DRAFT ZANZIBAR NATIONAL STANDARD  
 
 
Methods of sampling and test (physical and chemical) for water and 
wastewater - Part 2 : Mercury  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
ZANZIBAR BUREAU OF STANDARDS  
 
 
©ZBS 2022 - All rights reserved                First  Edition  
 
 
                                                        PCD 339 – 2:2022                                                       
ICS: 13.060.50  
                                                                                                                    PCD 339 – 2:2022                                                        
 
©ZBS 2022 - All rights reserved     ii 
 Foreword  
 
This draft Zanzibar National Standard has been developed by Water Quality Standards Technical 
Committee (TCE1).  In accordance with ZBS general procedures, this draft standard is presented to 
the public in order to receive any technical and editorial comment concerns.  
 
Technical Committee Representatives  
 
This Draft Zanzibar National Standard was prepared by Water  Quality Standards Technical 
committee  which consist of representatives from the following organizations:  
 
State University of Zanzibar (SUZA)  
Chief Government Chemist Agency (CGCLA)  
Zanzibar Urban Municipal Council (ZUMC)  
Zanzibar Environmental Management Authority (ZEMA)  
Department of Environment (DoE)  
Zanzibar Water Authority (ZAWA)  
Zanzibar Sugar Factory (ZSF)  
Zanzibar Bureau of Standards -Secretariat  
 
 
 
 
 
Zanzibar Bureau of Standard (ZBS)  
P.O.Box 1136  
Zanzibar  
Tel:  +255 24 2232225  
Fax: +255 2 4 2232225  
E-mail: info@zbs.go.tz  
Web: www.zbs.go.tz  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  PCD 339 – 2:2022                                                        
iii ©ZBS 2022 - All rights reserved    
 Introduction  
Organic and inorganic mercury salts are very toxic and the ir presence in the environment, especially 
in water, should be monitored. Therefore, it is desirable to a scertain the limit of mercury present in 
water and wastewater.  
In reporting the result of a test or analysis made in accordance w ith this standard, if the final value, 
observed or calculated, is to be rounded off, it shall be don e in accordance with  ZNS 9 4,Rounding o ff 
numerical values  . 
In the preparation of this standard, the reference was made to the following sources:  
 
IS 3025 ( Part 48 ) : 1994  (Reaffirmed 2019), Methods of sampling and test (physical and chemical) 
for water and wastewater Part 48 Me rcury.  PCD 339 – 2:2022                                                        
©ZBS 2022 - All rights reserved     1 
 Methods of sampling and test (physical and chemical) for water and 
wastewater Part 48 Mercury  
1 Scope  
 
This standard prescribes two methods for the  determination of mercury in water and wastewater.   
 
a) Cold vapour atomic absorption spectrophotometric method;  and 
b) Calorimetric dithizone method.  
2 Normative references  
 
The following referenced documents are indispensable for the application of this Zanzibar National 
Standard. For dated references, only the edition cited applies. For undated references, the latest 
edition of the referenced document (including any amendments) applies.  
 
ZNS 308  - EAS 32, Water quality — Vocabulary  
 
ZNS 356 -1, Water, sewage and industrial effluents – Glossary of terms – Part 1  
3 Terms and definitions  
 
For the purposes of this standard the definitions  given in IS 7022 (Part 1 ) : 1973 and IS 7022  
(Part 2 ) : 1979 shall apply.  
 
4 Sample preservation  
 
The sampling bottles should be cleaned thoroughly with di lute nitric acid ( 6 N ) prior to final  rinsing 
with water. The water samples should be collected and stored preferably in polypropylene  or 
chemically resistant glass containers. For preservation, the sample should  be acidified with 
concentrated nitric acid ( 2 mL  of AR grade nitric acid in one litre of the  sample just to bring down the 
pH below 2). For dissolved mercury, filter the sample in the field  and acidify the filtrate wit h nitric acid 
to a pH of 2 or lower.  
 
NOTE  1: Avoid excess nitric acid. Add 5 mL  or 0.1N iodine solution to avoid losses of volatile  
organo mercury components during handling and  digestion of samples.  
5 Cold vapour atomic absorption spectrometry  
 
5.1 Principle  
 
The flameless atomic absorption procedure is a  physi cal method based on the absorption of  
radiation at 253·7 nm by mercury vapour. The  mercury is reduced to the elemental state and  
aerated from solution in a closed system. The  mercury vapour passes through a cell positioned  
in the light path of mercury hall ow cathode  lamp of an atomic absorption spectrophotometer.  
Absorbance ( peak height) is measured  as a function of mercury concentration and  recorded.  
 
