Chemical methods in geology
3.
Practical field hydrogeochemistry
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Principles of good practice
• How to approach the
utilization of geochemical
properties of water?
• How do we get quality
data?
• How should we proceed?
• What to watch out for?
• Rules of good practice.
Introduction
• The use of geochemical data in hydrogeology
usually requires thoughtful sampling and analysis.
• The key is to keep in mind the goal of the
measurement
– For some studies, basic parameters are sufficient,
while others require a broad analytical approach
• If you are not sure about anything – the type of
sample container, the amount of sample, etc. –
consult with the laboratory.
Field measurements
• Properties that can easily change during
transport and storage
• Temperature
– Thermodynamic considerations, equilibria,
speciation
– EC and pH calibration
– Usually directly via conductometer or a pH meter
– The value is drifting (tempering of the devices)
Specific conductivity (conductivity)
• Property of a solution – a measure of the ability
to conduct an electric current
• SI unit is siemens per meter (S/m)
– In hydrogeology commonly μS/cm
• Conductivity is proportional to the content of
dissolved ions in the solution
– Water purity monitoring
• Deionized water 0.055 μS/cm
• Drinking water 50-500 μS/cm
• Sea water 50000 μS/cm
– Simple estimation of total dissolved solids (TDS)
• Basis for further sampling and analyses
Specific conductivity (conductivity)
• It is measured by determining the resistance of the
solution between two electrodes at a known (and
fixed) distance
• Alternating current (to avoid electrolysis)
• Conductometer
– A very diverse group of devices
– With fixed or exchangeable probe
– Necessary maintenance – especially cleaning the electrode
from deposits (calcite, dirt, bacteria…)
– Easy storage, long battery life of basic devices
– Choose types with automatic temperature compensation!
(The water in wells is cold :-))
• Induction is also used for industrial applications…
pH
• One of the most important parameters
– Measuring requires patience and diligence
• Necessary calibration using buffers
– For groundwater, a two-point calibration is usually
sufficient (pH 7 and 4 or 7 and 10)
• Glass electrodes – potentiometry
• The use of two separate electrodes (glass +
reference) is extremely impractical
• Combined electrode
• Contains both electrodes in one body
• Reference el. of a known potential to a
hydrogen electrode
• Glass measuring electrode
• The circuit is closed using a junction/diaphragm
(ceramic, Pt) – reference electrode / measured
solution
• The trend of miniaturization of the glass part,
plastic bodies, gel fillings
Measurement
pH meters
pH calibration – buffers
direct measurement of E – Zobell's solution
Combined pH
electrode
Combined electrode
pH-electrodes
Wear and tear
Compatibility
Calibration
• For the most accurate measurement
possible:
• Correct electrode storage (see
manuals)
• Replace on time – monitor the
quality of electrode calibration
• Rinse the electrode between
measurements
• Gentle movement of the
electrode in the solution
(prevents development of
potentials on the electrode)
• For some types of water (little
buffered, low TDS, low
alkalinity...) the measured value
can drift
• For water degassing CO2 or with
low redox (red. Fe and S) it is
advisable to avoid contact with air
(e.g. using flow cells)
pH meters
Redox potential
• Electrode measures potential (electromotive
force)
• No calibration
– Checking the correctness of the measurement
against a solution with a fixed Eh (e.g. Zobell's
solution)
Redox potential
• When measuring:
– Gentle movement of the electrode in the solution
(prevents the development of static voltage on the
electrode)
– Wait a few minutes for stabilization
– If the readings fall and then start to rise, there is
O2 contamination from the atmosphere – record
the lowest value reached
Redox potential
• Correcting the measured E to Eh :
– Add the standard electrode potential to the
measured value
– Standard potentials of argent-chloride electrodes:
Redox potential
• Determination problems
– Volatile values an long settling
– Clean the electrode (soapy water and a soft brush)
– Soaking in the preservation solution
– Measurement in a sealed container (constant
solution concentration)
Ion Selective Electrodes (ISE)
• Membrane potentials – solid/liquid
membrane
• Direct quantitative determination
according to the Nernst equation –
calibration line
• For anions: F−, Cl−, NO3
−, SO4
2−,
PO4
3−…
• Metals: Ca, Ag, Pb, Cd, Na, K...
