Cooling System clean with Zitronensäure(Citric acid)? HOW TO USE IT?

[JohnV270]

I have an inappropriate coolant inside the car and i want to change it with a 325.0 one but first i think it will be good to clean the system. I bought some Zitronensäure (CITRIC ACID) from MB Dealership but i don't know how to use it, mix it, pour it etc...
1)Can someone who did this, write me a step by step plan? Please?
I don’t have a thermostat that can stay open all the time to perform this cleaning. Is it enough to run 40 minutes with the heater ventilation on MAX?
2)Doesn’t the thermostat open when you put heater on max?
3) Does anyone know how much coolant mix enters in a CLK 270 CDI W209?
In the owner’s manual it is 12.5 but I think it is to much...I mean like a CLK 55 AMG....and since the owner’s manual is in German I think I have misread something.

 

[BlackC55]

Why do you need to clean it. Has the head gasket gone or been done?

It is used for this purpose when oil is mixed with coolant.

 

[JohnV270]

The Gasket hasn't been replaced or done...But i read that it isn't only for oil mixed with coolant. I simply want to clean the system because i want all the bad coolant out and in the process to clean the bad stuff deposits. Can't i use it for that too? I Mean i don't want to have problems of using it for something that it wasn't meant to be used for.