Pigment Green 18 also known as chromium (III) hydroxide, is chemically classified as an inorganic colorant. This pigment is widely used in various industrial and cosmetic applications due to its distinctive color properties and chemical stability.

Chemical Name

Chromium (III) hydroxide

chromium(3+);oxygen(2-);dihydrate

Chemical Composition and Structure

Pigment Green 18 is an inorganic compound with the chemical formula Cr(OH)3. It consists of chromium (Cr) and hydroxide (OH) ions, forming chromium (III) hydroxide. This compound is characterized by its green color, which is derived from the trivalent chromium ions in the hydroxide matrix.

Physical Properties

This pigment typically appears as a fine green powder. It has a high density and good covering power. Pigment Green 18 is known for its chemical stability and resistance to light and heat, making it suitable for long-lasting applications. Its hydroxide structure provides the pigment with specific properties that enhance its performance in various uses.

Chemical Industrial Synthesis Process

• Preparation of reagents. The main raw materials include chromium (III) sulfate (Cr₂(SO₄)₃) and sodium hydroxide (NaOH).
• Synthesis of chromium (III) hydroxide. The production of Pigment Green 18 begins with the reaction between chromium (III) sulfate and sodium hydroxide in an aqueous solution. This reaction forms a precipitate of chromium (III) hydroxide.
• Filtration. The resulting suspension is filtered to separate the solid chromium (III) hydroxide from the aqueous solution.
• Washing. The chromium (III) hydroxide is washed with deionized water to remove any soluble impurities such as sodium sulfate.
• Drying. The washed chromium (III) hydroxide is dried at controlled temperatures to remove residual moisture and obtain a dry powder.
• Grinding. The dried chromium (III) hydroxide is ground to obtain a fine and uniform powder. This step may involve the use of ball mills or other grinding machinery.
• Classification. The dried powder is classified to ensure a uniform particle size. This step may involve sieving or the use of air classifiers.
• Stabilization. The Pigment Green 18 powder is stabilized to ensure its stability during transportation and storage, preventing aggregation and degradation.
• Quality control. The Pigment Green 18 undergoes rigorous quality testing to ensure it meets standards for purity, color intensity, and safety. These tests include chemical analysis, spectroscopy, and physical tests to determine particle size and rheological properties.

What it is used for and where

Cosmetics

Restricted cosmetic ingredient as IV/130 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009. Substance or ingredient reported:

• Chromium (III) hydroxide

Cosmetics - INCI Functions

Colorant. This ingredient has the function of colouring the solution in which it is inserted in a temporary, semi-permanent or permanent manner, either alone or in the presence of the complementary components added for colouring.

Pigment Green 18 is used in cosmetic products such as eyeshadows, eyeliners, and nail polishes due to its green color and opacity. 

Industrial Applications

Paints and Coatings: In the paint industry, chromium (III) hydroxide is used to achieve green shades and enhance the stability of paints. Its stability makes it ideal for both decorative and protective coatings.

Plastics and Polymers: This pigment is employed in the production of plastics and polymers to improve color uniformity and resistance to degradation from UV light and heat.

Ceramics and Glass: Pigment Green 18 is used in the production of ceramics and glass to achieve stable green colors. It is particularly valued for its resistance to high-temperature processes.

Inks: The pigment is also used in printing inks to produce vibrant and durable green colors, ensuring high-quality prints.

Safety

Pigment Green 18 is mentioned in some studies as a dye with contraindications (1) and, in general, chromium is a heavy metal that can create toxicity to the environment (2)

Pigment Green 18 is generally considered safe for use in consumer products when handled according to proper safety procedures. It is important to avoid inhalation of the powder and minimize direct contact with skin and eyes. Although chromium (III) hydroxide is less toxic than other chromium compounds, environmental considerations include controlling emissions during production and ensuring proper waste disposal.

Molecular Formula Cr(OH)₃

Molecular Weight   106.042 g/mol

CAS  1308-14-1 12001-99-9

EC number 215-158-8

UNII RV8FT8XF5R

DTXSID70894251

Synonyms: 

Pigment Green 18

Chromium Hydroxide

Hydrated chromium sesquioxide

CI 77289

References_____________________________________________________________________

(1) Jasnow SB, Smith JL. Microwave sanitization of color additives used in cosmetics: feasibility study. Appl Microbiol. 1975 Aug;30(2):205-11. doi: 10.1128/am.30.2.205-211.1975. 

Abstract. Microwave exposure has been explored as a method of microbiologically sanitizing color additives used in cosmetic products. Selected microbiologically unacceptable cosmetic color additives, D&C red no. 7 Ca lake (certified synthetic organic color), carmine (natural organic color not subject to certification), and chromium hydroxide green (inorganic color not subject to certification), were submitted to microwave exposure. Gram-negative bacteria were eliminated, as verified by enrichment procedures, and levels of gram-positive bacteria were reduced. Generally, analytical and dermal safety studies indicated no significant alterations in physical, chemical, and toxicological properties of the colors. Sanitization was also successfully performed on other colors (D&C red no. 9 Ba lake, D&C red no. 12 Ba lake, D&C green no. 5, and FD&C red no. 4); initial physical and chemical tests were satisfactory. Results indicated that this method of sanitization is feasible and warrants further investigation.

(2) Bellie, P. N., Thiruvenkadam, R., Panchanathan, K., & Sockan, V. (2014). Health Effects of Chromium, its Reduction and Removal. Internatiional Journal of Chemical And Pharmaceutical Analysis, 2(1), 48-53.

Abstract. Chromium is one of the heavy metal coming from various industries having high toxicity. The compounds of chromium especially Cr (VI) are known to be detrimental to human beings and animals. Extensive chemical analysis was done to detoxify Cr (VI) by reducing it to Cr (III) by using reducing agents such as sodium bisulphite, and the precipitation of chromium hydroxide by addition of base. Also experiments were conducted to study the effect of reduction time and pH on the reduction of chromium for optimization studies. In this work, hexavalent chromium is removed by reduction and precipitation reaction, by using Sodium bisulphite as reducing agent to convert Cr(VI) to Cr(III) and Calcium hydroxide (Ca(OH)2), Sodium hydroxide (NaOH) and combination of Sodium hydroxide + Calcium hydroxide are used as precipitating agents to precipitate Cr(III) as hydroxides. The reduction reaction is conducted at low pH (<3) and precipitation reaction is carried out high pH (>8) because hexavalent chromium is stable under oxidizing conditions, whereas trivalent chromium is stable under reducing conditions. The purpose of this work is to compare the efficiencies of precipitating agents in the maximum reduction and precipitation reactions of waste water on pH values