Tetramethylammonium Hydroxide (TMAH) is available in solution and in the pentahydrate form as a white crystalline solid. The pentahydrate CAS# is -65-4.
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TMAH solution is widely used in the electronics industry as a developer or cleaner. TMAH is typically one of several ingredients in etching / stripping mixtures, although it may also be used as a pure chemical. It is often used in solution in water, and less frequently, in methanol. These solutions are assigned the CAS# 75-59-2. The concentration of TMAH in commercially available developers used on Penn’s campus is <3% (according to chemical inventory records 9/). The highest concentration solution of TMAH that is commonly available commercially is 25%. TMAH is a strong base; the 25% solution in water has a pH of greater than 13.
The odor of TMAH has been described as a strong, ammonia-like smell. Although pure TMAH will have virtually no odor, solutions may give off a fishy smell from triethylamine, which is a common impurity.
Refer to a specific product’s Safety Data Sheet for more hazard details. An example Safety Data Sheet from Sigma-Aldrich for a 25% solution of TMAH can be found here: Sigma-Aldrich SDS for TMAH 25%
The health hazards of TMAH pentahydrate (solid) are very similar to those of the solution, however the solid is a GHS hazard category 3 for dermal acute toxicity, whereas the solution is a GHS hazard category 2 (higher hazard) for dermal acute toxicity. This difference in hazard category is likely due to the increased risk of dermal absorption of the chemical in solution.
GHS Classification Information from the Sigma-Aldrich SDS for the TMAH 25% Solution in Water:
Acute toxicity, Oral (Category 2), H300
Acute toxicity, Dermal (Category 2), H310
Skin corrosion (Category 1), H314
Serious eye damage (Category 1), H318
Specific target organ toxicity - single exposure (Category 1), Central nervous system, H370
Specific target organ toxicity - repeated exposure, Dermal (Category 1), thymus gland, Liver, H372
Acute aquatic toxicity (Category 2), H401
Chronic aquatic toxicity (Category 2), H411
TMAH is extremely corrosive to skin, eyes, and mucous membranes and will cause serious burns to eyes, and skin on contact.
In addition to causing chemical burns, TMAH can cause systemic neurotoxicity leading to respiratory failure by ganglion block that occurs through skin absorption. No antidote has been developed yet.
The following exposure information is from the U.S. Library of Medicine Toxicology Data Network (TOXNET):
INHALATION EXPOSURE: Mild exposure may cause cough and bronchospasm. Severe inhalation may cause upper airway edema and burns, stridor, and rarely acute lung injury.
OCULAR EXPOSURE: Ocular exposure can produce severe conjunctival irritation and chemosis, corneal epithelial defects, limbal ischemia, permanent visual loss and in severe cases perforation.
DERMAL EXPOSURE: Mild exposure causes irritation and partial thickness burns. Metabolic acidosis may develop in patients with severe burns or shock. Prolonged exposure or high concentration products can cause full thickness burns.
Systemic toxicity is likely to occur with dermal exposure.
When heated to decomposition, TMAH emits toxic fumes of NOx and ammonia.
A Hazard Control Plan is recommended for procedures involving TMAH over 5%, and one may be required by EHRS under certain circumstances.
Contact EHRS for assistance with your hazard assessment.
All work with TMAH requires the approval of the P.I. The P.I. must ensure that the person or team who will be working with the chemical writes a task-specific Hazard Control Plan (HCP) if required by EHRS or the P.I.'s hazard assessment.
The HCP must be sent to EHRS for review. EHRS will upload the HCP to the “documents” section of the lab’s BioRAFT page.
The P.I. must also ensure that the person or team who will be working with the chemical understands the hazards and has received adequate training and supervision for the procedure.
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All of the handling procedures detailed in SOP: Acutely Toxic Chemicals and SOP: Corrosives apply to all work involving tetramethylammonium hydroxide solutions.
Because TMAH exposures to skin and eyes will result in serious burns, and because dermal absorption may result in fatal systemic toxicity, risk assessments for work involving TMAH may determine a level of required personal protective equipment (PPE) that is beyond the minimum mandatory laboratory PPE requirements, i.e. safety glasses, lab coat, disposable nitrile gloves, closed-toed shoes, and long pants.
Additional recommended PPE includes:
*Note that Glove-manufacturer’s compatibility charts do not often include data specifically for TMAH; however, the 4-mil disposable nitrile rubber gloves commonly used in the laboratory will provide adequate protection against incidental contact with TMAH solutions. This is based on the information from the Sigma-Aldrich SDS. Testing was done on a 0.11 mm (4.33 mil) nitrile glove.
Full contact Material: Nitrile rubber Minimum layer thickness: 0.11 mm Break through time: 480 min Material tested:Dermatril® (KCL 740 / Aldrich Z, Size M)
All exposures or potential exposures to TMAH require immediate medical attention.
General emergency response information can be found at Emergency Info
General procedures for chemicals spill response can be found in Section X: Chemical Spills in this CHP.