The cold vapour atomic absorption spectrophotometric  method is suitable for all types of  samples 
such as natural waters, potable waters,  domestic and industrial wastewaters. Lower  detection limit of 
0·2 µ g/l may be achieved.  
  PCD 339 – 2: 2022                                                        
 
2 ©ZBS 2022 - All rights reserved    
 5.2 Apparatus  
5.2.1  Atomic Absorption Spectrometer (AAS)  and Associated Equipment  
 
Instrument settings recommended by the manufacturer  shall be f ollowed. Instruments designed  
specifically for the measurement of mercury  using the cold vapour technique may be substituted  
for the AAS.  
 
5.2.2    Mercury Vapour Generation Assembly  
 
Consists of an absorption cell, peristaltic pump,  flow meter, aeration tubing and a drying tube  
containing magnesium perchlorate.  
 
5.2.3    Mercury Hollow Cathode Lamp  
 
5.2.4  Recorder I/Printer / Display Meter  
 
Any multi -range variable. recorder that is cornpatible  with the UV detection system is suitable . 
 
5.3 Reagen ts 
 
5.3.1  Sulphuric Acid - Concentrated.  
 
5.3.2   Nitric Acid - Concentrated.  
 
5.3.3   Stannous  Chloride Solution  
 
Dissolve 2 5 g of stannous chloride ( SnCI 2 ) in water containing 50 mL  of concentrated hydrochloric  
acid and dilute to 250 mL . If a suspension  forms, stir reagent continuously durin g use.  
 
5.3.4   Sodium Chloride - Hydroxylamine sulphat e solution.  
 
Dissolve 12g of sodium chloride and 12 g of  hydroxylaminesulphate (NH 2OH ) 2H2SO 4, in distilled 
water and dilute to 100 mL . 
 
5.3.5   Potassium Permanganate Solution  
 
Dissolve 5 g of potassium pertnanganate in  distilled water and dilute to 100 mL . 
 
5.3.6  Potassium Persulphate Solution  
 
Dissolve 5 g of potassium persulphate in distilled  water and dilute to 100 mL . 
 
5.3.7  Stock Mercury Solution  
 
Dissolve 1.354 g  of mercuric chloride in about  700 mL of distilled water. Add 10 mL  of concentrated  
nitric acid and make up to 1 000 mL ( 1 mL =  1 mg Hg ).  
 PCD 339 – 2:2022                                                        
©ZBS 2022 - All rights reserved     3 
 5.3.8  Standard Mercury Solution  
 
Prepare a series of' standard mercury solutions  containing 0 to 5 µg/L  by appropriate dilution  of stock 
mercury solu tion (5.3.7) with water  containing  10 mL of concentrated nitric  acid per litre. Prepare 
standards daily.  
 
NOTE: Use mercury free dist illed water for the  preparation of reagents and standards . 
 
 
 
Figure 1: Schematic arrangement of equipment  for   measurement  of mercury  by  
cold vapour atomic absorption technlque  
 
5.4  Procedu re 
 
5.4.1  Instrument Operation  
 
Follow the procedure of the manufacturer's  operating manual. Connect the mercury vapour  
gene rating assembly as shown in Figure  1 . 
 
5.4.2   Standardization  
 
Transfer 100 mL of each of the 1.0, , 2.0 and 5. 0 µg/ L standard mercury solution and a blank of  100 
mL water to 300 mL BOD bottles. Add 5 mL of concen trated sulphuric acid and 2·5 mL  of 
concentrated nitric acid to each bottle. Add  15 mL of potassium permanganate solution to each  bottle 
and let stand for at least 15 minutes.  Add 8  mL  of potassium persulphate ( K 2S2O8 ) solution to each 
bottle and heat for 2 hours in  a water  bath at 95°C. Cool and add 6 mL  of sodium chloride -
hydroxylamine su lphate solution  to reduce the excess permanganate. After  decolourization add 5 mL  
of stannous chloride  solution and attach the bottle immediately to  the aeration apparatus forming a 
closed system.  As mercury is volatilised and carried into the  absorption c ell, absorbance will increase 
to a maximum within a few seconds. As s oon as  recorder re turns approximately to the base line 
remove stopper holding the aeration fr it from the reaction bottle and replace with a bottle  containing 
distilled water. Flush the sys tem for a few seconds and run the next standard in the same manner. 
Construct a standard calibration  curve by plotting absorbance (peak  height) versus mercury  
concentration in  µ g. 
 
