Titrimetric Determination and Precision Analysis of Saponification Value of Commercial Coconut Oil

Introduction

Fats and oils are essential components of human nutrition and serve critical roles in food processing, cosmetic formulation, and industrial manufacturing. Their characterization through physicochemical parameters is fundamental to quality assurance, authenticity verification, and regulatory compliance.^1–3 Among these parameters, the saponification value (SV) is one of the most widely utilized, providing information on the average molecular weight and fatty acid chain length distribution of triglyceride constituents.^4–6

The saponification value is defined as the milligrams of potassium hydroxide (KOH) required to neutralize free fatty acids and saponify the esters present in one gram of fat or oil.^7–9 SV exhibits an inverse relationship with average molecular weight: oils rich in short- and medium-chain fatty acids have higher SVs because of the greater number of ester bonds per unit mass, while long-chain fatty acid oils yield correspondingly lower values.^10–12 The theoretical basis derives from the alkaline hydrolysis stoichiometry, which requires three equivalents of KOH per mole of triglyceride.¹³

Coconut oil (Cocos nucifera L.) is distinguished among edible vegetable oils by its high medium-chain triglyceride (MCT) content—approximately 60–70% of total fatty acids—dominated by lauric acid (C12:0, 45–52%), myristic acid (C14:0, 16–21%), caprylic acid (C8:0, 5–10%), and capric acid (C10:0, 4–8%).^14–19 This fatty acid profile confers a characteristically high SV, typically 248–265 mg KOH/g per international standards.^20–22

SV determination serves multiple analytical objectives. In quality control, it enables adulteration detection through dilution with oils of differing fatty acid profiles.^23–25 It also reflects hydrolytic rancidity, as free fatty acid liberation alters saponification stoichiometry.^26–28 In industry, precise SV data are required for soap manufacturing alkali calculations.^29–31

The classical titrimetric method—exhaustive alkaline hydrolysis under reflux followed by back-titration of excess KOH with standardized acid—is codified in ISO 3657 and AOAC Official Methods.^32–37 Result accuracy depends on completeness of saponification, reagent stability, and endpoint detection precision.^35–37

Despite the fundamental importance of SV in oil quality assessment, systematic evaluation of method precision and analytical variability remains insufficiently addressed in the literature. This study therefore aimed to determine the SV of commercial coconut oil using the classical titrimetric method and to comprehensively evaluate the precision, repeatability, and reliability of the procedure through rigorous statistical analysis.

Materials and Methods

Chemicals

All reagents were of analytical grade. Potassium hydroxide pellets (≥85% purity) were dissolved in 95% ethanol to prepare the alcoholic saponifying solution. Hydrochloric acid (37%) was diluted and standardized as the titrant. Potassium hydrogen phthalate (KHP, 99.95% purity) served as primary standard for NaOH standardization. Phenolphthalein indicator (1% w/v in ethanol) was used for endpoint detection. Absolute ethanol (95% v/v) was employed as the solvent for alcoholic KOH preparation.^42,43

Sample Material

Commercial coconut cooking oil (SPRING brand) was obtained from local retail sources. Prior to analysis, the oil sample was filtered through Whatman No. 1 filter paper to remove particulate matter and traces of moisture that might interfere with the saponification reaction. Coconut oil was selected as the analytical substrate due to its characteristically high proportion of medium-chain triglycerides, which significantly influence the SV and facilitate evaluation of method performance.^14–16

Preparation of Reagent Solutions

0.5 N Alcoholic Potassium Hydroxide Solution

Potassium hydroxide pellets (28.05 g) were dissolved in 20 mL of distilled water with cooling to prevent excessive heat generation, then diluted to 1000 mL with 95% ethanol and mixed until homogeneous. The solution was stored in a tightly sealed polyethylene container to minimize atmospheric CO2 absorption.⁴⁴

0.5 N Hydrochloric Acid Solution

Concentrated hydrochloric acid (approximately 41 mL) was carefully diluted to 1000 mL with distilled water. The precise normality was determined by standardization against sodium hydroxide solution of known concentration.

1% Phenolphthalein Indicator

Phenolphthalein (2 g) was dissolved in 100 mL of absolute ethanol and mixed thoroughly.

Standardization Procedures

Standardization of Sodium Hydroxide

Potassium hydrogen phthalate (approximately 0.5 g, accurately weighed) was dissolved in 50 mL of distilled water and titrated against sodium hydroxide solution using phenolphthalein indicator until a persistent pink color was achieved. The normality was calculated as:

N_NaOH = m_KHP / (M_KHP × V_NaOH)

where m_KHP is the mass of KHP (g), M_KHP is the molecular weight of KHP (204.22 g/mol), and V_NaOH is the volume of NaOH consumed (L).

