How pure $\ce{NaCl}$ is typical table salt? Is it pure enough for research-level experiments in chemistry? How is $\ce{NaCl}$ purified if need to be?

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Table salt is often iodized (iodine is an essential micronutrient, and this is one way to introduce it into the diet in the right amount). According to Wikipedia,

Edible salt can be iodised by spraying it with a potassium iodate or potassium iodide solution. 60 ml of potassium iodate, costing about US $1.15 (in 2006), is required to iodise a ton of salt. Dextrose is added as a stabilizer to prevent potassium iodide from oxidizing and evaporating. Anti-caking agents such as calcium silicate are commonly added to table salt to prevent clumping.

Elsewhere, Wikipedia mentions other anti-caking agents, like sodium ferrocyanide, sodium aluminosilicate, and magnesium carbonate. And they point out that salt may also be "fortified" with iron or other micronutrients.

The certificate of analysis for "ultra pure NaCl" from American Bio shows TRACE amounts of aluminum, arsenic, bromides, heavy metals, iron, magnesium, phosphates, and sulfates. Those impurities may well be expected in table salt, too.

From all that, you'd think that table salt is fairly impure. Let's look at the numbers to put the amounts of these impurities in perspective. The box of Morton iodized salt in my kitchen gives me the amount of sodium per serving. If all of the sodium is in the sodium chloride, I estimate the purity as

$$\frac{0.590~\mathrm{g}~\ce{Na+}}{1.5~\mathrm{g}~\text{table salt}}\frac{1~\mathrm{mol}~ \ce{Na+}}{22.989769280~\mathrm{g}~\ce{Na+}} \frac{1~\mathrm{mol}~\ce{NaCl}}{1~\mathrm{mol}~\ce{Na+}}\frac{58.44277~\mathrm{g}~\ce{NaCl}}{1~\mathrm{mol}~\ce{NaCl}}\cdot 100\% = \fbox{99.99% NaCl}$$

where I'm assuming the label information is actually a bit more precise than reported (exactly 1.5 g of table salt contains approximately 590 mg Na). This is just a BOE (Back of the Envelope) calculation; I should round my answer to three or possibly two significant figures.

In fact there are sometimes other sodium salts present (anti-caking estimates such as Tivolex contain sodium) so this purity is a slight overestimate. One salt company (WA Salt supply) lists "typical" purities for the table salt it provides at 99.72%; this salt contains uses Tivolex. Morton Salt claims that their iodized table salt contains 0.04% dextrose and "less than 0.5% calcium silicate", so the mg Na per serving on the label is a little bit higher than it actually is; I should get something above 99.46%, considering that additional information.

Still, that's pretty pure. The assay for "ultra pure" NaCl linked above gives 99% to 100.5% NaCl. Sigma-Aldrich has reagent-grade NaCl (>98%), ACS grade NaCl (>99.5%) and TraceSelect NaCl for doing heavy metal analysis that is more than 99.999% pure.

Is 99% purity "pure enough for high level experiments in chemistry?" There is no one answer. It depends on what those experiments are. How many significant digits do you need in your results? Are the results going to be affected by the presence of tiny amounts of iodide, iodate, glucose, calcium, or silicate, or the other impurities listed above? Maybe, maybe not. If a specific impurity interferes, find a way to lock up, remove or otherwise neutralize its effects.

