I recently wrote a paper related to enteral feeding tubes so I had a reference handy...
This is from the Journal of Parenteral and Enteral Nutrition: Enteral Nutrition Practice Recommendations
quotes are from p. 23
"The appearance and pH of aspirates from a feeding tube mayprovide useful clues to an enteral access device location.
For example, fluid withdrawn from a tube that has perforated
into the pleural space typically has a pale yellow
serous appearance and a pH of 7 or higher, while fasting
gastric fluid typically is clear and colorless or grassy green
with a pH of 5 or less.12-16 However, appearance and pH of
aspirates are not sufficiently accurate to distinguish
between gastric and bronchopulmonary placement."
"It is difficult to obtain an aspirate from an enteral
access device when the tip lies in the esophagus. On the
occasions when an aspirate can be obtained, it is likely
refluxed gastric juice or swallowed saliva. Thus, observing
the aspirate's pH and appearance is of little or no
benefit in this situation. The auscultatory method also
cannot differentiate between the esophagus and stomach."
Sorry for bad formatting but I have to get ready for work!
link to document: http://pen.sagepub.com/content/early....full.pdf+html