This problem most likely indicates that a portion of your powder has decayed from iron (III) or ferric oxalate to iron (II) or ferrous oxalate. Ferrous oxalate is poorly soluble in water; ferric oxalate is slightly soluble in water.
The brownish color of the dissolved FO in the beaker is what it should look like. Not all ferric oxalate will be that color, but the B&S powder will. How old is your ferric oxalate powder? I see you got it from ArtCraft. Dana at B&S told me recently that ArtCraft gets their FO from B&S (and B&S gets something, I forgot what, from ArtCraft). Is it possible that your FO is more than a couple years old? That is a problem I ran into a couple of years ago with a jar that was about 3 years old. A fresh supply from B&S solved the problem, but now I don’t keep more than I anticipate using in a year on hand.
180°F is 82°C, so it seems if it takes 2-3 hours to get to 178 there is maybe something wrong with the hotplate. My simple kitchen hot plate will boil 100ml of water in 5-10 minutes. A microwave oven is useful for this too.
I suppose it’s possible that being exposed to the sodium vapor light overnight could have caused this to happen, but it seems unlikely. I hadn’t thought about it before, but I think I always mix fresh solutions last thing and then turn the lights out.
But, I see another possible issue, and I guess that’s why I asked about the formula you are using. As you approach the saturation point it takes longer for the remaining solute to dissolve. The saturation point for ferric oxalate is around 33% if I remember right, though I can’t find a reference so I may be thinking of something else. I think you are using too much ferric oxalate powder. Where did you get the idea to use 29gm? Are you actually using 100ml of water, or is that the final volume? If the latter, which is the correct way to make your solutions, you are using too much powder.
Your problem could be due to one of these factors or a combination.
I suggest that you start with 25gm of (fresh) ferric oxalate powder and 80ml of distilled water. Heat the water to ~ 180°F or even boiling (I use a microwave), add the powder, put it on your stirrer for a little while, then just let it sit. Don’t continue heating it. All of the powder should dissolve within a couple hours. Then transfer to a graduated cylinder (for more accurate measuring) and add distilled water to bring the final volume to 100ml.
Hope this isn’t too much information. 