I usually use the pinch a penny 31.45 percent acid but their hours suck and I couldnt make it over there. I bought a two pack of HD acid at the same concentration but I have doubts after using it. It didnt fume, didnt smell or burn my eyes or nose and there was a tiny bit of solid stuff at the bottom. I was actually able to look down into the white bottle to see the dark solids on the bottom. With the pinch a penny acid I would have hated life if I did this. I am going to test PH again when I get home to see what it did but I am confused.