He Sodium dichromate Is an inorganic compound of the formula Na 2 Cr 2 O 7. It is one of many compounds of chrome Hexavalent (Cr VI). Its structure is illustrated in figure 1, although usually the salt is handled in its dihydrated form whose formula would be Na 2 Cr 2 O 7 · H 2 O.
Has two Ionic bonds Between sodium molecules and negatively charged oxygen. The 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 relatively low production, less than 50,000 tons per year each, compared to the Kazakhstan 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 properties similar to potassium dichromate, however, the sodium salt is more soluble in water and has a lower equivalent weight to the potassium salt.
Sodium dichromate produces toxic fumes of chromium when heated. It is a strong oxidizing agent and is highly corrosive.
This compound can be found in sources of contaminated drinking water from different industrial processes such as galvanizing or Electroplating , Leather tanning and textile manufacturing.
Physical and chemical properties of sodium dichromate
Sodium dichromate is composed of monoclinic crystals of reddish orange color in their anhydrous and odorless form. 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
Figure 2 shows the appearance of sodium dichromate. 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, s.f.).
It is considered a stable compound if stored under recommended conditions and is not flammable. Since it is a strong oxidizing agent, it is corrosive, and in solution is acid having the ability to lower the pH to 4 in a solution of 1% w / v.
Production methods
Sodium chromate can be converted to dichromate by a continuous process which is treated with sulfuric acid, carbon dioxide or a combination of these two. Evaporation of the sodium dichromate liquor causes precipitation of sodium sulfate and / or sodium bicarbonate, and these compounds are removed before the final crystallization of sodium dichromate.
Sodium dichromate can be made in a three-step process:
- Alkaline roasting of chromite oxidation conditions
- Leaching. Extraction of the soluble matter from a mixture by the action of a liquid solvent
- Conversion of sodium monochromate to sodium dichromate by means of an acid.
Anhydrous sodium dichromate may be prepared by melting the sodium dichromate dihydrate, crystallizing aqueous dichromate solutions above 86 degrees C, or drying sodium dichromate solutions in spray driers.
Sodium dichromate solutions at 69 and 70% w / v are used as a convenient and cost-effective method of dispatching quantities, avoiding the need for manual manipulation or dissolution of crystals.
Reactivity and hazards
It is a strong oxidizing agent. Incompatible with strong acids. Contact with combustible materials may cause fires. Toxic fumes of chromium oxide may form in the presence of heat or fire.
The 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 the dichromate with the hydrazine is explosive, the reaction of the 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).
Inhalation of dust or mist causes respiratory irritation that sometimes looks like asthma. Septal perforation may occur. It is considered poison. Ingestion causes vomiting, diarrhea, and, very unusually, complications of the stomach and kidney. Contact with eyes or skin causes local irritation. Repeated exposure to the skin causes dermatitis.
Sodium dichromate is carcinogenic in humans. There is evidence that hexavalent chromium compounds or Cr (VI) 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 compound Teratogen Or reproductive risk, it is known that hexavalent chromium or Cr (VI) compounds are teratogenic and cause reproductive harm such as reducing fertility and interfering with menstrual cycles.
Sodium dichromate can cause liver and kidney damage and should be handled with extreme caution (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 skin or eyes should be treated as acid burns; Rinse eyes with water for at least 15 minutes. External injuries can be rubbed with a 2% solution of Sodium thiosulfate . In all cases a doctor should be consulted.
Uses and applications
Apart from its importance in the manufacture of other chromium chemicals, sodium dichromate also has many direct uses as an ingredient in the production of:
- Metal finish: it helps to resist corrosion and to clean the metallic surfaces, also favors the adhesion of the painting.
- 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 inhibitors of corrosion in the lower layers and primers.
- Ceramics: used in the preparation of colored glass and ceramic enamels.
- Textile: Used as a mordant for acid dyes to improve its fast coloring properties.
- Production of chromium sulphate.
(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 enamels 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 mainly used to produce other chromium compounds, but is also used in bentonite sludge used in the production of petroleum, in wood preservatives, in the production of organic chemicals and as a corrosion inhibitor.
When mixed with aluminum dichromate and potassium, using the aluminum-thermal process, the chromic oxide produces high purity metal chromium. This is a vital ingredient in the production of high performance superalloys used in the aerospace industry.
In organic synthesis, sodium dichromate is used as an oxidizing agent in reducing oxide 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 (V.K. 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 post exposure; Peak concentrations were reached 6 hours after exposure.
The ratio of whole blood chromium to plasma concentrations of chromium was significantly different for Cr (VI) and Cr (III) treatments. Therefore, chromium and blood chrome plasma assays should be used for the evaluation of chromium exposure.
Chromium was also detected in peripheral lymphocytes. Cr (VI), but no Cr (III) accumulated significantly in lymphocytes after treatment. These cells have potential to be used as biomarkers of exposure assessment to chromium compounds (Hooth, 2008).
References
- Chemical Datasheet Sodium Dichromate. (2016). Retrieved from cameo chemicals: cameochemicals.noaa.
- Hooth, M.J. (2008). Technical Report on Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrated. National Institute of Health USA.
- Kogel, J.E. (2006). Industrial Minerals & Rocks: Commodities, Markets, and Uses seventh edition. Littleton colorado: society of mining, metallurgyc and exploration inc.
- National Center for Biotechnology Information. (S.f.). PubChem Compound Database; CID = 25408. Retrieved from pubchem.com: pubchem.ncbi.nlm.nih.gov.
- New Jersey Department of Health. (2009, November). Hazartdous substance fact sheet sodium dichromate. Retrieved from nj.gov: nj.gov.
- Sodium dichromate. The building block for virtually all other chromium compounds. (2010-2012). Retrieved from elementis chromium: elementischromium.com
- K. Ahluwalia, R.A. (2004). Comprehensive Practical Organic Chemistry: Preparations and Quantitative Analysis. Delhi: University Press (India).