Thursday 21 January 2016

Standardisation of Stock Cyanide Solution

1. Principle:

CN- is titrated with standard silver nitrate to form the soluble cyanide complex, Ag(CN)2-. As soon as all CN- has be complexed and a small excess of Ag+ has been added, the excess Ag+ is detected by the silver-sensitive indicator, p-dimethylaminobenzal-rhodanine, which turns from a yellow to a salmon colour end-point.

2. Reagents:

2.1 Potassium cyanide (KCN), AR. Extremely toxic – read MSDS before handling.

2.2 Sodium hydroxide (NaOH), AR

2.3 Stock cyanide solution, ~1000 ppm: Dissolve 1.6 g NaOH and 2.51 g KCN in 1 L reagent grade water.

2.4 p-dimethylaminobenzal-rhodanine

2.5 Acetone, AR

2.6 Indicator solution: Dissolve 20 mg p-dimethylaminobenzal-rhodanine in 100 mL of acetone.

2.7 Standard AgNO3 solution; (see : Standardisation of Silver Nitrate)

3. Procedure:

To one 250 mL Erlenmeyer flask, add 20 mL of stock KCN solution plus approximately 100 mL reagent grade water. Add a couple of drops of rhodanine indicator; titrate with standard silver nitrate titrant to the first change in colour from a ‘canary’ yellow to a salmon hue. A blank must also be established containing the same amount of alkali and water. Record the results. Carry out standardisation in triplicate. Calculate normality of CN- as per section 5.

4. Frequency of Standardisation

Restandardise every week.

5. Calculation:

T    =    mL AgNO3 titrant
B    =    blank titre (mL)
N    =    Normality of AgNO3
V    =    Volume standard CN

6. Reference:

APHA 20th Edition: 4500 CN-, D. pp. 4-38-39.

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