High-precision measurement of pH in the full toothpaste using NMR chemical shift

The pH of toothpaste is a critical factor for product stability and customer acceptance. However, no reliable and consistent method is currently available to precisely quantify pH in toothpaste as it is. In this study, a new method to directly determine pH value in the full toothpaste contained sodium bicarbonate (NaHCO₃) was developed. Briefly, we utilized the ¹³C NMR chemical shift of a small molecule that has been formulated in the toothpaste as a sensitive probe to consistently respond to the pH value of the full toothpaste. The ideal pH probe molecule has the following characteristics: (1) its NMR chemical shift is sensitive to pH within a certain range, and (2) the chemical shift only responses to pH value, not to other factors, such as molecular interaction. NaHCO₃ is a common ingredient in many toothpaste products used as a mild abrasive and an effective pH adjustment compound. Its chemical shift is very sensitive to pH; therefore, it was used as a candidate molecule to test this concept. This technique was demonstrated on select toothpaste formula contained arginine and sodium bicarbonate with different abrasive bases. The result shows that the pH value of full toothpaste is significantly higher than the pH of the toothpaste slurry. Arginine is a key active ingredient in these toothpastes, and it does not interfere with the chemical shift of sodium bicarbonate. The traditional method to determine the pH of toothpaste using pH electrode in toothpaste slurry typically has a larger measuring error, ranging from 0.1 to 0.3. This new method greatly reduced the measuring error, providing a consistent way to reliably determine pH in the full toothpaste, and enabling the stability test of toothpaste with smaller variations. This newly developed method can be further extended to other low-water gel or paste products at a different pH range by using different probe molecules.