SODIUM DICHROMATE (NA2CR2O7): PROPERTIES, PRODUCTION, USES - CHEMISTRY - 2025

The sodium dichromate is an inorganic compound of formula Na2Cr2O7. It is one of many compounds of hexavalent chromium (Cr VI). Its structure is illustrated in figure 1, although the salt is usually handled in its dihydrated form, the formula of which would be Na2Cr2O7 · H2O.

It has two ionic bonds between sodium molecules and negatively charged oxygen. Chromium ore is extracted from sodium dichromate. Millions of kilograms of sodium dichromate are produced annually.

Figure 1: Structure of sodium dichromate

China is the largest producer of sodium dichromate, however Chinese chemical plants have a relatively low production, less than 50,000 tons per year each, if compared to the Kazakh plant that produces more than 100,000 tons per year.

Plants in Russia, the United States and the United Kingdom have an intermediate production between 50,000 and 100,000 tons per year (Kogel, 2006).

In terms of reactivity and appearance, sodium dichromate has similar properties to potassium dichromate, however, the sodium salt is more soluble in water and has a lower equivalent weight than the potassium salt.

Sodium dichromate produces toxic chromium fumes when heated. It is a strong oxidizing agent and is highly corrosive.

This compound can be found in contaminated drinking water sources from different industrial processes such as galvanizing or electroplating techniques, leather tanning and textile manufacturing.

Physical and chemical properties

Sodium dichromate is composed of crystals with a monoclinic structure of a reddish-orange color in its anhydrous form and lacking odor. Its molecular weight is 261.97 g / mol in its anhydrous form and 298.00 g / mol in its dihydrate form.

It has a melting point of 356.7 degrees Celsius, a boiling point of 400 degrees Celsius at which it decomposes. It has a density of 2.52 g / ml.

Figure 2: Appearance of sodium dichromate

The appearance of sodium dichromate is shown in Figure 2. Its solubility in water is 187 g per 100 grams at 25 degrees centigrade and its solubility in ethanol is 513.2 grams per liter at 19.4 degrees centigrade (National Center for Biotechnology Information, nd).

It is considered a stable compound if stored under recommended conditions and it is not flammable. Since it is a strong oxidizing agent, it is corrosive, and in solution it is acid having the ability to lower the pH to 4 in a 1% w / v solution.

Production methods

Sodium chromate can be converted to dichromate by a continuous process that deals with sulfuric acid, carbon dioxide, or a combination of these two.

The evaporation of the sodium dichromate liquor causes the precipitation of sodium sulfate and / or sodium bicarbonate, and these compounds are removed prior to the final crystallization of sodium dichromate.

Sodium dichromate can be made in a three-step process:

1. Alkaline roast chromite oxidation conditions
2. Leaching. Extraction of soluble matter from a mixture by the action of a liquid solvent
3. Conversion of sodium monochromate to sodium dichromate by means of an acid.

Anhydrous sodium dichromate can be prepared by melting sodium dichromate dihydrate, crystallizing aqueous dichromate solutions above 86 degrees C, or drying sodium dichromate solutions in spray dryers.

Sodium dichromate solutions at 69 and 70% w / v are used as a convenient and cost-effective method of shipping quantities, avoiding the need for manual handling or crystal dissolution.

Reactivity and hazards

It is a strong oxidizing agent. Incompatible with strong acids. Contact with combustible materials can cause fires. Toxic chromium oxide fumes can form in the presence of heat or fire.

The well-known "chromic acid mixture" of dichromate and sulfuric acid with organic residues gives rise to a violent exothermic reaction. This mixture in combination with acetone residues also leads to a violent reaction.

The combination of dichromate and sulfuric acid with alcohols, ethanol and 2-propanol, gives rise to a violent exothermic reaction. Due to the occurrence of many incidents involving the mixing of dichromate-sulfuric acid with oxidizable organic materials, it is probably best to avoid such interactions.

The combination of dichromate with hydrazine is explosive, the reaction of dichromate can be expected to be vigorous with amines in general. The addition of the dehydrated dichromate salt to acetic anhydride leads to a finally explosive exothermic reaction.

Boron, silicon, and dichromates form pyrotechnic mixtures. A mixture of acetic acid, 2-methyl-2-pentenal and dichromate leads to a rampant reaction (Chemical Datasheet Sodium Dichromate., 2016).

Breathing dust or mist causes respiratory irritation that sometimes resembles asthma. Septal perforation can occur. It is considered poison.

Ingestion causes vomiting, diarrhea, and, very unusually, stomach and kidney complications. Contact with eyes or skin causes local irritation. Repeated exposure to the skin causes dermatitis.

