There are many brands of pickling salt available and there is not much difference from one to another. The best pickling salts are not treated with iodine and come in the form of small, powder-like crystals that dissolve quickly in a pickle brine. All salts intended for pickling are composed of sodium chloride just as standard table salt is.
When choosing a pickling salt, it is important to find one that does not have iodine in it. Table salt usually does contain iodine, which is both an important nutrient and an additive that keeps salt from clumping when exposed to humidity. Using a salt treated with iodine in a pickle, however, can change the color of the pickles and the brine. Though there is nothing wrong with eating pickles discolored by iodine, the yellow or brown tint that the food takes on can make it look unappealing.
It is also important, when choosing a pickling salt, to pay attention to the size of the salt crystals themselves. Most pickling recipes assume that the pickling salt has small crystals that are about the size of grains of sand. Cooking instructions are often given in volume, which may make it difficult to use the correct amount of salt if the salt contains larger crystals. There isn't a problem with using kosher salt or flake salt in a pickle, though the amount of salt used will need to be adjusted, as an equal weight of these larger salts will take up more volume than pickling salt does.
Select a pickling salt that does not have any additives or extra ingredients. Sea salt, Hawai'ian salt, and other specialty salts are not ideal for use as pickling salts. These salts often have other chemicals in them that can affect the acidity of the pickle brine. Adjusting the acidity, even a small amount, can make it possible for bacteria to grow in the pickle. If this happens, people who eat the pickles could become ill.
Low sodium salt should never be used as a pickling salt. The salinity of the brine needs to be carefully regulated so that the pickle is safe to eat long after pickling. Using a brine that is not salty enough can also make it possible for bacteria to grow in the pickle. Additionally, only actual sodium chloride salt is appropriate for use as a pickling salt.