edited by
thumbnail h e t c h e ~ Chemical Additives in Common Table Salt Davld R. Tyler University of Oregon Eugene, OR 97403 Every chemist worth hisher salt knows that common table salt is the compound sodium chloride. However, as the list of ingredients on the side of every table salt carton shows, commercial table salt contains a surprising number of additives for a substance that is so chemically simple. What are these additives and why are they in table salt? lodlde and lodlde Stablllzers Potassium iodide is the most familiar additive in table salt. Not all commercial brands of table salt contain iodide, of course, but in those that do the iodide is added in amounts up to0.01% because i t is a necessary nutrient, being essential for the formation of the thyroid hormone thyroxine.' The use of iodized table salt is especially important in communities where the soil, and thus the local food, is iodide deficient and where there is no alternative source of iodide (e.g., fish); an iodide-deficient diet results in an enlareement of the thyroid gland, a condition called goiter. ~ e c a i s salt e is used so eenerouslv and freauentlv in cookine and as a condiment. iodized table salt is one of the easiest waysof supplementing the diet with iodide. thus vreventine this uarticular health problem. However. when iodide is added to table salt. other additives must also be included to stabilize the iodide ion (I). Stabilizers are necessary because iodide is slowly oxidized to molecular iodine in moist air.
-
41-
+ OZ+ 4H+
-
212
+ 2H2O
(1)
Formation of Iz is not desirable for tworeasons. First, molecular iodine is volatile; unstabilized iodized table salt gradually loses its iodide content over a period of time hecause of the volatilization and escape of Iz from the container. Secondly, Iz has an unpleasant, chlorine-like odor. Iodized (but unstabilized) salt cartons that have been sitting on the shelf
Brief descriptions of phenomena, topics, facts, etc.. that chemical educators have found to be of interest in lheir teaching, are presented in a "note-type" format throughout the JOURNAL.
1016
Journal of Chemical Education
MARY VIRGINIA ORNA,0.S.U &liege of New Rochelle New Rochelle. NY 10801
for a long time may smell unpleasant when opened because t be commercially of the build-UD . of b. - Such a ~ r o d u cwould unacceptable. Generally, iodide stabilizers are reducing agents that prevent the oxidation of iodide. Until recently, one of the common stabilizers was sodium thiosulfate, Na2S203 (I). The thiosulfate ion reduces molecular Iz to iodide as shown by the following equations. 212 I?
-
+ SzOa2-+ 60H-
+ 2S20a2-
-
21-
2S0s2-
+ 3H20 + 41-
(basic solution) (2)
+ S1O&
(neutral or acidic solution)
(3)
The iodide stabilizer replacing sodium thiosulfate in common use todav is d e x t r o ~ e Because .~ dextrose is a reducing sugar, it also prevents the oxidation of iodide to iodine. finds use in iodized table salts because i t is cheap and nontoxic. Such small quantities are added that the taste of the salt is not affected by the sweet taste of the dextrose. In to sodium thiosulfate or dextrose. bases are -~~addition -~~~~~ added to iodized table salt as iodide stabilieers.~otethat theoxidationof I- lean. (11) is favored inacidicsolutionand, basic solution. Should any Iz be consequently, inhibi&d formed. the bases serve an additional Dumose because the thiosulfate reduction of 1%(eqn. (2)) is favored in basic solution. Sodium bicarbonate is the base typically used to stabilize iodized table salt. Why is NaHC03 used as the base? The answer is m i t e simvle: i t is cheap and nontoxic. Other bases that are omasionaily used include sodium carbonate and calcium hydroxide. Occasionally, one runs across brands of iodized table salt containing disodium phosphate (NazHPOd, sodium pyrophosphate (Na4P20,), and/or other basic phosphates. These additives are also iodide stabilizers. The phosphate and py~
~~
~
in
' Interestingly. sodium iodide is not approved by the government as
a- food ~ -additive. ~ ~
~
The switch from sodium thiosulfate to dextrose apparently was made in order not to alarm label-reading consumers. Dextrose is a more innocuous-sounding chemical additive than sodium thiosulfate. The caking of sodium chloride is apparently amibutable to traces of magnesium~chloride impurity. Magnesium chloride is hygroscopic: it isthe water absorbed by this impurity which causes the salt to cake (seeref.(4)). Another source (5)says that water trapped in the sodium chloride crystals from the crystallization process is slowly released and contributes to the caking.
