Iodate or iodide?
The term "iodine" usually refers to the chemical element in a general sense without specifying its chemical form, but it is also used to denote the form I2. Occasionally salt or another vehicle is described as "iodated" when potassium iodate (KIO3) is added, or as "iodinated" when iodine (I2) is added to a vehicle such as water. WHO, ICCIDD, and UNICEF have recommended that the term "iodized" (also spelled "iodised") be used when iodine is added in any form).
Iodine is normally introduced as the iodide or iodate of potassium, calcium or sodium. This table shows important physical properties of these compounds. Developed countries use both potassium iodide (KI) and potassium iodate(KIO3) extensively for iodization of refined table salt. This table lists a few countries with the potassium compound used and iodization levels:
|Iodine ||I2 ||100 ||- ||- ||0.3 ||0.4 ||0.6
|Calcium iodide ||CaI2 ||86.5 ||646 ||676 ||690 ||708 ||740
|Calcium iodate ||Ca(IO3) 2.6H2O ||65.0 ||- ||1.0 ||4.2 ||6.1 ||13.6
|Potassium iodide ||KI ||76.5 ||1280 ||1440 ||1520 ||1600 ||1760
|Potassium iodate ||KI O3 ||59.5 ||47.3 ||81.3 ||117 ||128 ||185
|Sodium iodide ||NaI.2H20 ||85.0 ||1590 ||1790 ||1900 ||2050 ||2570
|Sodium iodate ||Na IO3 ||64.0 ||- ||25.0 ||90.0 ||150 ||210|
|Australia ||Potassium Iodate ||65
|Cameroon ||Potassium Iodate ||50
|Canada ||Potassium Iodide ||77
|China ||Potassium Iodate ||40
|Ecuador ||Potassium Iodide ||40
|Germany ||Potassium Iodate ||25
|India ||Potassium Iodate ||30
|Indonesia ||Potasssium Iodate ||25
|Kenya ||Potasssium Iodate ||100
|Nigeria ||Potassium Iodate ||50
|Panama ||Potassium Iodate/Iodide* ||67-100
|USA ||Potassium Iodide ||77
|Zimbabwe ||Potassium Iodate ||50 (at point of entry)|
* If KI is used it must be guaranteed that there is no significant iodine loss.
The high solubility of KI enables dispersion by atomized sprays on very dry crystals. However, KI in salt is not very stable and can easily be lost by oxidation to iodine if the iodized salt is subjected appreciably to any of the following conditions: (1) moisture in the salt, (2) humid or excessively aerated environment; (3) exposure to sunlight; (4) exposure to heat; (5) acid reaction in the salt; or (6) presence of impurities. It can also be lost if the iodized salt packages become damp, resulting in the migration of iodide from the salt to the fabric, and subsequent evaporation if the fabric is pervious. This loss can be lessened when the salt iodized with KI is very pure (+ 99.5%) and dry (moisture less than 0.1%), and by the addition of stabilizers such as sodium thiosulfate and calcium hydroxide, and/or drying agents such as magnesium carbonate or calcium carbonate. However, in most impure salt, KI stability is poor due both to oxidation and to migration and segregation in the presence of moisture.
Most people in iodine deficient areas use unrefined salt that can be effectively supplemented with KIO3 without added carrier agents or stabilizers. Iodate is more stable under adverse climatic conditions than iodide and does not require stabilizers. It is also less soluble than iodide and less likely to migrate from the bag but is only sparingly soluble in water at low temperatures. However, solutions of up to 40 g/L (4% approximately) are readily obtainable. Such a solution is sufficient for salt iodization even at iodine levels of 100 mg/kg. The addition of 0.1% moisture to crude salt, which may already contain 1-5% moisture has no adverse effect. Potassium iodate breaks down rapidly in the human body and effectively delivers iodide to the thyroid gland for the synthesis of thyroid hormone. It is not toxic, and has been approved and recommended by the Joint FAO/WHO Expert Committee on Food Additives as safe when used within the Provisional Maximum Tolerable Daily Intake (PMTDI) for iodine of 1 mg from all sources. Even at the highest dosages currently used, iodine intake through iodized salt is unlikely to exceed 20% of this value.
As indicated in, iodine makes up a greater fraction of the weight of KI than it does of KIO3. KI is also cheaper than KIO3. However, when used in impure salt the overall cost of using KI may be higher owing to its relative instability. (Calcium iodate (Ca(IO3)2) is also stable in impure salts but has not been used extensively because of its very low solubility in water).
The following table illustrates the specific gravity of dilute solutions of potassium iodate. Potassium iodate is a heavy salt and solution strengths can be checked with precision hydrometers if desired. It shows the specific gravity of potassium iodate solutions (in water, at 18 0C, from "International Critical Tables")
|6 ||1.05153 |
Salt Iodine levels
The recommended minimum daily requirement of iodine varies from 150 to 200 µg. To visualize this quantity, a particle the size of a pinhead is sufficient for one person for one month. There is no universal specification for the level of iodine to be added to salt to achieve this recommended iodine intake. Numerous factors influence the selection of an appropriate level for a given population, including: (1) per-capita consumption of salt in the region; (2) the degree of iodine deficiency in the region; (3) transit losses; and (4) shelf life required. Per-capita consumption of salt in different countries varies over a wide range, from about 3 to 20 gms per day. Table 7.4 offers a sample calculation for fixing the level of iodization of salt with KIO3:
- Assume that the per capita daily requirement of iodine is 200 µg;
- Assume that the per capita salt consumption is 10 g per day.
