Is there an App to lock Whatsapp?

Is there an app i can download fro apple store to lock Whatsapp? I have an iPhone 4. Please give one that is guaranteed and i don't mind if it costs money as long as it works and sets a password to my Whatsapp.

Yeah just type into the app store ''whatsapp lock'' then it will come up with many different apps. You can even download an app called ''app lock'' which will lock any app