WATER QUALITY--Preservation of nutrient samples by addition of mercuric chloride.


In Reply Refer to:                             September 19, 1980
EGS-Mail Stop 412


QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 80.26

Subject: WATER QUALITY--Preservation of nutrient samples by 
                        addition of mercuric chloride.

Beginning October 1, 1980, water samples submitted to the National 
Water Quality Laboratories for analysis of nitrogen and phosphorus 
compounds in filtered and unfiltered samples must contain 40 mg/L 
mercuric ion (Hg2+) added as mercuric chloride (HgC12) and be 
chilled to 4 degrees C. Samples should not be frozen.

Tablets containing about 10 mg Hg2+ as HgC12 in a NaCl base have 
been prepared and will be distributed to the District Offices. 
Instructions for obtaining the HgC12 tablets and amber bottles for 
nutrient samples are given in Central Laboratory Newsletter No. 
80.02 which was published in late August or early September.

One (l) tablet is to be added to each FC and RC bottle submitted 
for nutrient analysis. Sample volume must be at least 200 mL and, 
ideally, 250 mL in order for the NaCl concentration to be within 
critical limits for analysis.

After October 1, 1980, the nitrogen and phosphorus analyses at the 
Central Laboratories will be calibrated using standard solutions 
containing 40 mg/L Hg added as HgC12. The analysis of a sample 
submitted without HgC12 added or with an Hg2+ concentration 
outside the range 20-60 mg/L will have an indeterminate error. The 
lab should be informed in advance of samples requiring special 
handling; if not, the analyses may be delayed. A surcharge will be 
applied to the analytical cost and billed to a district account to 
cover special handling for projects that need to arrange "special" 
sample handling without HgC12 tablets. "Specials" must be arranged 
prior to sampling.

Explanation

The rationale behind the decision to use HgC12 and chilling for 
nutrient samples preservation is summarized below. We invite you 
to read the briefing paper and annotated bibliography for further 
detail. Comments and new information are welcomed.

On the average, water samples are in-transit and awaiting analysis 
for at least 3-5 days, and often longer, from time of collection. 
In order for the analytical results to fairly represent 
concentrations of nitrogen and phosphorus compounds in the water 
at the time of collection, chemical and biological activity in the 
sample must be stopped for the period between time of collection 
and time of analysis.

Studies performed over the past 10 years by both Survey and 
academic researchers suggest that chilling alone, the only 
preservation techniques now used by WRD, is not reliable. That is 
to say the method preserves some samples adequately but not all. 
Moreover, it is not possible to determine under what circumstances 
the method is adequate and under what circumstances it is not.

These same studies also suggest that there are no completely 
reliable techniques for preserving nitrogen and phosphorus 
concentrations in water samples for 3-5 days. But reviews of the 
state-of-the-art in 1972 and 1976 both concluded that chilling 
plus addition of a biocide is a more reliable preservation 
techniques than is chilling alone. The best biocide among nearly 
10 is HgC12, which appears to be effective in most samples studied 
at concentrations of 25-40 mg/L Hg2+ ion. Some studies indicate 
that 40 mg/L Hg2+ is not completely effective in samples having 
organic carbon concentrations of 20 mg/L or more, a concentration 
encountered in only 7 percent of NASQAN sample analyses since 
1972.

Freezing and addition of HgC12 was found to be a more reliable 
method than chilling and addition of HgC12 in some studies but, in 
freezing, there exists a risk that ortho-phosphates will 
coprecipitate with calcite or amorphous silica causing 
irreversible losses of ortho-phosphate from solution. This risk is 
judged to be unacceptable. For this reason, freezing is 
discouraged as a routine preservation method.

Potential for Cross-Contamination

Concern has been expressed by WRD personnel that routine use of 
HgC12 will inevitably lead to dispersion of mercury in field 
vehicles, filtration equipment, and shipping containers, thereby 
greatly increasing the potential for contamination of the FA and 
RA samples with mercury. The possibility definitely exists but can 
be minimized by the following procedures which should be adopted 
by all field personnel:

1. The tablets, which are hard and individually packaged, should 
be kept in a closed, metal container where they cannot be crushed 
or abraded. The package of a crushed or noticeably abraded tablet 
should be discarded unopened in a safe place outside of the field 
vehicle or service laboratory.

2. When not in use, tablets should be kept in a cool place. 
Mercuric chloride has a slight vapor pressure at room temperature. 
Keeping the tablets cool will reduce the vapor pressure and extend 
their effective life.

3. The bare tablets should not be handled. The package can be 
opened and the tablet dropped into the sample bottles without 
being touched.

4. When FA or RA and FC or RC samples are being prepared at the 
same site, nitric acid should be added to the FA or RA sample 
first, hands thoroughly washed with clean water to remove traces 
of nitrlc acid, then add a HgC12 tablet added to the FC or RC 
sample. Finally the hands should be thoroughly washed again.

5. Caution: A sample to which HgC12 has been added should never be 
filtered. Doing so may contaminate the filtration equipment so 
completely that it cannot be cleaned satisfactorily and must be 
discarded.

Clearly great care must be exercised in the field to avoid cross-
contamination. Each HgC12 tablet contains only 10 mg of mercury. 
But ambient mercury concentrations in water are usually less than 
10 ug/L, and contamination at the 0.1 ug/L level would 
substantially change the mercury concentration of most samples.

Recent literature has suggested that metallic mercury vapor can 
pass through the walls of polycarbonate bottles and contaminate 
samples. This finding raises the question of contamination of 
samples stored in field vehicles or gage houses from mercury used 
to service manometers or spilled from broken themometers. The 
Quality of Water Branch is continuing to study the issue and will 
report findings at a later date. The Branch will also do 
periodically check the mercury concentration reported at NASQAN 
stations for signs of mercury contamination.

Safety

Mercuric chloride is extremely toxic to animals. The Merck Index 
reports the LD50 dose to rats as 37 mg/kg body weight. One or two 
grams ingested orally--equivalent to about 75 to 150 tablets--is 
often fatal to humans. Other mercuric compounds cause inflamation 
of skin and mucous membranes. There is also evidence that mercury 
accumulates in the body.

The mercuric chloride tablets must be treated with respect. The 
best safety measure to follow is to avoid contact. Following the 
practices and precautions listed above will help prevent 
contamination of acidified samples and will also ensure a good 
margin of safety for all who use the tablets.

The health hazard to workers using the tablets is so small that 
they meet or exceed all OSHA and pharmaceutical standards. The 
comfortable margin of safety is, however, no license for sloppy 
technique.



                              R. J. Pickering
                              Chief, Quality of Water Branch

Attachment: Briefing Paper

Memoranda superseded: QW Branch Technical Memos 69.07, 70.02, 
                      72.09

Keywords: Water quality, sampling, nutrients, preservation

WRD Distribution: A, B, S, FO, PO