How can I perform the suitability test described in general chapter 2.5.12.?

Answer:

You may proceed as described below. The sequential additions are about the same size for each step, which usually corresponds to about 50 to 100 percent of the amount found in the sample.

1. Titrate the sample (M mg of water).

2. Add the first amount of water (x1 mg). Titrate (y1 mg).

3. Add the second amount (x2 mg). Titrate (y2 mg).

4. Add the third amount (x3 mg). Titrate (y3 mg).

5. Add the fourth amount n (x4 mg). Titrate (y4 mg).

6. Add the fifth amount (x5 mg). Titrate (y5 mg).

7. Calculate the recovery r for each addition and the mean recovery. 

8. Calculate the regression line for the data where the cumulative water added (X1=x1; X2= x1 + x2; X3=x1 + x2 + x3; X4=x1 + x2 + x3 + x4; X5= x1 + x2 + x3 + x4 + x5) on the x-axis is plotted against the sum of the initial water content determined for the substance (M) plus the cumulative water determined (Y1=M + y1; Y2=M + y1 + y2; Y3=M + y1 + y2 + y3; Y4=M + y1 + y2 + y3 + y4; Y5= M + y1 + y2 + y3 + y4 + y5) on the y-axis.
Calculate the slope (b), the intercept with the y-axis (a) and the intercept of the extrapolated calibration line with the x-axis (d) (which is always negative), and the percentage errors (e1 and e2).
In an ideal case a and │d│ are equal to M.