Table 1.
Typical mitragynine absorbance at various pH and determination of pKa using the UV spectrophotometer and microplate spectrophotometer methods.
| Target pH | UV Spectrophotometer Method | Microplate Spectrophotometer Method | ||||
|---|---|---|---|---|---|---|
| Actual pH achieved | Absorbance (d) | pKa | Actual pH achieved | Absorbance (d) | pKa | |
| 7.6 | 7.52 | 0.8436 | 8.0219 | 7.55 | 0.4993 | 8.0860 |
| 7.8 | 7.78 | 0.8186 | 8.1617 | 7.76 | 0.4823 | 8.1124 |
| 8.0 | 7.96 | 0.7786 | 8.1726 | 7.94 | 0.4527 | 8.0256 |
| 8.2 | 8.13 | 0.7367 | 8.1810 | 8.15 | 0.4350 | 8.0870 |
| 8.4 | 8.34 | 0.6677 | 8.1281 | 8.32 | 0.4170 | 8.1015 |
| 8.6 | 8.45 | 0.6232 | 8.0489 | 8.50 | 0.3910 | 8.0257 |
| 8.8 | 8.71 | 0.6148 | 8.2692 | 8.66 | 0.3877 | 8.1477 |
| 9.0 | 8.92 | 0.5347 | 7.9303 | 8.80 | 0.3707 | 8.0567 |
| Dm | 0.4917 (pH 12) | 0.3393 (pH 12) | ||||
| Di | 0.9544 (pH 5) | 0.5460 (pH 5) | ||||
| pKa (Mean ± SD) | 8.11 ± 0.11 | 8.08 ± 0.04 | ||||
Note: pKa was calculated using the following formula: pKa = pH + log (d − dm/di − d) for basic drug.