Methods Manual for Salt Lake Studies/Looking deeper - preparing to analyse

Authors: PSJ Coleman,

Overview of analytical preparation

Chemical analysis of brines maybe complicated by salinity effects and by the interfering effects of the various ions in solution.So, before actually running an analysis, stop and consider your sample matrix and the tests you wish to undertake.

The basics

Unpack the container of brine/water samples and look for any analysis requests, sampling logs or chains of custody and other documentation.Make a note of which tests are required and the quantity of the sample.

Is there enough sample to conduct all tests separately? If not, are there some tests that can be run sequentially on samples without affecting later tests? For example, SG and temperature may be read with a hydrometer and thermometer prior to other tests. Be aware that reading pH with a meter should be conducted AFTER other tests such as EC and SG, as the reference electrode in the meter contains KCl.

What analytical equipment will you need? If you need an oven, spectrophotometer or scales, these require warming up. Turn them on now.

The sample

Consider the samples. What do you know about them? Do you know enough about the sample matrix to determine the dilutions required to prevent interferences from affecting the method you plan to use?

Is there any literature that describes existing typical analyses (major ions) of the sample matrix? Seawater-derived brines of known specific gravity are well described in Baseggio (1974). Brines from inland salt lake brines are less well known, but if there have been any typical analyses conducted on your brine previously, they will helpfully inform your analytical decisions. Information on brines derived from groundwaters may be available from government agency online bore monitoring databases. For example, in South Australia PIRSA and DLWBC have online databases that contain information from groundwater bores that form a network across the state.

If there is no existing information on the sample matrix, what tests will you need to conduct to determine the level of interferences? You may need to analyse first for the interference you suspect, take an EC/pH reading, and/or run the samples along with some matrix spikes. At a minimum, determining the specific gravity of the brine with a hydrometer will give you a helpful pointer regarding these necessary salinity dilutions you may need.

Dilutions, sample size and extra preparation

Very high dilutions are not good practice. Each time you halve the concentration of the sample you essentially double the size of the minimum practical quantation limit. Use the smallest dilution that will provide you with reliable results. Using the test methods you have available, will any dilutions be small enough to still provide a useful minimum practical quantation level? You may need to utilise a quick 'dip test' such as the Merck phosphate strips to characterise the sample, allowing you to estimate appropriate dilutions. If you are not sure that you have selected an appropriate dilution, consider "bracketing" the sample dilution. that is, make 3 dilutions including your best estimate, a large dilution and a smaller dilution. The use of standard additions is also recommended if you are unsure about the salinity and other interferences your sample matrix presents. See your test method, Eaton et al 1995, or any good chemistry manual for details on the technique of standard addition.

When doing titrations, use the information obtained from your consideration of the samples to determine what sample size you need to test and what approximate volume of titre and size of burette will be appropriate for the sample.

Determine if the samples need additional preparation before they can be tested (eg water or acid extraction for soils, and digestions for some water/brine parameters).

Method familiarisation

Get out the test method documentation (including any extraction/digestion processes) and become familiar with all the requirements. Read it right through and consider what you will need to do in each step.

Check:

1. that you have laid out a page in your lab notebook with the sample details and the tests in the order you will conduct them,
2. that you have sufficient reagents to test all your samples,
3. that you have all the necessary labware for the number of samples for each step of the procedure,
4. that all the equipment (scales, ovens, spectrophotometers, environmental chambers, incubators etc) is ready, turned on and/or warmed up,
5. that any scales being used have been checked against a known mass and calibrated if necessary,
6. that any meters being used have been calibrated,

Ready, steady, go...

Lay out the equipment and reagents for the first analysis, and start the analysis.