Preheat the oven to 170°C fan forced. Add the baking soda and then pour the orange juice in a large bowl. It will start to froth, just mix it to ensure all the baking soda is dissolved otherwise you will end up with white spots in the cake.
Spray the baking dish with some oil all around or you may choose to line it with baking paper. I used a rectangular baking dish measuring 26x21x5cm in size, see notes2. Place the baking dish on the middle rack in the oven for 60-65min or until it is cooked through.
1. As an indication I needed 5 small oranges but that may vary depending on the juiciness of oranges used.
2. If the baking dish you use is larger, then the cake may bake faster due to the increase in surface area to volume ratio, so keep that in mind.