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  • $\begingroup$ A sense of social responsibility dictates removal of the (!) next to arsenic unless you have verified that it is in a harmful form and present in a harmful concentration (or the addition of a ! to all other compounds listed). Similar to the way that brussel sprouts could land you in the hospital is an irresponsible tag line for an article. $\endgroup$ – Jason C Feb 8 '15 at 22:19
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    $\begingroup$ @Jason C, you're right. What was I thinking? Exclamation point REMOVED. $\endgroup$ – Fred Senese Feb 8 '15 at 23:21
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    $\begingroup$ @FredSenese no way table salt is 99.99% NaCl see wasalt.com.au/Table.html and spexcertiprep.com/knowledge-base/files/AppNote_GourmetSalts.pdf $\endgroup$ – DavePhD Feb 9 '15 at 12:11
  • $\begingroup$ @DavePhD The first reference gives the purity as 99.72% for their product, and they use an anti-caking agent that contributes sodium to the mix. The second reference deals mainly with "Gourmet Salts" which contain many minerals. The table salt they analyze is quite clearly not at all like the Morton Iodized salt I'm discussing above; they don't give %NaCl at all, did I miss that? The top contaminant in table salt they list is potassium at 0.0006 - 0.029 mg/g. $\endgroup$ – Fred Senese Feb 9 '15 at 13:37
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    $\begingroup$ I have to ask as it is a bit of a bugbear of mine but how can anything be 100.5% pure - does it contain negative amounts of some impurities? It reminds me of a certain avert "Not less than 100% beef, (also contains..)" $\endgroup$ – Steve Barnes Feb 9 '15 at 13:40

Salt purity is measured on "dry salt" - which is to say, water does not count towards the purity. In the UK, 99% pure NaCl is used for ice-treating areas that might be subject to staining by less pure gritting methods.

99.9% purity is used for food: http://www.british-salt.co.uk/Products/PureDriedVacuum.aspx

This is the same purity as pharmaceutical-grade salt, except that pharm. grade also requires a moisture content of 0.01% or less.

For export markets, British Salt can then add the required impurities for that market's requirements.

The British Salt company provides about 2/3 of domestic cooking salt in the UK.

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Some table salt sold for consumption contain additives which address a variety of health concerns, especially in the developing world. The identities and amounts of additives vary widely from country to country. Wikipedia

Table salt is hardly even close to pure.

In summary, a variety of additives is used in common table salt. In addition to potassium iodide, the additives can be categorized as either iodide stabilizers or anticaking agents. Iodide stabilizers act to prevent the oxidation of iodide to iodine, while anticaking agents keep salt free-flowing. Abstract of this article

Thus, table salt can result in an approximation in the experiment you're conducting. You can look for kosher salt though.

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  • $\begingroup$ I did not know there's a thing such as Kosher salt. Thanks for the tip! $\endgroup$ – dash Feb 8 '15 at 18:41

See Analysis of Gourmet Salts for the Presence of Heavy Metals which reports analysis of table salt (noniodized), 12 gourmet salts, and reagent grade NaCl using inductively coupled plasma mass spectrometry.

Considering metals, other than sodium what is present, by far the greatest other metal ion present in table salt was potassium at 1000 ppm (1000 micrograms per gram of sample). Next was calcium at 218 ppm. Considering that pure NaCl is 393000 ppm Na, the presence of K and Ca alone brings the purity of table salt down to 99.7%

There is no indication that the above study tested for carbon, oxygen, silicon or hydrogen, so levels of water, sugar such as dextrose, silicates or carbonates should be considered unknown.

The Pennsylvania State University agricultural extension service advises against using table salt for canning because up to 0.04% dextrose and up to 0.5% calcium silicate are added.

CODEX Alimentarius is an international set of food standards endorsed by the World Health Organization and Food and Agriculture Organization of the Unitied Nations.

The CODEX STANDARD FOR FOOD GRADE SALT is linked and requires

The content of NaCl shall not be less than 97% on a dry matter basis, exclusive of additives.

The remainder comprises natural secondary products, which are present in varying amounts depending on the origin and the method of production of the salt, and which are composed mainly of calcium, potassium, magnesium and sodium sulphates, carbonates, bromides, and of calcium, potassium, magnesium chlorides as well. Natural contaminants may also be present in amounts varying with the origin and the method of production of the salt. Copper shall not exceed 2 mg/kg

The name "salt" shall have in its close proximity a declaration of either "Food Grade" or "Cooking Salt" or "Table Salt"

So "Table Salt" as internationally recognized can have impurities of up to 3%, plus additives (such as for anti-caking or iodine supplementing), plus moisture.

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