The primary standard used for the standardization of hydrochloric acid (HCl) is typically sodium carbonate (Na₂CO₃).

Sodium carbonate is a suitable primary standard because it is:

• Available in high purity: It can be obtained in a highly pure form, which is essential for accurate standardization.
• Stable: It is stable and does not readily absorb moisture from the air (although it can, so drying is usually required).
• Has a high equivalent weight: This reduces the impact of small weighing errors on the accuracy of the standardization.

Other substances like borax (sodium tetraborate decahydrate) can also be used, but sodium carbonate is more common.

Here is how sodium carbonate is used to standardize HCl:

1. A known weight of dried, pure sodium carbonate is dissolved in water.
2. This solution is titrated against the HCl solution using an appropriate indicator, such as methyl orange or bromocresol green.
3. The endpoint of the titration is reached when the indicator changes color, signifying the complete neutralization of the sodium carbonate by the HCl.
4. From the volume of HCl used and the known weight of sodium carbonate, the molarity (concentration) of the HCl solution can be calculated.

The reaction between sodium carbonate and hydrochloric acid is:

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)