Silver nitrate test For the detection of seawater contamination in cargoes
A solution of silver nitrate in distilled water is regularly used as a simple test for the detection of seawater contamination of ships’ cargoes. Whilst the test is a useful preliminary test for saline contamination it should be considered as indicative only. It has limitations and should not be relied on as definitive proof of saltwater contamination; more detailed laboratory analysis will be required to confirm
this.The reaction is shown below.
This reaction results in the clear test solution turning a cloudy white as insoluble silver chloride is displaced out of the solution.
Silver nitrate + Sodium chloride (salt) (clear solution)
(contamination)
●
Quantitative determination of anions (i.e. chloride, nitrate etc) and cations (sodium, potassium etc) by ion chromatography.
●
Quantitative determination of metals, such as sodium, magnesium, calcium, potassium and strontium, using either inductively coupled plasma emission spectroscopy (ICPES) or atomic absorption spectroscopy (AAS).
Test procedures Quantitative chloride determination by titration
Two methods are available, one involving titration against standard silver nitrate solution using potassium chromate as an indicator and the other, involving titrating against a standard solution of a soluble mercury salt using diphenylcarbazone as an indicator. The chloride concentration is calculated from (i) the volume of standard silver nitrate solution used (ii) the volume of sample taken and (iii) normality/molarity (strength) of the standard solution used.
Determination of anions and cations by ion chromatography
Sodium nitrate + Silver chloride (clear solution)
(insoluble white precipitate)
In order to undertake a ‘spot’ test in situ, surveyors will normally carry a small dropping/pipette bottle containing acidified silver nitrate solution. A few drops of the solution should be dropped onto both the suspected contaminated and apparently uncontaminated areas of the subject cargo.
Alternative samples of the cargo both apparently contaminated and uncontaminated should be obtained, placed into sealed plastic sample bags, and removed for later testing.
It is essential to use distilled water to wash the test samples as the presence of chloride in some waters may produce anomalous results.
The test solution should also be acidified with nitric acid to prevent the formation of other silver salts.
Laboratory identification of water origins
The source, or origin, of a particular water contamination can be identified in the laboratory by a number of different testing procedures. The testing procedures are as follows:
● Quantitative chloride determination by titration. 27
In this instrumental technique a sample (i.e. water) is introduced via an injection loop into the eluent passing through the system. The sample is then pumped through an analytical ion exchange column, which will separate the various ions (i.e. anions or cations). The
Typical anion chromatogram and standard
Sample Name: Vial Number: Sample Type:
Control Program: Quantif. Method: Recording Time: Run Time (min):
5.00 S 2 - Cl - 7.217 4.00
WS03/1601 5
unknown Anions
Anions_7a
8/12/2003 18:55 30.00
Injection Volume: Injection Volume: Injection Volume: Injection Volume: Dilution Factor: Injection Volume: Injection Volume:
10.0
ECD_1 n.a. n.a.
1,0000 1,0000 1,0000
3.00
2.00
3 - NO3 - 14,163 1.00
4 - SO3 - 23.903
0.00
1 - F - 4.053 min 0.0 5.0 10.0 15.0 20.0 25.0 30.0
No. Ret. Time Peak Name Height min
S
1 4.05 F 2 7.22 Cl 3 14.16 NO3 4 23.90 SO4
Total:
0.036 4.093 0.993 0.926
6.048
Area Rel. Area Amount Type S* min %
ppm
0.005 0.946 0.464 0.792
0.22
42.88 21.01 35.88
2.207 100.00
0.040 BMB* 10.759 BMB* 9.752 BMB* 12.370 BMB*
32.920
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