At this moment, i am researching about Solubility test of Ciprofloxacin HCl at pH 6.8 (Phosphate Buffer according to USP 37). I have a problem with my result.

Beforehand, I will describe in detail about my procedure:

I did solubility test of Ciprofloxacin HCl (100 mg/100 mL and 25 mg/100 mL) at pH 6.8 for 4 hours using shaker (167 motion/minutes), and I took the sample of each point of 30 minutes, 60 minutes, 120 minutes and 240 minutes using independent samples. After sampling each point, then letting the sample for 10 minutes. I took the upper solution, and passed the solution through PVDF 0.2 µm filter (disposed 5 mL of filtrate) and then use this solution as the sample solution. This Solution was analyzed by UFLC. I've done it on a truly protected from light. I got the results that solubility of ciprofloxacin in pH decreased at pH 6.8 starting from 30 minutes – 240 minutes. The result of Solubility test of Ciprofloxacin HCl (100 mg/100 mL) decreased until 70% for 240 minutes, while the result of Solubility test of Ciprofloxacin HCl (100 mg/100 mL) decreased until 90% for 240 minutes. I didn't get straight trend or saturated trend.

Can you help me for giving me suggestion about my experiment or my procedure?