The pins of receiver are inverted?
The longer is negative and the shorter is positive?
Yes and what of it?
It is a result of having a pull up resistor and the photo transistor pulling down. It makes no odds, you either cope in your code with smaller numbers meaning more light or if you really can't cope with that then subtract your reading from 1023 to invert the values. This sort of thing is normal in embedded systems.
So you think that the IR receiver and emmiter are more appropriate for this project?
No, this thread has been cleaned up by the moderators due to cross posting. You didn't mention your application in your initial thread.
White light is preferable because turbidity is a visible effect, so it makes no sense at all to use IR to measure it.
I tried 10k but the output is frozen, and keep showing 14.
That suggests it it too sensitive and the photo transistor has saturated. So reduce the value of the pull up resistor, try 1K.
I have a IR kit 5mm LED, is it possible to modulate the LED signal?
Yes, but if you modulate it then you must include at the sensor end an amplifier with a frequency response that rejects DC signals and responds only to the modulation frequency. That helps with stray light, but for proper measurements your equipment must be made to measure this in the dark to have any meaning.
Given you are supposed to be a University student then I would have thought that this was blindingly obvious.
You have the arrow pointing in the wrong direction in you block diagram you gave in the first post in the thread. Again not a mistake one would expect from a University student.