Standardization of Hydrochloric Acid

Aliquots (25.0 mL) of hydrochloric acid solution were titrated against standardized sodium hydroxide using phenolphthalein indicator. Normality was calculated as:

N_HCl = N_NaOH × V_NaOH / V_HCl

Saponification Value Determination

The determination was performed according to modified ISO 3657 methodology.⁴⁵ Oil samples (2–5 g, accurately weighed) were transferred to 250-mL round-bottom flasks. Alcoholic KOH solution (25–50 mL) was added from a calibrated burette, and contents were swirled to ensure complete dissolution. A blank determination was prepared simultaneously by adding an identical volume of alcoholic KOH to a separate flask without oil. Both flasks were fitted with reflux condensers and heated in a water bath at gentle boiling for one hour to ensure complete saponification. After cooling, phenolphthalein indicator (2–3 drops) was added. Each flask was titrated with standardized HCl until the pink color disappeared completely, indicating the endpoint.

Calculation of Saponification Value

The saponification value was calculated as

SV = [(B – S) × N × 56.1] / W

where SV = saponification value (mg KOH/g); B = volume of HCl consumed by blank (mL); S = volume of HCl consumed by sample (mL); N = normality of HCl (mol/L); 56.1 = molar mass of KOH (g/mol); W = mass of oil sample (g).

Statistical Analysis

All determinations were performed in triplicate. Results were evaluated using descriptive statistics including mean, standard deviation (SD), variance, and relative standard deviation (%RSD). Method precision was assessed per ISO 5725 repeatability criteria,⁴⁶ with the repeatability limit calculated as:

r = 2.8 × SD

The factor 2.8 ensures that 95% of differences between repeated measurements under repeatability conditions fall below the repeatability limit.^46,47

Results

Standardization of Sodium Hydroxide

The results of sodium hydroxide standardization using KHP as primary standard are presented in Table 1. The mean normality of sodium hydroxide was 0.4452 N, which was subsequently used for hydrochloric acid standardization.

Table 1: Standardization of Sodium Hydroxide Using Potassium Hydrogen Phthalate (KHP) as Primary Standard

Standardization of Hydrochloric Acid

Table 2 summarizes the standardization of hydrochloric acid against sodium hydroxide. The mean normality of HCl was 0.4884 N with minimal inter-trial variation, indicating satisfactory reagent preparation.

Table 2: Standardization of Hydrochloric Acid Against Standardized Sodium Hydroxide

Saponification Value Determination

Experimental data for SV determination are presented in Table 3. The blank titration consumed 30.8 mL of standardized HCl, representing the total unreacted KOH in the absence of oil.

Table 3: Experimental Data for Saponification Value Determination of Commercial Coconut Oil

Statistical Analysis

Descriptive statistics and precision parameters for the SV data are summarized in Table 4.

Table 4: Descriptive Statistics and Precision Parameters of Saponification Value Results

The repeatability limit was calculated as: r = 2.8 × 12.96 = 36.29 mg KOH/g

Repeatability Assessment

Inter-trial differences were evaluated for compliance with ISO 5725 repeatability criteria (Table 5).

Table 5: Inter-Trial Differences and ISO 5725 Repeatability Assessment

All inter-trial differences were below the repeatability limit of 36.29 mg KOH/g, confirming that results satisfy the ISO 5725 criterion that no more than 5% of differences between independent single test results obtained under repeatability conditions should exceed the repeatability limit.^46,47

Discussion

Interpretation of Saponification Values

The experimentally determined SV ranged from 228.04 to 253.77 mg KOH/g, with a mean of 241.80 mg KOH/g. These values fall slightly below the Codex Alimentarius standard of 248–265 mg KOH/g for coconut oil³⁸ but remain consistent with reported values for refined and commercial coconut oil products.^48–50 Milani and Folegatti⁴⁹ reported SVs of 243–262 mg KOH/g for commercial coconut oils, and Maurikaa, Jaganivash, and Shanmugasundaram⁵⁰ documented 252 ± 0.75 mg KOH/g for refined, bleached, and deodorized (RBD) coconut oil—closely approximating the upper range observed here. The relatively high SV confirms the predominance of medium-chain fatty acids characteristic of coconut oil. As established by Marina, Che Man, and Amin¹⁴ and Prasanth Kumar and Gopala Krishna,⁴⁰ coconut oil contains approximately 60–70% MCTs, primarily lauric acid (C12:0) and myristic acid (C14:0), which yield higher SVs due to the greater number of ester bonds per unit mass.^10–12

Precision and Repeatability Assessment

The statistical analysis demonstrated acceptable analytical precision. The %RSD of 5.36% falls well below the commonly accepted threshold of 10% for manual titrimetric methods, indicating satisfactory reproducibility.^59,60 According to Miller and Miller,⁶¹RSD values in this range are typical for titrimetric determinations relying on visual endpoint detection. All inter-trial differences fell below the calculated repeatability limit of 36.29 mg KOH/g per ISO 5725, confirming that the observed variability represents normal analytical scatter rather than systematic methodological deficiency.^46,47

