Abstract: A new fixed pH method for the continuous determination of total acidity and crude protein in Rumex dentatus whole plant was established. The acids in samples were extracted by formaldehyde with the aid of ultrasound, and a fixed pH titration method was used to determine the total acidity using KOH as a standard solution. After the determination of the total acidity, the excessive formaldehyde in the sample solution was removed and then digested with concentrated sulfuric acid. The extra sulfuric acid was neutralized and the NH4＋dissociated into H+ by formaldehyde. The crude protein was determined by fixed pH titration method using KOH as the standard solution as well. The total acidity (H＋) of a dried sample was 0.9178 mol/kg with a relative standard deviation less than 5% (n= 6), and the crude protein was 15.30 g/100 g with the relatively standard deviation less than 3% (n = 6). These results indicated no obvious difference from those determined by the national standard method. In conclusion, the method has been proven to has many advantages such as simplicity, rapidity, easy operation, low regent consumption, short assay time, thus deserving to be popularized.

Key words: Rumex dentatua L., fixed pH method, total acidity, crude protein, continuous determination