AN IMPROVED PROCESS FOR THE PREPARATION OF SILDENAFIL CITRATE (Viagra) IN ITS POLYMORPHIC FORM The present invention provides an improved process for preparation of 1-[[3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H –pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulphonyl]-4methypiperazine citrate (Sildenafil Citrate) ofPolymorphic form I)