Rafael, Dune Vienis Karen N. November 20, 2009BS-Pharmacy 4A Toxicology Lab

Activity No. 02HYDROCYANIC ACID

I. Objectives:1. To detect the presence of chlorofrom by performing Schönbein Pagenstecher’s test, Prussian

blue test, Leibig’s test, picric acid test, silver nitrate test and Vortmann’s nitroprusside test.2. To understand the principles behind each test, and3. Interpret the results after performing each method of detection.

II. DATA:

Type of Test/Method of Detection

Observations/Data

1. Schönbein Pagenstecher’s Test Actual: the guaiac-copper paper turned black after it was warmed.Ideal: The paper is turned blue to bluish green (Warren W. H.,1920).

2. Prussian Blue Test Actual: Upon the addition of potassium hydroxide and two drops of freshly prepared ferrous sulfate the distillate turned its color to blue.Ideal: The precipitate of Prussian blue appeared. But if the quantity is small, the solution will have merely a blue or bluish green color. After a long time, a flocculent precipitate of Prussian blue will settle to the bottom of the test tube (WarrenW. H., 1921).

3. Leibig’s or Sulfocyanate Test Actual: The color of the solution turned to light rusty brown.Ideal: Reddish to blood-red color will appear due to ferric sulphocyanate (Warren W. H., 1921).

4. Picric Acid Test Actual: The distillate turned to rusty brown in color.Ideal (+): The solution will become blood-red due to the formation of potassium isopurpurate (Warren W. H., 1921).

5. Silver Nitrate Test Actual: The acidified distillate with dilute nitric acid and drops of silver nitrate produced a white precipitate.Ideal (+): A white, curdy precipitate of silver cyanide (AgCN) will appear (Warren W. H., 1921).

6. Vortmann’s Nitroprusside Test Actual: The filtrate upon the addition of dilute ammonium sulfide turned the solution to green.Ideal (+): A violet color will appear and pass through blue green and yellow (Warren W. H., 1921).

Data Analysis:

Schönbein-Pagenstecher Test Mechanism of Action (Warren W. H., 1921)Hydrocyanic acid has nothing to do with this reaction. But rather, it forms ozone with copper

sulphate and that turns the guaiaconic acid of guaiac resin blue. Cupric cyanide (a) is an intermediate product which furnishes ozonized oxygen as shown below (b):

(a) CuSO4 + 2HCN = Cu(CN)2 + H2SO4

(Cupric cyanide)(b) 6Cu(CN)2 + 3H2O = 6CuCN + 6HCN + ↑ O3 Turns guaiac-copper paper to blue

(Ozone)Why is it called a “Preliminary Test”?

Neither hydrocyanic acid nor potassium cyanide is present, unless the paper is turned blue or bluish green. But the only conclusion to be drawn from a positive test is that hydrocyanic acid, or easily decomposable cyanide, may be present. Further conclusions should not be drawn from a positive result, since other substances like ammonia, volatile ammonium compounds, hydrochloric acid and especially oxidizing agents like ozone, hydrogen dioxide, nitric acid and chlorine will turn the paper blue.

Prussian Blue Test Mechanism of Reaction (Warren W. H., 1921)Hydrocyanic acid and potassium hydroxide form potassium cyanide which with ferrous sulfate

produces ferrous cyanide (a). The latter combines with more potassium cyanide, forming potassium ferrocyanide (b) which with ferric chloride precipitates Prussian blue (c), the ferric salt of hydroferrocyanic acid (H4Fe(CN)6).

(a) FeSO4 + 2KCN = Fe(CN)2 + K2SO4

(Ferrous sulfate) (Potassium cyanide) (Ferrous cyanide)

(b) Fe(CN)2 + 4KCN = K4Fe(CN)6

(Ferrous cyanide) (Potassium cyanide) (Potassium ferrocyanide)

(c) 3K4Fe(CN)6 + 4FeCl3 = Fe4[Fe(CN)6]3 ↓ + 12KCl(Potassium ferrocyanide) (Ferric chloride) (Ferric ferrocyanide/Prussian blue)

Prussian blue will not appear in presence of alkalies, since they decompose as it follows:Fe4[Fe(CN)6]3 + 12KOH = 3K4Fe(CN)6 + 4Fe(OH)8

Sulphocyanate Test Mechanism of Reaction (Warren W. H., 1921)Hydrocyanic acid and potassium hydroxide form potassium cyanide which takes sulfur form

yellow ammonium sulfide and becomes potassium sulphocyanate (a). The latter with ferric chloride forms ferric sulphocyanate (b):

(i) HCN + KOH = KCN + H2O(Hydrocyanic acid) (Potassium hydroxide) (Potassium cyanide) (Dihydrogen oxide)

(a) KCN + (H4N)2Sx = KSCN + (H4N)2Sx-1

(Potassium cyanide) (Yellow ammonium sulfide) (Potassium sulphocyanate)(b) 3KSCN + FeCl3 = Fe(SCN)3 + 3KCl

(Potassium sulphocyante) (Ferric chloride) (Ferric sulphocyanate) (Potassium chloride)

If the distillate contained hydrocyanic acid, a reddish to blood-red color will appear. This is due to ferric sulphocyanate. The limit of delicacy is 1:4,000,000.

Silver Nitrate Test Mechanism of ReactionThe presence of hydrocyanic acid results to appearance of a white, curdy precipitate of silver

cyanide. The limit of delicady is 1:250,000 (Warren W. H., 1921).

(i) HCN + AgNO3 AgCN ↓ + HNO3

(Hydrocyanic acid) (Silver nitrate) (Silver cyanide) (Nitric acid)

III. ANSWERS TO QUESTIONS:1. What are the laboratory safety measures in handling hydrocyanic acid?

Safety glasses, gloves, good ventilation. Only use if no suitable alternative chemical is available. Do not work on your own. Keep a cyanide poisoning kit available at all times, and ensure that fellow workers know

how to use it. Do not release into the environment.

2. In storing, what other acids are incompatible with hydrocyanic acid? Nitric acid Alkali

IV. CONCLUSION:

In whatever way applied, hydrocyanic acid is absorbed, even from the skin. So rapid is the absorption of this poison that there is evidence of intoxication after a few seconds, or a few minutes at most. Part of the poison thus absorbed passes from the body unchanged by way of the lungs. Another part, usually much less, is eliminated by the kidneys and passes into the urine. Sweat also is said to contain hydrocyanic acid. With this in mind, pharmacists play a vital role in the management in cases of hydrocyanic acid intoxication. With this activity, I understood and acquired skills on the detection of chloroform in the laboratory aside from management using pharmaceutical means. Chloroform is detected in the laboratory by performing Schönbein Pagenstecher’s test, Prussian blue test, Leibig’s test, picric acid test, silver nitrate test and Vortmann’s nitroprusside test

Bibliography

Warren, W. H. (1921). Laboratory Manual for the Detection of Poisons and Powerful Drugs. New Jersey: Read Books.