Thanks so much for all your help
I have just looked at the bag of Sodium Carbonate on my shelf. It's Dri-Pak.
Use it with confidence. It must show the ingredients in full, but judging by the appearance of the crstals, I would consider that the other ingredients are impurities at a level not much more than trace.
I have been using that bag of Dri-Pak Soda Crystals for years. It is still almost full.
Your call, but I'm not going to buy the laboratory grade. On the other hand, we use so little per litre that you might take the opposite view, that a small amount of lab grade Sodium Carbonate, though expensive, will be likely to last a lifetime.