Development of a Differential Normal Pulse Voltammetric Method for Phosphate Determination in Water

Abstract

Determination of phosphate levels in water is important in assessing the quality of drinking water and remediation of phosphate in water sources. Visible spectrophotometry is the standard technique that has been used for analysis of phosphates. However, this method is affected by interferences such as turbidity, salinity and high concentration of ions (silicate, arsenate and germanate). These interferences make analysis of phosphates at low concentrations difficult. The objective of this study was therefore to develop a voltammetry-based sensitive method for analysis of phosphates in water that would overcome interferences. Phosphate is not redox-active. Its electrochemical studies depend on its association with molybdate salts to form phosphomolybdate complex. Differential Normal Pulse Voltammetry (DNPV) and Cyclic Voltammetry (CV) were used to analyse the redox behaviour of the complex. Analysis by CV gave two redox centres with formal redox potentials of 0.167  0.02 V and 0.357  0.02 V with diffusion coefficients (D) of 1.408 ×10−4 𝑐𝑚2𝑠−1 and 5.629×10−7 𝑐𝑚2𝑠−1. Analysis of the complex using DNPV gave two responses, namely, 0.02  0.001 V and 0.33  0.001 V. The complex response varied with the concentration which made it possible to apply the technique for quantitative analysis of phosphates. Linearity of the method occurred between 0 – 8 mg/L of phosphate concentration, with a linear correlation coefficient of 0.9816. The limit of detection (LOD) and the limit of quantitation (LOQ) were found to be 0.06586 𝑚𝑔/𝐿 and of 0.21952 m𝑔/𝐿 respectively. Accuracy was determined in terms of percent recovery and was found to range from 89 % to 102 %. Precision was also determined in terms of percentage relative standard deviation (percent RSD) of ten replicate measurements of a single concentration and the percent RSD was found to be 7.93 %. Both precision and accuracy were within acceptable limits for electrochemical methods of analysis. The developed method was then applied to natural water samples collected from lake Naivasha. Fifty water samples were collected in 50 ml plastic bottles and analysed using the developed method. The concentration of phosphates was found to be 0.6156±0.1046 𝑚𝑔/𝐿 (at 95% confidence level), which was higher, compared to the previous studies. This could be attributed to the intensified human activities especially agricultural activities around the lake region over time. The study has shown that the method developed using DNPV could be more sensitive than CV. This is because it has a lower limit of detection, allowing it to be applied for the determination of low levels of phosphate. Further, the simplicity of the instrumentation is of significance in enhancing the possibility of sensor development for onsite use of the technique.