Preparation of Ammonium Chloride and Determination of Nitrogen Content

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Ammonium chloride is an inorganic substance with the chemical formula NH₄Cl, which refers to the ammonium salt of hydrochloric acid, and is mostly a by-product of the alkali-making industry. Containing 24%~26% nitrogen, it is a small white or slightly yellow square or octahedral crystal. There are two dosage forms of powder and granular. Granular ammonium chloride is not easy to absorb moisture and is easy to store, while powdered ammonium chloride is mostly used as basic fertilizer for compound fertilizer production.

Experimental Principle

1. Preparation of ammonium chloride

In this experiment, ammonium chloride is prepared by the action of sodium chloride and ammonium sulfate:

2NaCl+(NH₄)₂SO₄=Na₂SO₄+2NH₄Cl

According to the principle of their solubility and the difference affected by temperature, measures such as heating, evaporation, and cooling are adopted to transform the dissolution into crystallization, thereby achieving separation.

Reaction involves solubility values of substances at different temperatures

Temperature/℃	0	10	20	30	40	50	60	70	80	90	100
NaCl	35.7	35.8	36	36.2	36.5	36.8	37.3	37.6	38.1	38.6	39.2
Na₂SO₄●10H₂O	4.7	9.1	20.4	41
Na₂SO₄				49.7	48.2	46.7	45.2	44.1	43.3	42.7	42.3
NH₄Cl	29.7	33.3	37.2	41.4	45.8	50.4	55.2	60.2	65.6	71.3	77.3
(NH₄)₂SO₄	70.6	73	75.4	78	81	84.8	88	91.6	95.3	99.2	103.3

It can be seen from the table that the solubility of ammonium chloride, sodium chloride and ammonium sulfate in water increases with the increase of temperature. However, the solubility of sodium chloride solution is not greatly affected by temperature; the solubility of ammonium sulfate is greatest at both low and high temperatures. There is a turning point in the solubility of sodium sulfate. The solubility of sodium sulfate decahydrate also increases with the increase of temperature, but when it reaches a certain temperature, it will be dehydrated into Na₂SO₄. The solubility of Na₂SO₄ decreases with increasing temperature. Therefore, as long as sodium chloride and ammonium sulfate are dissolved in water, heated and evaporated, Na₂SO₄ will crystallize and be filtered while it is hot. Then the filtrate is cooled, and NH₄Cl crystals are gradually precipitated as the temperature drops, and the NH₄Cl product is obtained by suction filtration at about 35℃.

2. Determination of nitrogen content

NH₄Cl is a commonly used nitrogen fertilizer, which is a salt of strong acid and weak base. Because the acidity of NH₄⁺ is too weak (Ka=5.6X10^-10), it cannot be directly titrated with NaOH standard solution. The formaldehyde method is widely used in production and laboratories to determine the nitrogen content in ammonium salts. The formaldehyde method is based on the following reaction:

4NH₄⁺+6HCHO=(CH₂)₆N₄H++6H₂O+3H⁺.

(CH₂)₆N₄H⁺+4OH^-=H₂O+(CH₂)₆N₄

The generated H⁺ and (CH₂)₆N₄H⁺ (Ka=7.1X10^-6) can be titrated with NaOH standard solution, the product at the metering point is (CH₂)₆N₄, its aqueous solution is slightly alkaline, and phenolphthalein can be used as an indicator.

Experimental Content

1. Preparation of ammonium chloride

1) Weigh 11g of NaCl and place it in a 100mL beaker, then add 30~40mL of distilled water. Heat and stir to dissolve. If there are insoluble substances, filter and separate them with a regular funnel, and hold the filtrate in an evaporating dish.

2) Add 13g (NH₄)₂SO₄ to the NaCl solution. Heating and stirring in a water bath promotes its dissolution. During the concentration process, a large amount of Na₂SO₄ crystallized out. When the solution is reduced to about 35mL, stop heating and suction filter while it is hot.

3) The filtrate is quickly poured into the 100mL beaker and cooled, the NH₄Cl crystal is gradually precipitated, cooled to about 35℃, and filtered.

4) Put the filtrate back on the water bath to heat and evaporate until more Na₂SO₄ crystals are precipitated and filtered. Pour the filtrate into a small beaker, let it cool to about 35℃, drain, and repeat twice.

5) The NH₄Cl crystals obtained three times were combined and weighed together, and the yield was calculated.

6) Identification of the product.

Take 1g of NH₄Cl product, put it at the bottom of a dry test tube, and heat it.

NH₄Cl impurity content = (G after burning - G empty test tube) / 1gx100%

2. Determination of nitrogen content

1) Treat the formaldehyde solution with NaOH to make the formaldehyde solution neutral.

2) Weigh three portions of 0.08~0.10g NH₄Cl into conical flasks.

3) Dissolve the weighed NH₄Cl in 20~30mL of water, add 5mL of neutral formaldehyde solution, add 1~2 drops of phenolphthalein, and titrate with 0.1mol/L NaOH until it becomes light red and does not change color within half a minute, which is the end point .