Why must the Hospira UK Ltd atracurium besilate 10 mg/ml solution for injection/infusion product not be mixed with alkaline solutions in the same syringe?

The Hospira UK Ltd atracurium besilate 10 mg/ml solution for injection/infusion product has an acid pH, so it must not be mixed with alkaline solutions such as barbiturates in the same syringe or administered simultaneously with them through the same intravenous infusion needle.