Do not hesitate to call EHRS for assistance with spill cleanup for Acutely Toxic and Corrosive Materials
24 hours: 215-898-
Contact Penn Police (511) only if the spill involves a fire, imminent risk of fire, an injury requiring an ambulance, or if there is a hazard that may affect others in the building.
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Danger[1] H300+H310, H314, H370, H372, H411[1] P260, P273, P280, P303+P361+P353, P304+P340+P310, P305+P351+P338[1] NFPA 704 (fire diamond) Safety data sheet (SDS) Sigma-Aldrich MSDS for TMAH·5H2O Related compounds Other anions tetramethylammonium chloride Other cations tetraethylammonium hydroxide Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N (what is YN ?) Chemical compoundTetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with molecular formula N(CH3)4+ OH−. It is commonly encountered in form of concentrated solutions in water or methanol. TMAH in solid state and its aqueous solutions are all colorless, but may be yellowish if impure. Although TMAH has virtually no odor when pure, samples often have a strong fishy smell due to presence of trimethylamine which is a common impurity. TMAH has several diverse industrial and research applications.
TMAH is most commonly encountered as an aqueous solution, in concentrations from ~2–25%, and less frequently as solutions in methanol. These solutions are identified by CAS number 75-59-2. Several hydrates such as N(CH3)4OH·xH2O. have been crystallized.[3] These salts contain well separated Me4N+ cations and hydroxide anions (Me is an abbreviation of methyl group). The hydroxide groups are linked by hydrogen bonds to the water of crystallization. Anhydrous TMAH has not been isolated.
One of the earliest preparations is that of Walker and Johnston,[4] who made it by the salt metathesis reaction of tetramethylammonium chloride and potassium hydroxide in dry methanol, in which TMAH is soluble, but potassium chloride is not:
Where Me stands for the methyl group, –CH3.
This report also provides details for isolation of TMAH as its pentahydrate, noting the existence of a trihydrate, and emphasizes the avidity which even the former exhibits for atmospheric moisture and carbon dioxide. These authors reported a melting point of 62–63 °C for the pentahydrate, and solubility in water measured averagely around 220 g/100 mL at 15 °C.
TMAH is a stable compound, with a half-life longer than 61 h in 6 M NaOH at 160 °C.[5]
TMAH undergoes simple acid-base reactions to produce tetramethylammonium (TMA) salts whose anion is derived from the acid used. Illustrative is the preparation of tetramethylammonium fluoride:[6]
TMAH and many other TMA salts containing simple anions thermally decompose into trimethylamine.[8] Dimethyl ether is a major decomposition product rather than methanol.[9] The idealized equation is:
TMAH is a very strong base.[10]
One of the industrial uses of TMAH is for the anisotropic etching of silicon.[11] It is used as a basic solvent in the development of acidic photoresists in the photolithography process, and is highly effective in stripping photoresists. TMAH has some phase transfer catalyst properties. It is also used as a surfactant in the synthesis of ferrofluids and to inhibit nanoparticle aggregation.
TMAH is one of the most common reagents used in thermochemolysis, an analytical technique involving both pyrolysis and chemical derivatization of analytes.[12]
TMAH belongs to the family of quaternary ammonium hydroxide (QAH) solutions and is commonly used to anisotropically etch silicon. TMAH is preferred over sodium or potassium hydroxide in applications that are sensitive to metal ion contamination.[13] Typical etching temperatures are between 70 and 90 °C and typical concentrations are 5–25 wt.% TMAH in water. In case of (100) silicon etching rates generally increase with temperature and decrease with TMAH concentration. Etched (100) silicon surface roughness decreases with increasing TMAH concentration, and smooth surfaces can be obtained with 20% TMAH solutions. Etch rates are typically in the 0.1–1 micrometer per minute range.
Common masking materials for long etches in TMAH include silicon dioxide (LPCVD and thermal) and silicon nitride. Silicon nitride has a negligible etch rate in TMAH. The etch rate for silicon dioxide in TMAH varies with the quality of the film, but is generally on the order of 0.1 nm/minute.[11]
The tetramethylammonium ion [14] affects nerves and muscles, causing difficulties in breathing, muscular paralysis and possibly death.[15][16] It is structurally related to acetylcholine, an important neurotransmitter at both the neuromuscular junction and autonomic ganglia. When it acts as an agonist, this structural similarity is reflected in its mechanism of toxicity – it binds to and activates the nicotinic acetylcholine receptors, although they may become desensitized in continued presence of the agonist. The action of tetramethylammonium is most pronounced in autonomic ganglia, and so tetramethylammonium is traditionally classified as a ganglion-stimulant drug.[17]
The ganglionic effects may have contributed to deaths following accidental industrial exposure. "Chemical burns" induced by this strong base are also severe. There is evidence that poisoning and even death can occur through skin-contact with low concentration solutions of TMAH.[16][18]
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