rophosphate ions form basic solutions when added to water; thus, they will inhibit I- oxidation for the same reason as does NaHCOq. In addition the phosphate and D V ~ O D ~ O S phate ions areUsequesteringagents ( 2 ) : ~sequesteifng agent is a compound capable of formina- verv. stable coordination compounds with metallic ions, thus preventing their normal reactions. They bond to the trace metal ions, present as impurities in the salt, that catalyze the oxidation of iodide ion to free iodine. Once sequestered, the trace metal ions cannot catalyze the iodide oxidation. In addition to being a reducing apent, thiosulfate can also act as a sequestering agent. Anticaklng Agents Sodium chloride will absorb atmospheric moisture and, as a consequence, will "cake," making i t difficult to shake i t from the salt shaker and commercially undesirable. NaCl cakes so easily because of its crystalline form: it crystallizes in a cubic lattice much like the solid packing of children's blocks, and the crystals themselves are tiny cubes (3).Because they are cubes, the crystals absorb water from the atmosphere, those that are face to face codissolve and then fuse together.3 The caking process is facilitated by the cubic structure because the faces provide a large surface area for contact between two crystals. Several methods are used to prevent caking. One of the simnlest wavs is to add a desiccant to the salt which will absorb traces of moisture more readily than the salt, thereby keeping the salt dry. Obviously, such desiccants must be water insoluble, nontoxic, and should not affect the taste of the salt. Common desiccants in table salt include magnesium carbonate, calcium silicate, dicalcium phosphate, tricalcium phosphate, calcium carbonate, and sodium alumina
-
' Tne star-shaped cn/stals. known as dendrlte salt, form when the
sodwm ferrocyanlde is added a1 a ieve of about 13 ppm Below tnls concentrat on, oenorite salr ooes not form, 0-1 the so0 dm ferrocyanide is still an effective anticaking agent, presumably by acting as an adsorbent for water (7). It is interesting to point out that sodium ferrocyanide is approved as a food additive. One might think that any complex with cyanide would be quite poisonous; however, ferrocyanide is not poisonous because the cyanide ligands are strongly bonded to the iron atom. The ferrocyanide ion contains low-spin, d 6 iron (8).The complex is low spin because the cyanide is a strong field ligand. The low-spin, d e configuration is an inert configuration so the complex is quite unreactive. Hence, ferrocyanide is safe to use as a food additive (at least in small amounts). According to one source, people have been known to ingest several grams of Fe(CN)64-without apparent harm (9). Although sodium ferrocyanide is not added to table salt for the purpose of stabilizing iodide ions, it is interesting to note that ferrocyanide will reduce iodine to iodide:
+
I? 2 Fe(CN)e-'
-
2 1-
+ 2 Fe(CN)&
silicates. A typical concentration of these anticaking agents is 0.5%. An old fashioned noncommercial way to prevent caking by desiccation is to add small amounts of table sugar (sucrose) or several grains of rice to a salt shaker. Such desiccants also inhibit the oxidation of iodide to iodine (eqn. (1))since the oxidation requires water. The mechanism by which this anticaking occurs involves more than adsorotion of moisture. Desiccants, which are added to the salt i n fine powder form, actually coat the salt separate the crystals and crvstals (6) and therebv. ~hvsicallv . . the contact necessary for the caking process to occur. A slightly more sophisticated method to keep salt freeflowing involves a modification of the shapes of the salt crvstals themselves bv alterine the crvstal faces. which in turn decreases the coitact and'becreases the caking. Crystal faces can be modified either bv altering the rate of growth of the crystals or by addition of specific impurities. Table salt manufacturers use both methods to ~ r o d u c ecrystals that are not cubes. For example, when salicrystals are grown by r a ~ i devaooration of a brine solution, the crvstals have a step-like &ucture (36). These crystals are ialled hopper crystals. Salt composed of hopper crystals does not cake so readily because of the numerous edges on the crystalsthere is no convenient way for the crystals to pack. The second method of modifying the sodium chloride crystal habit is to crystallize the salt in the presence of an impurity. Manufacturers sometimes add a small amount of sodium ferrocyanide, NaFe(CN)6 .lOHzO (called yellow prussiate of soda on the container label) to their brine solution. Evaporation of the water from these brine solutions prbducescrystals that are star-shaped and thus less likely to pack.4 In summary, avariety 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. Given the constraints that the additives must be nontoxic and tasteless, the formulators of commercial table salts have been quite successful with their products. Literature Cited 111 ~ ~D. W.. "Sodium ~
Chloride: f
Reinhaid ~ Puhiishinz ~ ~
Cow,~ New York.~ 1960, ~
.
pp. 27fi;nd 678.
121 Furia. T.. in~Eneveiomdiaof Chamieai Technolam,"voi. 11.3.d ed., (Editor Furia T.).~ii~~-l~te&&ee. New Yark, 1980, p. 161. (31 (a) Cotton. F. A , and Wilkinson, 0.. "Advanced InolganicChemisVy,"4thsd., Wiieyhtpraripnee. NewYork. i98O.o. 13: IhICsrkon. E. H..J. G~ol.Edue.,2i.160(1973). dhem& in E V ~ ; Y ~ ~LiYving: American Chemical (4) Toy, A. D. F., Society, Wruhington, DC. 1976, p. 54. ~
Hei~s,J.F.,andKuhsjek,E.J..in"En~iopedisofCh~mieaiTeehnoiagy~vol.21,3d ed.. (Editor FuciaT.1, Wiley-intcncience,Nea.York. 1980.p.215. (61 Seer&111, p. 276. (7) See ref. (21. p. 150. (8) For a general discussion of the relationship hetween eicetronic conruuration and iigand~subnlitutionreactivity (ar lack f h e r d l , see Baaaio. F., and Pearson, R. G.. "Mechanisms of Inorganic Reactions: 2nd ed., Wiiey. New York, 1968, p. 145.
(5)
(9) .'The Merck index,"fifh
Volume 62
ed..lEdiror:Steehcr,P.G.),Merckand Co.. Inc.,Rahwsy,NJ,
Number 11 November 1985
1017
