Rather simple.
compute once the values in global variable.
Lets say:
play field width is 800
drop from edge is 100
sprite width is 30
drop width is computed
drop offset is computed
var fw= 800
var de= 100
var sw= 30
var dw= fw-2*de
var do= de-sw/2
and in your function, change to this line
Chlorine.position = CGPoint(x: CGFloat (arc4random_uniform (dw )+do) ,y: self.size.height)
variable names are up to you.
Update: I don't know swift, but this change should fit
func spawnChlorine(){
var Chlorine = SKSpriteNode(imageNamed: "Chlorine.png")
var fw= self.size.width
var de= 100
var sw= 30
var dw= UInt32(fw-2*de)
var ok= UInt32(de-sw/2)
Chlorine.position = CGPoint(x: CGFloat (arc4random_uniform (dw )+ok) ,y: self.size.height)
let action = SKAction.moveToY(-20, duration: 5.0)
let actionDone = SKAction.removeFromParent()
Chlorine.runAction(SKAction.sequence([action, actionDone]))
Chlorine.runAction(SKAction.repeatActionForever(action))
Chlorine.physicsBody = SKPhysicsBody(rectangleOfSize: Chlorine.size)
Chlorine.physicsBody?.categoryBitMask = PhysicsCatagory.Chlorine
Chlorine.physicsBody?.contactTestBitMask = PhysicsCatagory.Shot
Chlorine.physicsBody?.affectedByGravity = true
Chlorine.physicsBody?.dynamic = true
self.addChild(Chlorine)
}