- Level of iodine required in salt is 200 µg per 10 g (1 g = 1 million µg) or 20 parts per million (ppm);
- Assume that half of the iodine may be lost in transit and storage;
Level of iodation required
= 40 ppm iodine;
= 40 x 1.685 ppm KIO3;
= 67 ppm KIO3.
Since levels of salt consumption vary and the amount of iodine lost from salt will depend on climate, packaging material and storage time, it is not possible to establish a global standard for the quantity of potassium iodate which should be added to salt. Current levels of iodization in different countries vary from 100 parts of iodine per million parts of salt, which corresponds to 170 grams of potassium iodate per ton (where salt quality and packing is very poor coupled with low salt intake) to 20 ppm iodine, which is equal to 34 grams per ton (high quality salt, good packing, high salt intake). Most countries have fixed levels around 50 ppm iodine (which corresponds to 85 ppm potassium iodate).
In a given country the fortification level may be changed over time, in response to changes in average daily consumption of salt and reduction in iodine losses during distribution and storage. WHO/UNICEF/ICCIDD-recommended levels of iodine in salt for different salt consumption levels, environmental and packing conditions are summarized in the next table. National authorities should establish suitable levels in consultation with the salt industry. National regulations should establish a minimum level of iodine at production level and a lower level at consumption level to allow for storage and transit losses e.g. minimum permitted level at production 40 ppm, minimum level at consumption 20 ppm.
Discussions and regulations about iodine levels in salt must clearly specify whether they refer to total content of iodine alone or to content of iodine compound (KIO3 or KI). From the example above, 40 ppm as iodine is the same as 67 ppm as KIO3 or 52 ppm as KI, offering a ready source for confusion unless the chemical form is clearly identified. In general we recommend that the level be expressed as content of iodine alone, which emphasizes the physiologically important component (iodine) and facilitates comparison of its different chemical forms. The following table shows the WHO/UNICEF/ICCIDD recommended levels of iodine in salt, expressed as mg iodine per kg salt (ppm):
|- ||Bulk / Retail (< 2 kg) ||Bulk / Retail (< 2 kg) ||Bulk / Retail (< 2 kg) ||
|Warm, moist || || || ||
|5 g ||100 / 80||90 / 70||80 / 60||50
|10 g ||50 / 40||45 / 35||40 / 30||25
|Warm, dry or cool, moist || || ||
|5 g ||90 / 70||80 / 60||70 / 50||45
|10 g ||45 / 35||40 / 30||35 / 25||22.5
|Cool, dry || || ||
|5 g ||80 / 60||70 / 50||60 / 45||40
|10 g ||40 / 30||35 / 25||30 / 22.5||20|
Iodine production in the world is limited to a few countries. Japan and Chile are the principal producers and exporters. The current (1994) CIF (Cost insurance and freight) price of elemental iodine is around $8-8.50/kg, and about $7.40-7.50/kg for KIO3. For IDD control programs, iodine is usually imported in the form of potassium iodate.
For salt iodization, potassium iodate should conform to the following quality specifications:
1. Physical Appearance: White to almost white crystaliine porwder
2. Particles retained on 100 mesh B.S. sieve: 5% max, w/w
3. Solubility: Soluble in 30 parts water
4. Reaction: A 5% solution in water shall be neutral to litmus
5. Iodine (I2) Max w/w: 0.005%
6. Sulphate, Max w/w: 0.02%
7. Heavy Metals (as Pb): Less than 20 ppm
8. Iron: Less than 10 ppm
9. Bromate, Bromide, Chloride and Chlorate Max % w/w: 0.5
10. Insoluble matter, Max % w/w: 0.5
11. Loss on drying at 105 0C Max % w/w: 0.5
12. Assay (on dry basis): 99.0% potassium iodate min
13. Packing: Plastic bag or paper drums with closed seal 50 kg
Normally for procurement of potassium iodate, U.S. Food Codex Specifications are followed. Quality specifications for potassium iodate are set out in this table. If the country's requirement is large (at least 30 tons/year), it may be more economical to import elemental iodine and convert it to KIO3.
Conversion of Iodine to Potassium Iodate
Potassium iodate is produced by the electrochemical reaction of elemental iodine with potassium hydroxide (KOH): 3I2 + 6KOH =3D KIO3 + 5KI + 3H2O
Iodine is dissolved in potassium hydroxide solution and the potassium iodide so obtained is electrolytically oxidised in an annular flow cell. At the end of the electrolysis, the electrolyte is cooled and potassium iodate is obtained.Approximately 80% of KIO3 crystallizes from the reaction mixture. After filtration, the electrolyte is recycled to the cell feed. One kg of iodine will yield 1.55 kg of KIO3.The total capital outlay for a 30 Tons per year plant has been estimated in India to be approximately $200,000. Further technical details can be obtained from the National Research Development Corporation of India, New Delhi, India.