A single novel PVC membrane for dual determination of sulphadimethoxine and malachite green in aquatic environment

2.2.1 Reference sample

Sulphadimethoxine sodium was kindly supplied from PHARMA SWEDE-Egypt and its purity was certificated to be 99.7%. Malachite green oxalate salt was obtained from Sigma–Aldrich, Germany with certificated purity of 96.9%.

2.2.2 Pharmaceutical formulation

Trimethoxin® was manufactured by Chemifarma and was purchased from local market. Each 1 mL was claimed to contain 200 mg sulphadimethoxine and 40 mg trimethoprim, Batch number (B.N.): A120009.

2.4.1 Stock solutions of SDM and MG (each, 1 × 10⁻² M)

They were freshly prepared by transferring 0.166 g of SDM and 0.4645 g of MG oxalate, separately, into two 50-mL volumetric flasks. Each compound was separately dissolved in 10 mL bi-distilled water, and the volume was then completed and protected from light.

2.4.2 Working solutions of SDM and MG (1 × 10⁻⁷ to 1 × 10⁻³ M)

Different solutions of each compound were freshly prepared by serial dilution of its corresponding stock solution using bi-distilled water. The prepared solutions were kept in well-closed tight containers and protected from light.

2.5.1 Aquatic sample preparation and collection

2.5.2 Precipitation-based technique for the preparation of PVC-membrane sensor

A volume of 20 mL aliquot of saturated aqueous sulphadimethoxine sodium (SDM) solution was mixed with 20 mL of saturated aqueous malachite green oxalate salt (MG) with continuous stirring. The resultant precipitate (ion pair complex) formed was filtered using Whatmann No. 42 paper, washed with bi-distilled water, dried at room temperature (about 25 °C) and grinded to fine powder. In a glass Petri dish (5 cm diameter), 10 mg of MG–SDM ion pair was thoroughly mixed with 0.4 mL of DOP and 0.19 g of PVC. The mixture was dissolved in 5 mL of THF. The Petri dish was covered with a filter paper and left to stand overnight to allow solvent evaporation at room temperature. A master membrane with a thickness of 0.1 mm was obtained. From the master membrane, two disks (8 mm diameter) were cut using a cork borer and pasted using THF, to two interchangeable PVC tips that were clipped into the end of the two electrode glass bodies to construct sensors 1 and 2.

Sensor 1 was then filled with an internal solution of equal volumes of 10⁻² M SDM and 10⁻² M KCl, while sensor 2 was filled with 10⁻² M MG and 10⁻² M KCl. Ag/AgCl wire (1 mm diameter) was used as an internal reference electrode. Each sensor was conditioned by soaking in 10⁻² M of its aqueous solution for 24 h, and then stored in the same solution when not in use.

2.5.3 Effect of pH

The effect of pH on the potential values of the two sensors was studied over pH range of 3–10 at 1 pH interval by using 10⁻⁴ and 10⁻³ M solutions for sensors 1 and 2, respectively. The pH was gradually increased or decreased by adding aliquots of dilute sodium hydroxide or dilute hydrochloric acid solutions. The potential obtained at each pH value was recorded.

2.5.4 Sensor calibration

Each sensor and the double junction Ag/AgCl were immersed in its corresponding aqueous solutions in the range of 1 × 10⁻⁷ to 1 × 10⁻² M. They were allowed to equilibrate while stirring and recording the EMF readings within ±2 mV. The membrane sensors were stored in distilled water between measurements. The mV-concentration profiles were plotted.

2.5.5 Sensor selectivity

The potentiometric selectivity coefficient – $\text{log}{\text{K}}_{(\text{Primary ion,interferent})}^{\text{pot}}$ was used to evaluate the extent to which a foreign ion would interfere with the response of an electrode to its primary ion.

Selectivity coefficients were calculated by the separate solution method (IUPAC, 2008), where potentials were measured for 10⁻³ M aqueous solutions for sensors 1 and 2, respectively and then for 10⁻³ M aqueous interferent solution separately then potentiometric selectivity coefficients were calculated using the following equation: $$\text{log}{\text{K}}_{\text{A,B}}^{\text{pot}}=\left({\text{E}}_{\text{B}}-{\text{E}}_{\text{A}}\right)/\text{S}+\left(1-{\text{Z}}_{\text{A}}/{\text{Z}}_{\text{B}}\right)\text{log}{\text{a}}_{\text{A}}$$ where ${\text{K}}_{\text{A,B}}^{\text{pot}}$ is the selectivity coefficient, E_A and E_B are the potentials of the drug and the interferent solutions respectively, S is the slope of the calibration plot, a_A is the activity of the drug, Z_A and Z_B are the charges on the drug and the interfering ions, respectively.

2.5.6 Direct potentiometric determination of SDM in its pharmaceutical formulation (Sensor 1)

From Trimethoxin® bottle, suitable dilutions were taken using bi-distilled water to obtain serial of 10⁻⁴–10⁻³ M SDM. Procedure was then completed as under Section 2.5.4. From the recorded potential calculate the concentration of SDM from its corresponding regression equation.

2.5.7 Direct potentiometric determination of SDM and MG in fish water

In volumetric flasks of 25 mL, the filtered fish water was spiked with standard drug solutions and the volume was completed to the mark with the fish water. Then the EMF values of these spiked water samples were recorded and the concentrations of SDM and MG were calculated from the corresponding regression equation.