Analytical Limitations and Sources of Variability

Several sources of analytical variability should be explicitly acknowledged. First, incomplete saponification during the one-hour reflux period represents a primary source of systematic negative error, as unreacted triglycerides reduce the calculated SV^51,52.Completeness depends on adequate mixing, temperature maintenance, and sufficient reagent excess.⁵³ Second, atmospheric CO₂ absorption by the alcoholic KOH solution converts KOH to potassium carbonate, reducing effective saponifying agent concentration and leading to SV underestimation.⁵⁴ Phenolphthalein is insensitive to potassium carbonate, compounding this effect.⁵⁵ Third, endpoint detection subjectivity inherent in phenolphthalein-based titration introduces inter-trial variability through differences in operator perception and titration rate; the relatively small volume difference between blank and sample titrations amplifies the proportional impact of such errors.⁵⁶ Fourth, sample-related factors including RBD processing may slightly alter the fatty acid profile relative to virgin coconut oil,⁵⁷ and the presence of partial glycerides or free fatty acids affects SV stoichiometry.⁵⁸ Collectively, these factors adequately explain the slight deviation from Codex standard values observed in this study.

Implications for Quality Control

The SV is a reliable indicator of coconut oil authenticity and quality.^23–25 The mean of 241.80 mg KOH/g, though marginally below the Codex minimum, is consistent with values reported for refined commercial products and does not indicate adulteration or significant quality defects. In soap manufacturing, the high SV of coconut oil indicates greater KOH consumption per unit mass relative to long-chain oils, which is characteristic of its MCT-rich composition and contributes to stable lather in hard soaps.^29–31,62

Comparison of Analytical Approaches

The classical titrimetric method remains the reference procedure in ISO 3657 and AOAC Official Methods.^42–45 While gas chromatographic fatty acid analysis with mathematical SV calculation and 1H-NMR spectroscopy offer advantages in throughput and non-destructive operation,^63,64 the titrimetric method retains advantages of simplicity, cost-effectiveness, and accessibility for routine quality control. Ivanova et al.¹⁰ demonstrated strong correlation between NMR-derived and titrimetric SVs for dairy fats, supporting the continuing relevance of the titrimetric approach.

Conclusion

The SV of commercial coconut oil (SPRING brand) was successfully determined by classical titrimetric analysis. The experimentally obtained values ranged from 228.04 to 253.77 mg KOH/g, with a mean of 241.80 mg KOH/g, SD of 12.96 mg KOH/g, and %RSD of 5.36%. Although the mean falls slightly below the Codex Alimentarius range of 248–265 mg KOH/g, results are consistent with published data for refined commercial coconut oil and are attributable to known analytical sources of variability, including incomplete saponification, CO₂ absorption by reagents, and endpoint detection subjectivity. Statistical evaluation per ISO 5725 confirmed method repeatability, with all inter-trial differences below the calculated repeatability limit of 36.29 mg KOH/g. The titrimetric method is confirmed as reliable and suitable for routine quality assessment, authenticity verification, and industrial evaluation of edible oils. The high SV reflects the predominant MCT composition of coconut oil and supports its applications in food processing, cosmetics, and soap manufacturing.

Recommendations

Based on the findings and identified analytical limitations, the following measures are recommended to improve method performance:

Reagent preparation and storage: Alcoholic KOH should be freshly prepared or stored in tightly sealed containers with minimal headspace to prevent CO₂ absorption. Standardization should be performed immediately before use.

Saponification conditions: Extending the reflux period beyond one hour or employing a higher reagent excess may improve completeness of saponification, particularly for samples of unknown composition.

Sample mass optimization: Using larger oil samples (5–10 g) increases the volume difference between blank and sample titrations, reducing the proportional impact of endpoint detection errors on precision.

Instrumental endpoint detection: Potentiometric titration with automated endpoint determination eliminates subjective color assessment and may improve inter-analyst reproducibility.

Comprehensive quality assessment: SV should be evaluated alongside complementary parameters—acid value, iodine value, and peroxide value—to provide holistic oil quality characterization.

Application

Saponification value determination finds application across multiple sectors: food industry quality control for oil identity verification, adulteration detection, and regulatory compliance; soap and detergent manufacturing for alkali requirement calculations; cosmetic formulation for selection of oils with appropriate physicochemical properties; biodiesel production for feedstock composition assessment; and nutritional research for fatty acid chain length characterization and MCT content estimation.

Acknowledgement

The author acknowledges the laboratory facilities and technical support provided by the Natural and Applied Sciences Department, College of Arts and Sciences, Nueva Ecija University of Science and Technology. Appreciation is extended to the Bachelor of Science in Food Technology (BSFT) students who contributed to data collection and preliminary analysis.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflict of Interest

The author(s) do not have any conflict of interest.

Data Availability Statement

This statement does not apply to this article.

Ethics Statement

This research did not involve human participants, animal subjects, or any material that requires ethical approval.

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