*2.4. Electrical Conductivity and pH*

The electrical conductivity meter was first calibrated with the use of standard calibration fluid supplied by the manufacturer. On calibration, the standard fluid was measured at 25 ◦C (in triplicates) and an average of 1413.6 <sup>µ</sup>S·cm−<sup>1</sup> was recorded. Thereafter, the electrical conductivity of DIW and hybrid nanofluids was determined at temperatures of 15–55 ◦C (at 10 ◦C intervals). Buffer solutions were used to calibrate the pH meter at a pH values of 4, 7, and 10. Thereafter, the measurement of pH values of the hybrid nanofluids was conducted. The uncertainty of the electrical conductivity and pH measurements was 6.2% and 0.04%, respectively. In comparison with DIW, the relative electrical conductivity and the improvement recorded for MWCNT-Fe2O3/DIW nanofluids were evaluated as expressed in Equations (2) and (3), respectively.

$$
\sigma\_{rel} = \frac{\sigma\_{hff}}{\sigma\_{bf}} \tag{2}
$$

$$
\sigma\_{enhan}(\text{\textquotedblleft}\_{0}\text{\textquotedblright}) = \left(\frac{\sigma\_{lnf} - \sigma\_{bf}}{\sigma\_{bf}}\right) \times 100\tag{3}
$$

where

*σhnf* = measured electrical conductivity (hybrid nanofluids) and *σbf* = measured electrical conductivity (DIW).
