What is pH Value of ddH2O?

The pH of double-distilled water (ddH₂O) is theoretically around 7.0 — perfectly neutral — because pure water at 25°C autoionizes slightly into H₃O⁺ and OH⁻ ions.

But in practice, ddH₂O often measures a pH of around 5.5 to 6.5.
This is because it rapidly absorbs CO₂ from the air, forming a small amount of carbonic acid (H₂CO₃), which lowers the pH slightly.

So basically:

• Freshly prepared ddH₂O: close to 7.0
• Exposed to air for a while: usually ~5.5–6.5