• Alkali metals – membranes contain
molecular cavities with the exact
dimensions of the given element
• Degradation of electrodes
(relatively limited lifetime) – more
intensive than for pH and ORP
• Typically for frequent
measurements of e.g. nitrates or
sodium (measurement of water
quality, spread of pollution,
passage of contamination cloud,
etc.)
Examples of measuring range:
0.1 to 14000 mg/L NO3
-
0.002 to 23000 mg/L Na+
1.8 to 35500 mg/L Cl-
0.02 to 40100 mg/L Ca2+
Multimeters
WTW marketers idea
of using a multimeter
¯\_(ツ)_/¯
Measurement of conductivity, temperature, pH, redox , oxygen, ISE
Logger functions (data recording), direct connection with a PC
Generally applies to all devices – the more functions, the greater the
demand on the battery
Simplicity could lead to pure perfection
Less common properties
• Radioactivity of water
– Dissolved gases (Rn)
– Activity [Bq]
• Amount of dissolved oxygen
– Oxidation-reduction properties
– Biota
– Electrochemically (electrodes)
CHEMICAL ANALYSES
Filtration
• Water contains unsuitable particles (suspension)
– Chemical interactions may occur
– Filtration directly in the field
– Syringe attachment (various sizes)
– Most commonly 0.45 µm
• Particles, clays, part of oxohydroxides Fe and Mn
• Bacteria
• Does not capture viruses and most organics (e.g. fulvic and
humic acids)
– For colloids up to 0.1 µm
– Suction can cause degassing - forcing is better
Major anions
• Laboratory measurements
• Gravimetric (sulphates), titration
(nitrates), liquid
chromatography, possibly also
MS (sulfur)
• Carbonates by titration
(determination of alkalinity)
• Direct field measurement
– Briefcase sets
– ISE
– Portable photometers
(Cl 0.01-10 mg/L, ammonia 0.01-
50 mg/L, NO3
− 0.2-133 mg/L)
– Lower accuracy – suitable for
orientation measurements, rapid
development monitoring or
routine measurements
Samples for anion analysis
• Usually 5 mL is enough for one analysis, better
around 50 mL (repetition)
• PE or PP bottles, rinse with sampled water,
filter
• Store in a cool and dark place – microbial
activity (oxidation-reduction of sulphur),
precipitation
Sulfides and sulfates
• H2S and HS− very soluble
• They are formed under reducing conditions
from sulfates
• They oxidize quickly – even O2 diffusing
through the plastic of the bottle
– Necessary stabilization during sampling –
precipitation as Zn and Cd sulfides
• Distortion of sulfate content – it is necessary
to separate also for the determination of SO4
2−
Major cations + trace elements
• Field determination via ISE (Na+, K+, Ca2+, Mg2+)
• Today usually ICP-AES, ICP-MS, AES or AAS
• Spectrophotometry (complexing cations)
• High precision, small samples (limit ca. 25 mL)
• Samples can be acidified – preventing
precipitation of carbonates and oxides (typically
small amounts of HNO3)
– Keeps metals in solution
• The acidified filtered sample is stored in a PP or
PE bottle
Sampling for isotopic analyses
• δ18O and δ2H
– Not subject to significant chemical and biochemical processes –
can withstand long-term storage
– High content in water – resistence to dilution and interaction
(isotopic exchange)
– A large air space above the water
• Tightly closed bottle with minimal bubbles – depending on sample
volume
– No filtration, tightly sealed (due to evaporation), refrigeration
recommended
– Various methods (LRS; IRMS)
• Tritium
– Depending on the method, it may not require special sampling
(LSC scintillation methods)
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia
chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Resources
• Most pictures are public domain or taken from
manufacturer‘s catalogues