Magnesium chloride hexahydrate is a highly soluble form of magnesium used across food production, dietary supplements, road maintenance, agriculture, skincare, and medical solutions. Its versatility comes from the fact that it dissolves readily in water and delivers magnesium ions efficiently, making it useful in situations where other magnesium compounds fall short.
Tofu Production and Food Use
One of the oldest and most widespread uses of magnesium chloride hexahydrate is as a coagulant in tofu making. Known by its Japanese name “nigari,” it curdles heated soy milk into solid curds. The coagulation process depends on concentration: below about 3 mmol/L, nothing happens to the soy proteins, but between 3 and 8 mmol/L the proteins begin to clump together. Stable coagulation occurs between 8 and 50 mmol/L. Too much, and the curds actually break apart again. Tofu makers prize nigari for producing a tender, slightly sweet product with a more uniform internal structure compared to other coagulants like calcium sulfate (gypsum).
Magnesium Supplements
Magnesium chloride hexahydrate is a common ingredient in oral magnesium supplements, including well-known sustained-release tablets. Among inorganic magnesium salts, it stands out for its high solubility. Magnesium oxide, by comparison, packs more elemental magnesium per gram but dissolves poorly, which limits how much your body actually absorbs. Magnesium chloride sits in a middle ground: it dissolves very easily, though organic forms like magnesium citrate still tend to edge it out in absorption studies.
The current tolerable upper intake level for supplemental magnesium in adults is 350 mg per day, set by the U.S. Institute of Medicine. Going above that threshold increases the risk of gastrointestinal side effects, particularly loose stools and abdominal cramping. In one study of 21 people taking 360 mg per day of magnesium chloride, two reported diarrhea. That side effect profile is notably milder than what’s seen with magnesium oxide, where a study of 50 participants at 476 mg per day saw 18 develop diarrhea and 6 drop out because of it.
Topical Magnesium Products
Dissolved in water, magnesium chloride hexahydrate creates what’s often marketed as “magnesium oil,” a slippery liquid applied to the skin. The idea is that magnesium can be absorbed through the skin, bypassing the gut and avoiding digestive side effects. The evidence for this is still limited but suggestive. A pilot study applying a magnesium cream over several weeks found an 8.5% increase in blood magnesium levels in the treatment group compared to 2.6% in the placebo group. In a subgroup of non-athletes, the increase was more dramatic: blood magnesium rose 22.7%, from 0.75 to 0.92 mmol/L, a statistically significant change. Athletes in the study showed smaller gains, possibly because their baseline levels were already higher.
These results are promising but come from a small trial of 24 people. Topical magnesium products remain popular for muscle soreness and sleep support, though the scientific case for skin absorption is not as firmly established as it is for oral supplements.
De-Icing Roads and Walkways
Magnesium chloride hexahydrate is widely used as a road de-icer because it lowers the freezing point of water and generates heat as it dissolves. When incorporated into road surface materials, it can push the freezing point down to around minus 13°C (about 9°F), releasing roughly 0.43 kilojoules of heat per gram as it dissolves. That exothermic reaction actively melts ice on contact rather than simply preventing new ice from forming.
Compared to rock salt (sodium chloride), magnesium chloride works at lower temperatures and is considered less corrosive to concrete and metal. It also attracts moisture from the air, which helps it stay active on road surfaces longer. Municipalities and transportation departments use it both as a pre-treatment (applied before a storm) and as a reactive de-icer during freezing conditions.
Correcting Magnesium Deficiency in Crops
Magnesium is the central atom in chlorophyll, the molecule that makes photosynthesis possible. When crops run low on it, the tissue between leaf veins turns yellow while the veins themselves stay green, a pattern called interveinal chlorosis. Tomatoes are especially prone to this problem.
Magnesium chloride hexahydrate dissolves cleanly in water, making it practical for foliar feeding, where a dilute solution is sprayed directly onto leaves. University of Delaware Cooperative Extension recommends a 0.5% to 2% fertilizer solution for vegetable crops. Foliar application corrects deficiency faster than soil amendments because the magnesium enters the plant directly through the leaf surface rather than waiting for root uptake.
Medical and Pharmaceutical Uses
In hospital settings, magnesium chloride hexahydrate serves as a source of magnesium ions in intravenous and renal replacement fluids. The FDA lists it as an ingredient in solutions used during continuous renal replacement therapy, where precise electrolyte concentrations are critical. These solutions typically contain magnesium at concentrations between 1 and 1.5 mEq/L to maintain safe blood levels in patients whose kidneys cannot regulate electrolytes on their own.
Beyond critical care, magnesium chloride appears in various pharmaceutical formulations where a stable, highly soluble magnesium source is needed. Its molecular weight of 203.30 g/mol (including the six water molecules bound to the crystal) makes dosing calculations straightforward for pharmacists and manufacturers.
Why the “Hexahydrate” Matters
The term hexahydrate means each molecule of magnesium chloride is bonded to six water molecules in its crystal structure. This matters practically because the anhydrous (water-free) form is aggressively hygroscopic, pulling moisture from the air so forcefully that it’s difficult to store and handle. The hexahydrate form is the stable version at room temperature, easy to weigh, dissolve, and work with. When you buy magnesium chloride flakes for a bath, food-grade nigari, or a supplement containing magnesium chloride, you’re almost always getting the hexahydrate form.