The Catalans go into the encounter with a 2-0 advantage from the opening leg thanks to a Sergio Busquets effort and an own goal from Enaut Zubikarai at the Nou Camp.
But Barca coach Tata Martino is not taking anything for granted.
"Each time there is less margin for error," Martino said on www.fcbarcelona.com.
"We are at a crucial stage of the season and it's clear that each time there will be fewer chances to make changes in the team, although we will try to give opportunities to players that are in good condition."
Martino is set to field the same side-up that won 4-1 at Sevilla on Sunday, with Alexis Sanchez, Pedro and Lionel Messi in attack.
Last weekend's win allowed Barca to bounce back from their surprise 3-2 loss to Valencia and return to the La Liga summit, level on 57 points with Real and Atletico Madrid.
Neymar returned to training on Monday, having recovered from the injury to his right ankle that has seen him sit out seven games.
However, the Brazilian forward is unlikely to feature on Wednesday.
Real Sociedad know they face a monumental task to knock out Barca, but they have not written off their hopes.
"We know we face a huge task against Barca," Real Sociedad defender Jon Ansotegi said. "But for many of us this could be our only opportunity to play in a final.
"It's clear that we need to go all out and we are mentally prepared."
Jagoba Arrasate's side were held to a goalless draw by Levante at the weekend.
The Basques are sixth in the standings, six points behind Athletic Bilbao, who hold the last Champions League qualifying spot.
Sergio Canales, who joined Real Sociedad from Valencia last month, could earn his first start on Wednesday, having come on as a second-half substitute against Levante.
Barca captain Carles Puyol is doubtful with a back injury.
The winner of Wednesday's game will set up a meeting with either Real or Atletico in the final on April 19.