective than that of FNDPE. The electrode was applied for the determination of caffeine in some central stimulants.

Liposomes for Topical Use: A Physico-Chemical Comparison of Vesicles Prepared from Egg or Soy Lecithin

“…the dissociation of the lipid molecules. Recently, a similar observation was obtained also on the surfactant vesicles whose zeta potential did not change after the addition of caffeine [31]. …”

Electrochemical Determination of Caffeine Using Bimetallic Au−Ag Nanoparticles Obtained from Low‐cost Green Synthesis

“…To further probe the type of process taking place at the electrode surface, the dependence of the log of anodic peak current on the log of scan rate was plotted (graph not shown) and a straight line was obtained with a slope of 0.61, which is comparable with the theoretical slope of 0.5 for a diffusion-controlled process [58]. It was shown from previous reports that, caffeine is a weak base with a very low pK a value of approximately 0.7 [59,60]. Hence it was argued that the protonation of the caffeinium ion (imine nitrogen) present will be highly favoured in acidic media, a number of reports have proposed the mechanism for caffeine oxidation involving the transfer of four electrons (4e À ) and four protons (4H + ) [55,58,[60][61][62].…”

Quantitative Analysis of Active Pharmaceutical Ingredients (APIs) Using a Potentiometric Electronic Tongue in a SIA Flow System

“…Additional cation‐sensitive electrodes based on KTFPB in PVC/DOS membranes were introduced to the sensor array, since the studied mixtures of APIs contained also caffeine – a component commonly added to the pharmaceutical formulations with anti‐inflammatory drugs. However, due to the very low value of pK a , the neutral form of caffeine was predominant in solution under the studied pH range 25 and thus the electrodes showed rather small and non‐linear response to this component.…”

“…ISEs based on polymer membranes doped with the ion‐pair complex of tetraoctylammonium cation were developed for the analysis of ibuprofen 21, 22. Similar approach was proposed for the design of ion‐selective electrodes sensitive towards: naproxen 23, ketoprofen 24 and caffeine 25. It is worth to note, that the use of potentiometric sensors provided measurements in flow injection mode (see a review on the application of Flow Injection Analysis (FIA) and Sequential Injection Analysis (SIA) for pharmaceutical analysis in 26).…”