What is Sodium Bisulfite? - Formula, Calculation & Hazards

Instructor: Matthew Bergstresser
Sodium bisulfite is an inorganic salt that is also known as sodium hydrogen sulfite. In this lesson, we'll go through how its formula is determined, calculate its molar mass and discuss any potential hazards associated with it.

Sodium Bisulfite as a Food Preservative

If you have ever eaten dried apples or dehydrated potatoes or cooked with bottled lemon juice, you have ingested sodium bisulfite. It's used as a preservative to maintain the flavor and aroma of these foods. It is also used in the preservation of wine. If it's used in foods, it can't be too toxic, can it? Let's go through the basics regarding this compound then investigate any potential hazards associated with it.

Sodium bisulfite is used as a preservative in wine.
wine

Chemical Formula

Sodium bisulfite is an inorganic salt that also goes by another name, which is sodium hydrogen sulfite. The bisulfite polyatomic ion has the formula HSO3 -1 . The SO3 component of this polyatomic ion is sulfite, and normally has a -2 charge. When a hydrogen ion (H+1) is added to the sulfite ion we get bisulfite, and its oxidation state changes to -1.

A bisulfite ion and sodium ion combine to form sodium bisulfite.
sodium bisulfite

The sodium ion is Na+1, and it is electrically attracted to HSO3 -1 because their electric charges, or oxidation states, are oppositely charged. This is known as electrostatic attraction. Since the magnitudes of the charges are the same they combine in a one-to-one ratio. This results in the formula NaHSO3. Let's now look at the molar mass of the compound.

Molar Mass Calculation

Molar mass is the mass of one mole of a substance. To determine the molar mass of a compound, we add up the molar masses contributed by each element in the compound. In sodium bisulfite we have one sodium atom, one hydrogen atom, one sulfur atom and three oxygen atoms. Let's make a chart to keep each of these element's molar masses organized.

Molar Mass of Sodium Bisulfite
Element Quantity Molar Mass (g/mole) Total Molar Mass Contributed (g/mole)
Na 1 22.99 22.99
H 1 1.01 1.01
S 1 32.06 32.06
O 3 16 48

Total molar mass = 22.99 g/mole + 1.01 g/mole + 32.06 g/mole + 48 g/mole = 104.06 g/mole

Now let's get back to the potential hazards of this compound.

Hazards

The addition of any preservatives to foods has to be deemed safe by the Food and Drug Administration (FDA), and sodium bisulfite meets this designation. In fact, it has been included in foods in the United States since at least the 1800s. It's used as a preservative in a wide variety of foods, although the FDA prohibits the use of sodium bisulfite (and sulfites in general) on fresh fruits and vegetables.

Basically, sodium bisulfite is generally recognized as safe in the extremely tiny quantities required to use it as a preservative. At very high doses in its concentrated form, sodium bisulfite could severely irritate the skin, eyes and throat, but any quantity of sodium bisulfite greater than 10 parts-per-million (ppm) has to be listed in the ingredients list.

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