Darlington took the lead against the run of play in the 27th minute when a low cross from Nathan Mulligan was turned in at the far post by Purcell for his fourth goal in seven games.
Purcell then grabbed his second in the 53rd minute when his 20-yard free-kick took a wicked deflection before wrong-footing Chesterfield keeper Tommy Lee.
The hosts looked to be heading for a comfortable win until Chesterfield substitute Simon Whaley fired an angled drive under keeper Redmond in the 80th minute.
A stunning 30-yard strike from Drew Talbot a minute later put Chesterfield level and Darlington's misery was complete four minutes from time when central defender Richie Byrne fired past his own keeper.
Source: Team Talk
Source: Team Talk