Methyl Orange Powder CAS 547-58-0

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Methyl orange powder is an organic substance with the chemical formula C14H14N3SO3Na. It is a yellow to orange powder. One part of the product can be dissolved in 500 parts of water, easily soluble in hot water and alcohol, and difficult to be dissolved in ether. Methyl orange itself is weakly alkaline, and the discoloration range is between pH 3.1~4.4. The discoloration range of methyl orange is red when pH is ≤ 3.1, orange when pH is 3.1-4.4, and yellow when pH is ≥ 4.4. It is often used as an acid-base indicator. It is suitable for titration between strong acid, strong base and weak base. In addition, methyl orange is also a common chemical dye widely used in biological dyeing.

Methyl Orange (chemical formula C14H14N3NaO3S), as a classic azo dye, has expanded its application fields from textile dyeing to interdisciplinary fields such as analytical chemistry, environmental monitoring, and biomedicine since its synthesis in the 19th century. This article systematically sorts out the diversified uses of methyl orange from the dimensions of technical principles, application scenarios, and security regulations, and explores its future development trends.

Acid-base titration indicator

 

The core advantage of methy orange as an acid-base titration indicator lies in its clear color change range (pH 3.1-4.4) and color mutation characteristics. When the pH of the solution is below 3.1, the azo bond (- N=N -) in the molecular structure combines with hydrogen ions to form a protonated form, which appears red; When the pH is above 4.4, the azo bond deprotonates and turns yellow; In the range of 3.1-4.4, the molecule is in an intermediate state and appears orange.

 

Typical application scenarios:

Strong acid titrates weak base: for example, using hydrochloric acid to titrate ammonia water, the endpoint pH of titration is about 4.7, and the color change point of methyl orange powder (pH 4.4) is highly consistent with the stoichiometric point, with an error of less than 0.1%.
Metal ion precipitation titration: When determining the total amount of calcium and magnesium ions in water, use methy orange to indicate the pH adjustment endpoint before adding chromium black T indicator to ensure the suitability of the indicator.
Redox titration assistance: In indirect iodometric titration, methy orange is used to indicate the titration endpoint of excess sodium thiosulfate solution, and the reaction endpoint is determined by color change.

 

Technical parameter requirements:

Indicator concentration: 0.1% (w/v) aqueous solution, prepared with boiling distilled water to remove dissolved CO ₂.
Titration conditions: The temperature should be controlled at 20-25 ℃ to avoid temperature fluctuations causing color change point shift (for every 1 ℃ increase, the pH of the color change point decreases by about 0.02).
Interference factors: Strong oxidants such as nitric acid can damage the molecular structure of methy orange, so direct use in systems containing such substances should be avoided.

Quantitative analysis by spectrophotometry

 

The characteristic absorption peak (λ max=464 nm) of methy orange in the UV visible spectral range (350-550 nm) makes it an ideal reagent for photometric quantitative analysis. Its molar absorptivity (ε) reaches 1.2 × 10 ⁴ L · mol ⁻¹· cm ⁻¹ at 464 nm, making it suitable for trace substance detection.

Typical application cases:

Chloride ion determination: In acidic medium, chlorine gas oxidizes and fades methyl orange. The chlorine content is determined by colorimetric method, and the detection limit can reach 0.01 mg/L.

Bromine ion detection: Utilizing the oxidation effect of bromine on methyl orange, combined with flow injection analysis technology, to achieve rapid detection of bromide ions in seawater and industrial wastewater.

 

Metal ion complexation reaction: Methyl orange forms colored complexes with metal ions such as Cu ² ⁺ and Fe ³ ⁺, and the quantification of metal ions is determined by changes in absorbance. For example, the λ max of Cu ² ⁺ - methyl orange complex is 505 nm.
Key points of methodological validation:

Linear range: 0.02-2.0 mg/L (chloride ion determination)
Precision: RSD≤2%(n=10)
Accuracy: spiked recovery rate of 95% -105%
Anti interference ability: Common ions (such as SO ₄² ⁻, NO3 ⁻) do not interfere with the measurement when the concentration is ≤ 100 times

In the laboratory, the synthesis of methyl orange powder usually uses p-aminobenzenesulfonic acid as the raw material. First, sodium salt is formed in the alkaline environment of sodium hydroxide, and then the reaction intermediate is obtained by diazotization reaction under the action of sodium nitrite and hydrochloric acid. This diazonium intermediate reacts with N, N-dimethylaniline and glacial acetic acid, and finally the product methy orange is obtained.

The synthesis steps of methy orange powder are as follows:

Step 1. Put 10ml of 5% sodium hydroxide solution and 2.1g (about 0.01mol) of p-aminobenzenesulfonic acid crystal into the beaker, and then heat them in a water bath to completely dissolve the p-aminobenzenesulfonic acid crystal in the solution.

