Laboratory validation of a cost-effective walkley–black titrimetric method for the determination of soil organic carbon

Soil organic carbon (SOC) is a key indicator of soil health, fertility,
and sustainability, influencing nutrient cycling, soil structure, and biological
activity. This study validated a classical Walkley–Black titrimetric method for
routine SOC determination under laboratory conditions. The method is based on
wet oxidation of organic carbon using potassium dichromate in concentrated
sulfuric acid, followed by back-titration of residual dichromate with standardized
ferrous sulfate using diphenylamine as an indicator. Method validation included
assessment of linearity, precision, accuracy, limit of detection (LOD), and limit of
quantification (LOQ). Calibration was performed using glucose standards
covering 0.1–3.0% organic carbon. A BIPEA certified reference soil was used to
ensure traceability and evaluate trueness. Excellent linearity was obtained (R² =
0.999). Precision showed relative standard deviation values below 5%, while
accuracy assessed by standard addition yielded recoveries of 99.3–100%,
indicating negligible matrix effects. The calculated LOD and LOQ were 0.01%
and 0.04% OC, confirming that the method is sufficiently sensitive, reliable, and
cost-effective for routine SOC analysis in agricultural and environmental soils.