If the product is a commercial product sold in major US stores like drugstores, grocery stores, or Wal-Mart, etc., then it you can be assured that it has been tested to at least some degree since no company would take the risk of marketing an untested product on a mass scale like that. Also, a pH of 5.5 is not too acidic. Tap water in various parts of the US is slightly acidic (between pH=6 and 7). But naturally acidic food products like vinegar or lemon juice are even more acidic than 5.5 (closer to pH=3, actually). Vinegar and lemon juice are not irritating to most people's skin. Taking all of these facts into account, I would not be concerned about using the skin product.