Step 2. Take another beaker, add 0.8g (about 0.11mol) of sodium nitrite and 6ml of water, mix them evenly, and cool them with ice salt bath to 0~5 ℃

Step 3. Under constant stirring, slowly add 3ml of concentrated hydrochloric acid and 10ml of water into the above mixed solution, control the reaction temperature below 5 ℃, and then use starch potassium iodide test paper to check after dropping, and then place it in ice salt bath for 15 minutes to ensure complete reaction

Step 4. Take a test tube and mix 1.2g (about 1.3ml), 0.01molN, N-2 methylaniline and 1ml glacial acetic acid in it. Slowly add this solution to the above cooled diazonium salt solution with constant stirring. The dripping of the mixture is still slow

Step 5. After adding all the mixed solutions, continue to stir for ten minutes, and then slowly add 25ml of 5% sodium hydroxide solution until the reactant turns orange. At this time, the reaction solution is alkaline, and then crude methy orange will precipitate as fine particles

Step 6. Heat the reactant in a 60 ℃ hot water bath for five minutes, cool it to room temperature, and then cool it in an ice water bath, so that methy orange crystals can be separated completely

Step 7. Filter by suction, collect the crystals, wash them with a small amount of water, ethanol and ether in turn after being drained, and press the precipitation with a vacuum plug as far as possible

Step 8. Then we get methy orange. A small amount of boiling water dissolved in sodium hydroxide can be used for recrystallization, and an appropriate amount of active carbon can be added. After the crystallization is cooled and precipitated completely, it is filtered, and the precipitate is washed with a small amount of ethanol and ether in turn to obtain the product - orange flake shaped methy orange crystal.

Principle analysis of methy orange powder experiment：

1. In the experiment, the diazotization intermediate obtained after the raw material is diazotized is very active. In order to prevent the occurrence of side reactions and ensure the yield, all steps involved in the preparation of the reaction phase should be carried out in an ice water bath environment, and the reagents used should also be slowly added and fully stirred.

2. If the reactant contains inactive N, N-2 methylaniline acetate, after adding sodium hydroxide, N, N-2 methylaniline which is difficult to dissolve in water will be separated out, which will affect the purity of the product. In addition, the color of wet methy orange will quickly become dark after being exposed to light in the air, so that it will generally get purplish red or even brown black crude products. Therefore, the recrystallization operation can play a great role in this experiment.

3. If it is necessary to use recrystallization, the recrystallization operation should be as fast as possible, otherwise, because the product is alkaline, the product is easy to deteriorate at high temperatures, and the color becomes darker. The purpose of washing with ethanol and ether is to dry the product as soon as possible to prevent the color from becoming darker again.

Methyl orange powder industrial preparation method:

1. It is obtained by reacting p-aminobenzenesulfonic acid with N, N-dimethylaniline after diazotization. Dissolve p-aminosulfonic acid in 2.5% sodium carbonate solution, add sodium nitrite, after complete dissolution, add crushed ice and hydrochloric acid for diazotization reaction, and precipitate diazonium salt crystals. Add the mixture of N, N-dimethylaniline and glacial acetic acid into the diazonium salt solution, stir for 10min, and then slowly add 10% sodium hydroxide solution until it is alkaline. Cooling crystallization, filtering, washing with saturated brine, drying, recrystallization with water to obtain methy orange.

2. In an enamel container containing 6L 2mol/L sodium hydroxide, add 2kg p-aminobenzenesulfonic acid and heat it to completely dissolve it (the solution is alkaline to litmus test paper). Then add 800g sodium nitrite, stir and dissolve it, and cool it with ice to 5 ℃. The obtained solution is slowly added to the mixture of 5L 2mol/L hydrochloric acid and 10kg crushed ice under continuous stirring, cooled, and left for 15min. Determine the end point of diazotization reaction with starch potassium iodide test paper (the test paper turns blue). The prepared solution is added to the cold solution of 1200gN, N-dimethylamine and 10L 1mol/L glacial acetic acid prepared in advance under stirring. After mixing evenly, it is allowed to stand for 10min. Slowly add 20% sodium hydroxide solution until it is obviously alkaline, and yellow scale like methy orange crystals are precipitated. After the crude product is heated to be completely dissolved, add 2kg of chemically pure salt, heat and stir until it is dissolved, cool and crystallize with water, filter by suction, and then dissolve it with a small amount of hot distilled water (1g of crystal is dissolved in every 20mL of hot water), filter while hot, cool and crystallize the filtrate, suck it dry, wash it with a small amount of ethanol and ether in turn, and dry it to get the finished methy orange powder.

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