Sodium bicarbonate "dose-response"
We’ve been focusing on bicarbonate ions as one of the causes of discoloration. So, I made several sodium bicarbonate solutions and tracked discoloration of non-RTP’ed samples. I made 3.7 mg/mL NaHCO3 (=44mM, this is the concentration in DMEM), 1.85 mg/mL NaHCO3, 0.925 mg/mL NaHCO3, 0.4625 mg/mL NaHCO3 and 0.23125 mg/mL NaHCO3. I also ordered a different bicarbonate salt – potassium bicarbonate. I made a couple of dilutions of KHCO3 to test out, as well. All of these experiments were conducted in the incubator.
Discoloration is totally non-linear – only the highest concentration of sodium bicarbonate caused discoloration. In the bottom panel, 100% FBS was the positive control – only the highest NaHCO3 concentration caused discoloration. Since I knew that 3.7 mg/mL NaHCO3 caused rapid discoloration in the past (New HBSS), (DMEM salt), I only looked at lower concentrations in the next experiment (the top panel). There was no discoloration at NaHCO3 concentrations <= 1.85 mg/mL, even out to a week. At 4.4 mg/mL KHCO3 (=44mM), there was discoloration within a day but the lower concentration did not cause discoloration. This is the same result as NaHCO3.
If bicarbonate ions alone caused discoloration, I would have expected a more linear response.