 
 PCD 339 – 2: 2022                                                        
 
4 ©ZBS 2022 - All rights reserved    
 5.4.3   Analysis of Sample  
 
Transfer 100 ml of sample or portion diluted to  100 mL  containing not more than 5·0 µg/L  mercury  to 
a 300 mL  BOD bottle. Treat as in 5.4.1  samples containing high ( sea waters, brines and  effluents 
high in chloride) chlorides require as  much as an a dditional 25 ml potassium perma nganate  solution. 
Durin g this step, chloride is  
converted into free chlorine which is absorbed  at 253. 7 nm. Hence remove all free chlorine  before the 
mercury is reduced and swept into  the cell by using  an excess ( 25 mL  ) of hydroxylamine  sulphate 
solution.  
 
5.5  Calculation  
 
Determine peak height of sample from recorder  chart and read mercury value from standard  curve.  
 
6  Colorimetric dithizone method  
 
6.1  Principle  
 
Mercury ions react with dithizone solution in  chloroform to form an orange colour which is  
measured at 490 nm  using a spectrophotometer.  The colorimetric dithizone method is suitable  
for estimating higher levels of mercury in potable  waters (more than 2 µg/L ) and has a  
minimum detection limit of2 µg/L . 
 
6.2 Interference  
 
Copper, gold, palladium, divalent platinum and  silver react with dithizone in acid solution.  
Copper in the dithizone extract remains in the  organic phase while the mercury dissolves in the  
aqueous phase. The other contaminants usually  are not present. The mercury dithizonate  
being  photosensitive should be measured  quickly.  
 
6.3  Apparatus  
6.3.1  Spectrophotometer  
 
For use at 492 nm, providing a light path of  1 cm or longer.  
 
6.3.2   Separating Funnels  
 
250 and 1 000 mL with PTFE stopcocks.  
 
6.3.3   Glassware  
 
Clean all glas sware with potassium dichromate - sulphuric acid cleaning solution.  
 
6.4 Reagen ts 
 
6.4.1  Mercury Free Distilled Water  
 
Use redistilled or deionised distilled water for  preparing all reagents and dilutions.  
 
 PCD 339 – 2:2022                                                        
©ZBS 2022 - All rights reserved     5 
 6.4.2  Stock Mercury Solution  
 
Dissolve 135·4 mg of mercuric chloride ( HgCI 2 ) in about 700 mL of distilled water, add 1·5 mL 
concentrated ni tric acid and make up to 1000 mL with distilled water ( 1.0 mL = 100 µg  Hg ).  
 
6.4.3   Standard Mercury Solution  
 
Dilute 10·0 m l of stock solution to 1 000 mL with 
distilled water. Prepare freshly bef ore use  ( 100 mL= 1.00 µ g Hg ).  
 
6.4.4  Potassium Permanganate Solution  
 
Dissolve 5 g of potassium permanganate in 100 mL  of distilled water.  
 
6.4.5   Potassium Persulphate Solution  
 
Dissolve 5 g of potassium persulphate (K 2S2O8) in 100  mL  distilled water.  
 
6.4.6   Hydroxylamine Hydrochloride Solution  
 
Dissolve 50 g of hydroxylamine hydrochloride  (NH 20H.HCL) in 100 mL  water.  
 
6.4.7   Dithizone Solution  
 
Dithizone is often contaminated with the oxidation  product diphenyl thiocarbodizone or  with metals. To 
purify dithizone  dissolve  100 mg of dithizone in 50 mL of chloroform in a  beaker and filter under slight 
vacuum using  Whatman No . 42 or equivalent filter paper. Wash beak er and filter paper with 5 mL  port 
ions of chloroform.  Transfer the filtrate to 500 mL separatory  funnel. Add 100 mL  of 1 percent  
minimum hydroxide and shake moderately for  1 minute only. Transfer chloroform  m layer to  250 mL  
separatory funnel retaining orange -red aqueous layer in a 500 mL  funnel. Repeat  extraction process 
keeping each time chloroform  layers separately in 250 mL  separatory  funnels and using 1  percent a 
minimum hydroxide.  After three extractions, transfer aqueous  layer to 500 mL  funnel. Discard 
chloroform  layer.  
 