Sodium dichromate is carcinogenic to humans. There is evidence that hexavalent chromium or Cr (VI) compounds can cause lung cancer in humans. Sodium dichromate has been shown to cause lung cancer in animals.

Although sodium dichromate has not been identified as a teratogenic or reproductive risk compound, it is known that hexavalent chromium or Cr (VI) compounds are teratogens and cause reproductive harm such as reducing fertility and interfering with menstrual cycles..

Sodium dichromate can cause liver and kidney damage so it must be handled with extreme care (New Jersey Department of Health, 2009).

In case of ingestion, the victim should drink water or milk; never induce vomiting. In case of contact with the skin or eyes it should be treated as acid burns; eyes are flushed with water for at least 15 minutes.

External lesions can be rubbed with a 2% solution of sodium thiosulfate. In all cases a doctor should be consulted.

Uses and applications

Aside from its importance in the manufacture of other chromium chemicals, sodium dichromate also has many direct uses as an ingredient in the production of:

• Metallic finish: it helps to resist corrosion and clean metal surfaces, it also favors paint adhesion.
• Organic products: used as oxidizing agents in the manufacture of products such as vitamin K and wax.
• Pigments: used in the manufacture of inorganic chromate pigments where it produces a range of colors stable to light. Some grades of chromate are also used as corrosion inhibitors in undercoats and primers.
• Ceramic: used in the preparation of colored glass and ceramic glazes.
• Textile: used as a mordant for acid dyes to enhance their fast coloring properties.
• Chromium sulfate production.

(Sodium dichromate. The building block for virtually all other chromium compounds., 2010-2012)

Sodium dichromate Dihydrate, its use is ideal in various conditions including high temperature applications such as ceramic glazes and colored glass.

Chromic oxide, being harder than other metal oxides, such as titanium or iron, is ideal for environments where the temperature and process conditions are aggressive.

This substance is used primarily to produce other chromium compounds, but it is also used in bentonite sludge used in oil production, in wood preservatives, in the production of organic chemicals, and as a corrosion inhibitor.

When mixed with potassium aluminum dichromate, using the aluminum-thermal process, chromic oxide produces high-purity metallic chromium. This is a vital ingredient in the production of the high performance superalloys used in the aerospace industry.

In organic synthesis, sodium dichromate is used as an oxidizing agent in oxide reduction reactions in the presence of sulfuric acid.

Figure 3. Uses of sodium dichromate in organic synthesis.

For example, the oxidation of p nitrotoluene to form p nitrobenzoic acid, in the oxidation of n-butanol to form n-butaldehyde, in the formation of cyclohexanone from cyclohexanol and the formation of adipic acid as illustrated in figures 3.1, 3.2, 3.3 and 3.4 respectively (VK Ahluwalia, 2004).

Biochemistry

Intratracheal instillation of sodium dichromate (CrVI) and chromium acetate hydroxide (CrIII) in male rats resulted in increased concentrations of chromium in whole blood, plasma, and urine up to 72 hours after exposure; Peak concentrations were reached 6 hours after exposure.

The ratio of whole blood chromium to plasma chromium concentrations was significantly different for the Cr (VI) and Cr (III) treatments. Therefore, blood chromium and plasma chromium tests should be used for the assessment of chromium exposure.

Chromium was also detected in peripheral lymphocytes. Cr (VI), but not Cr (III) accumulated significantly in lymphocytes after treatment. These cells have the potential to be used as biomarkers for the evaluation of exposure to chromium compounds (Hooth, 2008).

References

1. Chemical Datasheet Sodium Dichromate. (2016). Retrieved from cameo chemicals: cameochemicals.noaa.
2. Hooth, MJ (2008). Technical Report on Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrated. National Institute of Health USA.
3. Kogel, JE (2006). Industrial Minerals & Rocks: Commodities, Markets, and Uses Seventh Edition. littleton colorado: society of mining, metallurgyc and exploration inc.
4. National Center for Biotechnology Information. (sf). PubChem Compound Database; CID = 25408. Retrieved from pubchem.com: pubchem.ncbi.nlm.nih.gov.
5. New Jersey Department of Health. (2009, November). hazartdous substance fact sheet sodium dichromate. Retrieved from nj.gov: nj.gov.
6. Sodium dichromate. The building block for virtually all other chromium compounds. (2010-2012). Retrieved from elementis chromium: elementischromium.com
7. K. Ahluwalia, RA (2004). Comprehensive Practical Organic Chemistry: Preparations And Quantitative Analyzes. Delhi: University press (India).