grab samples

D. Procedure
(I) Take 0.2-0.5 g dried sample and mix with 10 ml of aqua regia in a 250-ml Erlenmeyer flask.
(2) Allow to stand at room temperature until effervescence ceases and then heat for 2 min in a hot
water bath ( "'" 90°C) Cool
(3) Bring to 100 ml volume with deionized-distilled water.
(4) Add 15 ml potassium permanganate solution slowly. If all the permanganate is reduced (i.e.,
pink-purple colour disappears), add more permanganate.
(5) Digest solution for 30 min in a hot wa ter bath ( "'" 90°C) Cool
(6) Add 10 ml of hydroxylamine sulphate-sodium chloride solution and mix thoroughly to remove
precipitated manganous oxide. Sufficient solution must be added until the permanganate colour
disappears.
(7) Add 10 ml of stannous sulphate and immediately connect the flask to the aeration system. Sparge
mercury from the solution with clean compressed air at flow rate of2l!min.
(8) Vent vapours from absorption cell through two traps of 5% acidified potassium permanganate
solution.
HI. Procedure for Low Concentrations Found in Seawater
A. Apparatus
The reduction/aeration cell of Hawley and Ingle ( 1975) is the major variation from the Hatch and Ott
( 1968) method. The reduction cell is constructed from a "medium" grade fritted glass tube of 30 mm
diameter with a top injection surface of a no. 20 sleeve-type rubber stopper. The lower end of the tube is
connected via a drying tube to a compressed air supply. Gas and mercury is lead from near the top of the
glass tube through a heated glass tube into a heated glass detection cell set into an atomic absorption
spectrophotometer (Christmann and Ingle 1976). The gas is then vented through two traps of 5% acidified
potassium permanganate solution.
B. Procedure (see Note 2 below)
(I) Using \00 ml of seawater in a 250-ml graduated Erlenmeyer flask , add 5 ml of conc. sulphuric
acid and 2.5 ml conc. nitric acid and 15 ml of( 5%) potassium permanganate solution.
(2) Heatinawaterbathfor 120minat60°C.
(3) Add 5 ml of( 5%) potassium persulphate solution and allow to stand for 12 h.
(4) Add 10 ml of hydroxylamine sulphate-sodium chloride solution and mix thoroughly to remove
precipitated manganous oxide. Sufficient solution must be added until the permanganate (pink-purple)
colour disappears.
(5) Inject I ml of prepared sample into the reduction cell and then 0.1 ml of stannous sulphate
reductant. Sparge with compressed air at 140 ml/min flow rate. After detection, flush the system with I ml
of blank, then inject I ml ofHg (II) standard followed by 0.1 ml of reductant.
(6) Calibration is accomplished by injecting quantities of known dilutions of a 100 ppm stock
solution (0. 1354 g of mercuric chloride + 50 ml of conc. nitric acid diluted to I /) with the range of values
of the standards" bracketing" the vafue of unknown. Standards for analysis should be prepared daily by
dilution from stock solution.
(7) Hawley and Ingle ( 1975) recommend changing the rubber stopper every twenty injections. This
method is suitable for about thirty samples per hour.
(8) The optimum absorption cell size is 2 mm X 60 cm, with the optics aligned by optical rail.
Notes: (a) All glassware should be thoroughly soaked in a nitric acid-permanganate solution for a day
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