FREE Cyanide Assay Method

FREE Cyanide Assay Method

A simple example of the application of a complexation reaction to a titration procedure is the titration of cyanides with silver nitrate solution. When a solution of silver nitrate is added to a solution containing cyanide ions (e.g. an alkali cyanide) a white precipitate is formed when the two liquids first come into contact with one another, but on stirring it re-dissolves owing to the formation of a stable complex cyanide, the alkali salt of which is soluble:silver_nitrate

When the above reaction is complete, further addition of silver nitrate solution yields the insoluble silver cyanoargentate (sometimes termed insoluble silver cyanide); the end point of the reaction is therefore indicated by the formation of a permanent precipitate or turbidity.

The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide, precipitated by local excess concentration of silver ion somewhat prior to the equivalence point, is very slow to re-dissolve and the titration is time-consuming. In the Deniges modification, iodide ion (usually as KI, ca 0.01 M) is used as the indicator and aqueous ammonia {ca 0.2M) is introduced to dissolve the silver cyanide.

The iodide ion and ammonia solution are added before the titration is commenced; the formation of silver iodide (as a turbidity) will indicate the end point:titrate_nacn

During the titration any silver iodide which would tend to form will be kept in solution by the excess of cyanide ion always present until the equivalence point is reached:cyanide_assay_method

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni( II) salt in ammoniacal solution is heated with excess of cyanide ion, the

[Ni(CN)4]2- ion is formed quantitatively; since it is more stable than the [Ag(CN)2]- ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA.

Preparation

  • Prepare the necessary AgNO3 and H2SO4 solutions following instructions below.
  • Set up the burettes in good light in preparation for titration.
  • Determine the silver nitrate factor by titrating 0.01g NaCN in 25 ml distilled water as per 2.0. Record the factor.

Silver Nitrate Titration

  • Add 1/8 tsp potassium iodide (KI) to the flask.
  • Titrate the prepared silver nitrate solution to a pale yellow opalescence end point of silver iodide once free cyanide is no longer present.
  • Record the exact volume of silver nitrate added.
  • Calculate the amount of NaCN in the leach solution.

ml AgNO3  x  f (AgNO3)  x  volume of leach sol’n =  g of NaCN in leach sol’n
ml of aliquot titred

Sulphuric Acid Titration

  • Use the same sample aliquot as in Step 2.0.
  • Add 1-2 drops phenolthalein indicator, which gives a pink colour.
  • Titrate with H2SO4 solution to a colourless end point when the pink disappears.
  • Record the exact amount of H2SO4
  • Calculate the amount of lime in the leach solution.

ml H2SO4 x f (H2SO4) x volume leach sol’n = g CaO in leach sol’n
ml of aliquot titred

Silver Nitrate Standardization

  • Standardize AgNO3 solution by dissolving 0.0100 g of NaCN in about 10 ml de-ionized water and titrating per Steps 2.1 and 2.2.
  • Calculate the factor f = g NaCN / ml AgNO3.
  • Based on this calculation the Met Lab will add NaCN to the bottle test to replenish an amount equivalent to that consumed during the time interval.

Solution Preparation

Silver Nitrate Solution:

  • Weigh 1.76 g silver nitrate into 1 L volumetric flask.
  • Mix to volume using distilled water ensuring the AgNO3 is in solution. F
  • Standardize using 0.0100 g NaCN to determine factor

Sulphuric Acid Solution:

  • Weigh 1.75g H2SO4 in a 1 L volumetric flask.
  • Make up to volume and mix well.
  • This solution is not standardized. Stoichiometrically the factor

f (H2SO4)  = 0.001g CaO per 1 ml H2SO4.

Phenolthalein Solution – 1%

  • Weigh 1.0 g phenolthalein indicator into 50 ml ethanol.
  • Bulk to 100 ml with distilled water.

Chemicals and Reagents

  1. All Chemicals are of Reagent Grade unless specified otherwise.
  2. Silver Nitrate (AgNO3)
  3. Sulphuric Acid (H2SO4)
  4. Potassium Iodide (KI)
  5. Phenolthalein Indicator
  6. Sodium Cyanide (NaCN)
  7. Lime (CaO)

Solution Preparation:

  1. Silver Nitrate Solution (AgNO3)
  2. Sulphuric Acid (H2SO4)
  3. Phenolthalein Indicator Solution – 1%

DETERMINATION OF CYANIDES

The theory of the titration of cyanides with silver nitrate solution has been given above. All silver salts except the sulphide are readily soluble in excess of a solution of an alkali cyanide, hence chloride, bromide, and iodide do not interfere. The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide is often precipitated in a curdy form which does not readily re-dissolve, and, moreover, the end point is not easy to detect with accuracy.

There are two methods for overcoming these disadvantages. In the first the precipitation of silver cyanoargentate at the end point can be avoided by the addition of ammonia solution, in which it is readily soluble, and if a little potassium iodide solution is added before the titration is commenced, sparingly soluble silver iodide, which is insoluble in ammonia solution, will be precipitated at the end point. The precipitation is best seen by viewing against a black background.

In the second method diphenylcarbazide is employed as an adsorption indicator. The end-point is marked by the pink colour becoming pale violet (almost colourless) on the colloidal precipitate in dilute solution (ca 0.01 M) before the opalescence is visible. In 0.1 M solutions, the colour change is observed on the precipitated particles of silver cyanoargentate.

Procedure

NOTE: Potassium cyanide and all other cyanides are deadly poisons, and extreme care must be taken in their use. 

For practice in the method, the cyanide content of potassium cyanide (laboratory reagent grade) may be determined.

Method A

Weigh out accurately about 3.5 g of potassium cyanide from a glass-stoppered weighing bottle, dissolve it in water and make up to 250 mL in a graduated flask. Shake well. Transfer 25.0 mL of this solution by means of a burette and NOT a pipette to a 250 mL conical flask, add 75 mL water, 5-6 mL 6M ammonia solution, and 2mL 10 per cent potassium iodide solution. Place the flask on a sheet of black paper, and titrate with standard 0.1M silver nitrate. Add the silver nitrate solution dropwise as soon as the yellow colour of silver iodide shows any signs of persisting. When one drop produces a permanent turbidity, the end-point has been reached.

Method B

Prepare the solution and transfer 25 mL of it to a 250 mL conical flask as detailed under Method A. Add two to three drops of diphenylcarbazide indicator and titrate with standard 0.1 M silver nitrate solution until a permanent violet colour is just produced.

The diphenylcarbazide indicator is prepared by dissolving 0.1 g of the solid in 100 mL of ethanol.

1 mole AgN03 = 2 moles CN~

Procedure-to-Determine-FREE-CYANIDE-IN-WATER

Reference: Vogel’s – Quantitative chemical analysis