## 3 Results and Discussion
### Voltammetric Quantification of PG and TBHQ  
This kind of quantification presents a challenge in biolubricants, as this technology has been widely studied in biodiesel samples (where the particularities of each sample could also affect the operating conditions of voltammetry). However, it needs adaptation for the different physico-chemical conditions found in biolubricants, especially concerning the variable viscosity values found for each specific product. That is the reason why an initial study should be accomplished to optimize voltameric response. Firstly, preliminary studies were carried out to register different samples, including buffer solution, and biolubricant without antioxidant addition and with antioxidant addition (in this case PG), in order to assess the voltammetric (CV) response. Figure 9 shows the main results. As can be inferred from this figure, the signal corresponding to the buffer solution and biolubricant without antioxidant addition is similar, with no response being observed. Nevertheless, the biolubricant with 300 ppm PG shows an anodic response that is increased with PG addition, with a signal appearing at the same potential. Thus, PG in doped biolubricants could be quantified. Afterwards, different studies for WCOBL with PG and TBHQ addition at different concentrations were carried out to assess the effect on antioxidant signals in voltammetry. Consequently, calibration lines for each antioxidant could be obtained. For this purpose, different standards were prepared, as explained in Section 2, at different PG or TBHQ concentrations; this test was conducted in duplicate for each case (see Figures 10 and 11 for PG and TBHQ addition, respectively). Figure 9: PG quantification in WCO biolubricant. As expected, the signal corresponding to PG (an anodic peak at 0.5 V) increased with its concentration in WCOBL (both peak area and height), with no signal shown in the inverse scanning. Concerning TBHQ, anodic and cathodic peaks (at 0.5 and \(-\)0.02 V, respectively) were observed, with a linear trend when the peak and height signal were represented. In this case, the anodic peak is more intense than the cathodic one, with PG and TBHQ having very similar anodic signals that are very close to each other. As a consequence, mixtures of both antioxidants in biolubricant could be determined, where two peaks (one anodic due to the mixture and the other cathodic due to TBHQ) were obtained. In both cases (PG and TBHQ), the quality parameters for the calibration curves are included in Table 6.  
Figure 11: Different TBHQ concentrations and its voltammetric determination in WCO biolubricant. Figure 10: Different PG concentrations and its voltammetric determination in WCO biolubricant. The high quality of the parameters included in this table should be noted (especially with high linearity, obtained at different PG and TBHQ concentrations and conducting the experiments in triplicate), similar to previous studies where TBHQ was quantified in a biolubricant based on WCO, obtained through double transesterification with methanol and pentaerythntrol [41]. Figure 12 includes the main signals obtained when PG and TBHQ were added to WCOBL. Thus, obtaining the anodic peak of PG + TBHQ, it is possible to correlate the peak area of this figure to the TBHQ signal (Figure 11) to obtain the signal corresponding to PG according to Equations (1) and (2):  
\[\left|\frac{I_{pA,\ TBHQ}}{I_{pC,TBHQ}}\right|=2.14\ \pm 0.16 \tag{1}\]  
\[\left|\frac{A_{pA,\ TBHQ}}{A_{pC,TBHQ}}\right|=2.98\pm 0.32 \tag{2}\]  
where \(I_{p}\) is the peak intensity, \(A_{p}\) is the peak area, and subscripts \(A\) and \(C\) correspond to anodic and cathodic, respectively.