Limit test for Arsenic

 

Aim: To carry out the limit test for arsenic for the given sample.

Gutzeit's Apparatus -

It consists of a wide mouthed glass bottle of 120 ml capacity fitted with rubber bung through which a delivery tube of 20 cm length is placed having external diameter 8mm and internal diameter of 6.5 mm. The tube is constricted at lower end with a hole of 2 mm diameter and also a side hole of 1mm diameter. The upper end of the tube is fixed with two rubber bung such a way that a mercuric chloride paper is sandwiched in between them . The two rubber bung are held together tightly with a clip or a screw clamp. Inside the tube , 2 mm below the rubber bung lead acetate cotton is placed.

Gutzeit Apparatus

Principle:

The limit test for arsenic is based upon the conversion of arsenic impurity by a series of reaction to arsenic gas which react with mercuric chloride test paper to give yellow or brown color strain of mercuric arsenide.

         The stain produce by the sample is compared to the standard stain produce from specific amount of arsenic trioxide under the same reaction condition.

for the sample to pass the limit test of arsenic the sample stain should not be more than that of standard stain.

Reaction:

• Trivalent or pentavalent arsenic impurity is converting to arsenous acid and arsenic acid by dilute HCl.

           

• the arsenic acid is formed is reduce to arsenous acid by reacting with stannous chloride.

• aresnous acid is further reduce to arsenic gas by reacting with nascent hydrogen which is obtained from zinc and HCl

• Arsine gas react with mercuric chloride test paper to give yellow or brown stain of mercuric arsenide.

Preparation of the apparatus for the arsenic limit test

• The glass bottle( Gutzeit Apparatus) and the tube are first washed with dilute HCl  and rinsed with water.
• The delivery tube is then tightly packed with lead acetate cotton.
• The upper end of the tube is inserted into a rubber bung on which a square piece of mercuric chloride test paper is placed.
• The second bung is placed over this and the two bungs are tightly attached with spring clip.
• The lower end of the delivery tube is inserted into another rubber bung.

Procedure for standard

Place 50 ml of water in wide mouth bottle. Add 10ml of standard HCl, 1g of KI . Pipette out 10 ml of standard Arsenic trioxide and transfer to the bottle, add 10g of granulated Zinc and quickly place the prepare glass tube in its position. Allow to stand for 40 min at 40 degree celcius.

Procedure for sample 

Place 50 ml of water in wide mouth bottle, add 10 ml of standard HCl, 1g of KI and specified amount 

of sample in to the bottle. Add 10 g granulated zinc and quickly place the prepare glass tube in its position. Allow to stand for 40 min at 40 degree celcius.

Reagent and chemicals required

All the reagents and chemicals used arsenic limit test must be completely free from arsenic and labelled as AsT except the sample and standard solution.

Mercuric Chloride Test Paper

It is smooth white filter paper, not less than 25mm in width , which is first shocked in saturated solution of HgCl2 and finally dried at 60 degree celcius in dark.

Precaution:

1. The apparatus must be properly washed with dilute HCl  and the rinsed with water every time before and after the Arsenic limit test.
2. Mercuric chloride test paper must be freshly prepared by wetting the test paper in Mercuric chloride solution and drying under shade.
3. The delivery tube must not dip in to the mixture in the bottle. so that the Arsine gas allowed passing through it.
4. The reaction must be allowed to proceed at 40 degree celcius for 40 min.
5. The standard strain  and the sample strain should be prepare simultaneously and compare immediately after the reaction.
6. Lead acetate cotton plug should be keep freshly prepared.

Reasons

1. Stannous chloride is added because it acts as a reducing agent in presence of HCl. It converts arsenic acid to arsenous acid.
2. Granulated zinc is added to produce the nascent hydrogen gas by reacting with HCl. The nascent hydrogen gas is more reactive which convert arsenous acid to arsine gas.
3. Granulated zinc contains an impurity of sulphur which reacts with HCl and produce H2S gas. So lead acetate cotton is placed in the delivery tube to trap the H2S gas by reacting with them, which otherwise react with HgCl2 paper and produce black color and the stain produce by arsine gas will not be visible.

                                   H2S     +     HgCl2   -----------------> HgS  +  2HCl

                         H2S   +   Pb(CH3COO)2  ------------------> PbS   +  CH3COOH

      4. A side whole is provided in the delivery tube  to allow continuous passage for arsine gas in the                case where the lower whole blocked due to condensation of water vapor.

      5. The stains are immediately compared because they fade on exposure to light. They can be preserved by dipping the test paper in molten paraffin wax and drying.