Research Article
Determination of the Acidity Constants of Serotonin in the Ground and Excited States Using Spectroscopic Methods
Abdourahmane Khonte*
,
Abdou Dieng,
Coura Dione,
Latyr Dione,
Atanasse Coly
Issue:
Volume 14, Issue 2, April 2026
Pages:
19-28
Received:
15 March 2026
Accepted:
24 March 2026
Published:
14 April 2026
DOI:
10.11648/j.ajche.20261402.11
Downloads:
Views:
Abstract: Serotonin (5-hydroxytryptamine, 5-HT) is a biologically important neurotransmitter whose photophysical behavior depends on its protonation state. This study investigates the acid-base properties of serotonin in aqueous solution using UV-visible absorption and fluorescence spectroscopies. The ground-state acidity constant (pKa(S0)) was determined spectrophotometrically, while the excited-state acidity constant (pKa*(S1)) was estimated using the Förster thermodynamic cycle. Absorption spectra revealed an isosbestic point, indicating a simple two-state equilibrium between the protonated (R-OH) and deprotonated (R-O⁻) forms. The results show that serotonin behaves as a weak acid in the ground state (pKa(S0) ≈ 10.5–10.6), whereas its acidity increases significantly in the excited state (pKa*(S1) ≈ 4.7) due to electronic redistribution within the indole chromophore. Excited-State Proton Transfer (ESPT) occurs efficiently, influencing both fluorescence intensity and emission wavelength. These findings provide a comprehensive understanding of serotonin’s photophysical behavior and support its use as an intrinsic fluorescent probe for monitoring local pH variations in aqueous or cellular environments. The combination of UV-Vis and fluorescence measurements, with triplicate statistical validation, ensures reproducibility and accuracy of the determined acidity constants. This work contributes to a better understanding of neurotransmitter acid-base behavior under physiologically relevant conditions and demonstrates the potential application of serotonin in fluorescence-based pH sensing and molecular studies.
Abstract: Serotonin (5-hydroxytryptamine, 5-HT) is a biologically important neurotransmitter whose photophysical behavior depends on its protonation state. This study investigates the acid-base properties of serotonin in aqueous solution using UV-visible absorption and fluorescence spectroscopies. The ground-state acidity constant (pKa(S0)) was determined sp...
Show More
Research Article
Mathematical Modeling of Bio-Unit System for Oilfield Produced Water Treatment
Darlington Bon Nwokoma*,
Kenneth Kekpugile Dagde
Issue:
Volume 14, Issue 2, April 2026
Pages:
29-44
Received:
12 March 2026
Accepted:
20 March 2026
Published:
23 April 2026
DOI:
10.11648/j.ajche.20261402.12
Downloads:
Views:
Abstract: The Nigerian oil and gas extraction faces the challenge of treating excessive co-extracted Produced Water (PW) to fulfil reinjection or disposal specifications. Deploying a proposed modular bio-oxidation system (Bio-Unit) to treat PW necessitates predictive model for this biotreatment process. Thus, this work aimed at formulating mathematical models for predicting and simulating the Bio-Unit system. Experimentally determined biokinetic coefficients and other operating parameters were incorporated into the developed models and integrated numerically using fourth-order Runge-Kutta algorithm. Model predicted values of outlet chemical oxygen demand (COD), total organic carbon (TOC), and bioagent suspended solids (MLSS) were 14.7 mg/l, 7.02 mg/l, and 3252.0 mg/l, respectively. The percentage deviation of model predicted values from measured values was 4.3%, 3.1% and 7.9% for COD, TOC and MLSS, respectively. Linear regression between measured values and model predicted values gave the best fit value (R2) of 0.9923, 0.9890, and 0.9831 for COD, TOC, and MLSS, respectively, which indicates that the formulated model had a significant correlation with the Bio-Unit data and is 99.2%, 98.9%, and 98.3% dependable in predicting the parameters. The mean bias error for predicted MLSS, COD and TOC concentrations were -3.67, 4.79 and 3.24, respectively, which indicates that the model under-predicted the MLSS, while the COD and TOC concentrations were over-predicted. Model simulation showed that biosolid retention time of 21 days and hydraulic retention time of 1.0 day resulted in 98.3% and 98.5% COD and TOC removal. Hence, the formulated models are adequate and recommended for predicting and optimizing the Bio-Unit system.
Abstract: The Nigerian oil and gas extraction faces the challenge of treating excessive co-extracted Produced Water (PW) to fulfil reinjection or disposal specifications. Deploying a proposed modular bio-oxidation system (Bio-Unit) to treat PW necessitates predictive model for this biotreatment process. Thus, this work aimed at formulating mathematical model...
Show More
Research Article
Novel Soybean Soapstock Biodiesel Synthesis Through Esterification / Transesterification Reactions Using Methanol and N-hexane Solvents
Issue:
Volume 14, Issue 2, April 2026
Pages:
45-53
Received:
29 October 2025
Accepted:
12 November 2025
Published:
24 April 2026
DOI:
10.11648/j.ajche.20261402.13
Downloads:
Views:
Abstract: Soybean soapstock (SS) a by-product from the soybean processing food chain can serve as a viable feedstock for biodiesel production. This viability was attributed to its low costs and high conversion rates. Biodiesel from this feedstock required both esterification and transesterification reactions, using methanol and n-hexane as solvents. Determination of the component fatty acids was carried out using gas chromatography while functional groups were obtained using the FTIR (Fourier transform infrared spectroscopy) for both feedstock and biodiesel. The produced biodiesel had a higher percentage of unsaturated fatty acids when compared to its feedstock (soybean soapstock) with oleic acid (26.045%) and linolenic acid (22.344%) constituting the bulk of the unsaturated fatty acids. This conversion of saturated fatty acids into unsaturated fatty acids as confirmed in the GC analysis highlighted the effect of transesterification in biodiesel production. The effect of transesterification was further confirmed by the presence of cyclic ester compounds and carbonyl groups as observed in the FTIR analysis. Desirable fuel properties for the produced biodiesel were confirmed by comparing its physico-chemical properties with standard fuel properties. A high biodiesel yield of 96.8wt% and positive economic indices (38% ROI) obtained further validated soybean soapstock as a viable feedstock for biodiesel production.
Abstract: Soybean soapstock (SS) a by-product from the soybean processing food chain can serve as a viable feedstock for biodiesel production. This viability was attributed to its low costs and high conversion rates. Biodiesel from this feedstock required both esterification and transesterification reactions, using methanol and n-hexane as solvents. Determin...
Show More
