What, specifically, doesn't work?
Can you provide a more complete (testable) version of your code?

2. PImage i1,item1;
3. 
   
4. void setup()
5. {
6.   size(800,400);
7.   i1 = loadImage("1.png");
8.   item1 = createImage(28,10,RGB);
9.   
10.   
11.   //Items mit Transparenz versehen
12.   i1.loadPixels();
13.   item1.loadPixels();
14.   //Pixel kopieren
15.   for (int i = 0; i < i1.pixels.length; i++)
16.   {
17.     item1.pixels[i] = i1.pixels[i];
18.   }
19. 
    
20.   //Nun die Transparenz zuweisen
21.   for (int i = 0; i < item1.pixels.length; i++)
22.   {
23.     if (item1.pixels[i] == color(128, 0, 128))
24.     {
25.     item1.pixels[i] = color(0,0,0,0);
26.     }
27.   }
28.   i1.updatePixels();
29.   item1.updatePixels();
30. }
31. 
    
32. void draw()
33. {
34.  background(255); 
35.  image(i1,0,0,300,300);
36.  image(item1,400,0,300,300);
37. }

My problem is that there is no alpha value.

The problem is that none of the pixels in the image are actually the color (128, 0, 128). Take, for example, the first pixel. Its int representation is -8450435, compared with -8388480, which is what you are checking for. Perhaps you should use a threshold of so many values.