Saline Solution: When ‘Bad’ Chemicals Are Good For You

PART 1: What’s normal about “normal saline”?

Is there abnormal saline?

Normal saline (often abbreviated NS) is defined as a 0.9% concentration of sodium chloride dissolved in water. Sodium chloride is the primary ingredient in table salt.

NPR did a nice article on the historical origins of normal saline.  In 1883, the sodium chloride level in human blood was incorrectly measured as being 0.9%, and this concentration became the definition of a “normal” level of saltiness in medical liquids, whether internal (blood) or external (IV fluids).  In medicine as in other fields, once a historical mistake becomes entrenched in theory and in practice, it is exceedingly difficult to un-entrench it.

Normal saline has a pH of about 5.5 – 5.7.  For a geek-out discussion as to why, read this article.  Okay, I’ll read it for you.

Freshly distilled water has a pH of about 7, which is what we would expect because pure water is neither acidic nor basic.  However, water absorbs carbon dioxide from the atmosphere, where it is converted to carbonic acid.  A molecule of carbonic acid dissociates (falls apart) into one hydrogen ion and one bicarbonate molecule.  The free hydrogen ions liberated from carbonic acid acidify the water, causing the pH to drop to about 5.7.

When you add sodium chloride to distilled water, two opposing effects on pH occur.  First, less carbon dioxide is absorbed from the atmosphere, which acts to limit the decrease in pH.  Second, sodium chloride increases the dissociation of carbonic acid into hydrogen and bicarbonate, adding more free hydrogen ions to the solution, thus decreasing the pH.  As an added twist, sodium ions make hydrogen ions less acidic.  The net effect of adding salt to water is a minimal further reduction in pH by about 0.01.  This is pretty small, so let’s ignore it.

Straight-up normal saline ophthalmic products usually start out more acidic than your natural tears, because they were manufactured in earth atmosphere (not on the moon) and atmosphere contains carbon dioxide.  The pH of some normal saline products can be less than 5.  It is possible to have a manufacturing process in a space that is not filled with “air,” but rather something inert like nitrogen gas.  This is expensive, and saline eye wash needs to be cheap.  Also, the minute you open the bottle air rushes in, so why bother.

The pH of the normal saline continues to decrease as the bottle is being used and the liquid absorbs more carbon dioxide from room air.

PART 2: When bad-sounding chemicals are good for you

The pH of a normal saline eye drop can be adjusted to neutral (7.0 – 7.4 range) and kept at that neutral level by a buffer system.  A buffer system consists of a molecule that can release a hydrogen ion (an acid), and a companion molecule that can soak up a hydrogen ion (a base).  Some classic examples include citric acid and sodium citrate, or boric acid and sodium borate.  Don’t think of these “acids” as liquids that are burning holes in a lab bench or your clothing, and therefore your corneas.  Any molecule that can liberate a hydrogen ion is technically an acid.  Almost any molecule that starts with the word sodium is technically a base. 

As carbon dioxide is absorbed into the saline solution, carbonic acid is created.  The buffer system doesn’t stop this process – instead it stops the carbonic acid from acidifying the saline.  The loose cannon hydrogen ions that are liberated from the carbonic acid displace the sodium ions on the base, converting the base into its companion acid – for example, sodium citrate becomes citric acid.  The citrate molecule hangs on to the hydrogen ion tightly, thus the pH remains stable.  Having both an acid buffer and a base buffer in the bottle allows the hydrogen ions to shuttle around between them without ever managing to totally break free, thus preventing significant drift in the pH either up or down.

If you use saline to fill your scleral lenses, you should use buffered saline.  If you use unbuffered saline, you are trapping a liquid which probably has a pH much lower than your natural tears into contact with your corneas, for hours.

This isn’t good.

A resource guide for purchasing preservative free saline solution can be found here.