Comb ine all the extracts in a 500 mL  separatory  funnel. Ad d 1: 1 hydrochl oric acid in 2 mL portions, 
mixing each time till dithizone precipitates  and orange -red colour appears with  three 25 mL  portions 
chloroform. Dilute the  combined  extract to 1 000 mL  with chloroform;  1 000 ml = 100 µg dithizone. 
Dilute 60 mL of this solu tion with chloroform to 1 000 mL , ( 1 mL = 6 µ g dithizone ).  
 
6.4.8  Sulphuric Acid - 0·25 N.  
 
Dilute 25 mL of 1 N sulphuric acid to 100 mL with distilled  water.  
 
6.4.9   Potassium Bromide Solution  
 
Dissolve 40 g of potassium bromide in 100 mL distilled water.  
 
6.4.10   Chloroform  
 
  PCD 339 – 2: 2022                                                        
 
6 ©ZBS 2022 - All rights reserved    
 6.4.11   Phosphate -Carbonate Buffer Solution   
Dissolve  150  g of Na 2HPO 4.12H 2O and 38 g of  anhydrous potassium carbonate (K 2C03) in 1 litre of 
distilled water. Extract with 10 mL  portions of dithizone until the last portion  remains blue. Wash with 
chloroform to remove  excess dithizone.  
 
6.4.12  Sodium sulphate - anhydrous.  
6.5 Procedure  
6.5.1  Preparation of calibration c urve  
 
Pipette 0 ( blank ) , 2·0, 4·0, 6·0, 8·0 and 10·0 mL  of me rcury standard solutions into separate  
beakers. To each beaker, add 500  mL of distilled  water or that volume chosen for sample, 1 mL  
of potassium perm anganate solution and 10 m L of concentrated sulphuric acid solution. Stir  and boil if 
necessary, add more potassium  permanganate until a pink colour persists. After  boiling has  ceased 
continuously add 5 mL of potassium persulphate (K 2S2O8 ) solution and  let cool for 30 minutes. Add a 
few drops of  hydroxylamine hydrochloride (NH 2OH.HCI)  solution to discharge the pink colour. Cool  
and transf er each solution to individual 1  litre separating funnels. Add 25 m L of dithizone  solution. 
Shake t he separating funnel vigorously and trans fer each organic layer to 250 mL  separating funnel. 
Repeat this extraction at  least  three times making sure that the colour in the last dithizone extract is  
an intense blue as  that of the original  dithizone  solution. Wash accumulated  dithizone extracts in the 
250 mL  separating  funnel by shaking with 50 mLof 0·25 N sulphuric acid. Transfer washed dithizonate 
extract  to another250 mL separating funnel. Add 50 mL  or 0 25 N sulphuric acid and 10 mL of 
potassium  bromide solution a nd shake vigorously to transfer mercury dithizonate from organic layer to  
aqueous layer. Discard dithizone layer. Wash  aqueous layer with a small volume of chloroform  
and discard the chlor oform. Transfer 20 mL  of phosphate ca rbonate buffer solution to each separating 
funnel and 10 mL  standard dithizone  solution. Shake vigorously and after separation,  transfer the  
mercury dithizone to beakers.  Dry contents with anhydrous sodium sulphate.  Transfer mercury 
dithizonate solution to a  cuvette and record absorbance at 492 nm.  Plot absorbance versus mercury 
concentration in µg. 
 
6.5.2  Analysis of  s amples  
 
Use 500 mL of  sample and prepare an absorbance  blank consisting of all reagents. When  necessary, 
filter sample through glass wool into  the separating funnel after oxidation step.  Complete  procedure 
as descri bed in 6.5.1.  
 
6.6  Calculation  
 
6.6.1  Read mercury from calibration curve.  
 
6.6.2  After obtaining the above value, depending  upon the volume chosen for analysis,  calculate for 
1  000 mL  and report result in  µg/L Hg.