Water Analysis in field and lab (chromatography, AAS, IC, photometry)

Table of Contents

1 Introduction

2 Field work at Darmbach (May 11)
2.1 Multimeter measurements
2.2 Titration for alkalinity
2.3 Results from field measurements

3 Chromatography & salt experiment (May 18)
3.1 Chromatography
3.2 Salt experiment

4 Lab analysis with AAS, IC and Photometry (May 22)
4.1 Atomic absorption spectroscopy - AAS
4.2 Ion Chromatography - IC
4.3 Photometry

5 Synopsis of measurements
5.1 Schoeller diagram & ion balance
5.2 Stiff diagrams
5.3 Piper diagram
5.4 Discussion of Darmbach water properties

6 References

1 Introduction

The Water Analysis exercises (TuCaN 3214) are part of Special Modul SM9 “Hydrogeological Methods” of the MSc TropHEE and scheduled for the 1nd semester but had to be adjourned to the 2nd semester due to capacity bottlenecks in the lab. This course prepares for the Hydro­geological Field Course (TuCaN 3417) scheduled for the 2st semester. The Water Analysis course contains lectures and a practical part with surface water sampling, measuring water temperature, EC, pH, oxygen concentration and alkalinity in the field as well ion concentrat­ions in the lab with AAS, IC and Photometry. A salt concentration experiment (EC = f(salt con­centration) ) with 2 different salts and an introduction to chromatography was also part of the exercises.

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TropHEE Module Handbook names as goals of this course: “The students are enabled to apply basic field techniques to characterize groundwater levels, groundwater flow fields, and to characterize aquifers in term of hydraulic properties. Students acquire methodical skills to use standard laboratory equipment to analyze water samples and to evaluate the results. Through the hands-on field and laboratory work they gain soft skills such as organizational skills, team working skills, communication skills, and data presentation skills.”

The 25 TropHEE students were assigned to one of several groups that worked at different times on the different topics.

For our group 1 the work was divided into the following 3 days:

Monday, May 11 we walked along the stream Darmbach with a water sampling case with multimeter, several electrical probes, plastic bottles, titrator, At six stations we took water samples: 2 plastic bottles per station/location - one for anion-analysis and one for cation­analysis (with 1 cubic cm of acid HCI to stabilize the sample against degradation/precipitation before being analyzed in the lab) and measured water parameters like water temperature, EC, pH and oxygen concentration. Alkalinity (HC03+, C032+ ) was measured by titration with 1.6 normal sulfuric acid (H2S04) until the related indicator changes color (at pH 4.3)

Monday, May 18 we got an introduction to chromatography by Claudia Cosma and conducted an experiment dissolving increasing amounts of 2 salts and measuring the electric conduct¡- vity EC of the two solutions supervised by Dr. Marandi.

Friday, May 22 we analyzed our water samples from Darmbach with lab equipment like AAS, IC and Photometry supervised by Zahra Neumann. The samples from May 11 were stored in a fridge until opening the bottles in the lab on May 22.

2 Field work at Darmbach (May 11)

Water samples were taken alongside the stream Darmbach and water parameters like tempe­rature, EC, pH, oxygen concentration, and alkalinity were measured in field. The samples from 6 stations were stored cool and dark until later analysis in the lab. We walked from the spring of Darmbach south east of Darmstadt (spring = station 1 = eastern end) into the city of Darmstadt west of Lake Woog (post Woog = station 6 = western end):

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Fig. 3: Six sampling locations (western end = 6 = Post-Woog .. eastern end = 1 = spring) alongside stream Darmbach (produced w. ESRİ ArcGIS).

We took two 100 ml plastic bottles per event - one for anions (F, Cl, N02, Br, N03, P04, S04 ) and one for cations (Mn, Fe, Li, Na, K, Mg, Ca, Sr, Si,...) stabilized with 1-2 ml of acid e.g. 2- normal FICI (= 2-molar = 2 mol HCI/L = 73g FHCI/L) to lower pFH below 2 and to prevent degradation (precipitation of metals).

We got a WTM measurement case and instructions for calibrating multimeters with probes by the lab for inorganic chemistry:

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Fig. 4: Water analysis case for field measurements; picture from [Schiedek 2014].

The bottles are flushed several times with the water to be analyzed before the sample is taken. The bottles must be properly closed, marked/labeled so that they are clearly identified in the lab (ID, A_nion, C_ations, date, location, person, ...) and kept cool and dark before opened in the lab. Data related to the samples must also be documented in a field book.

2.1 Multimeter measurements

Fast changing parameters must be measured immediately in the field: temperature in °c, pFH (negative decimal logarithm of FI+ concentration), EC (electric conductivity in pS/cm), oxygen concentration in mg/L (and alkalinity due to escaping C02).

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Ideally several probes sticking in a Flow-Through-Cell (where pumped water flows through) measuring most of the field parameters at the same time and place. Different probes can be connected to the same multimeter. Unfortunately we had no pump and no Flow-Through-Cell. So we had to do the measurements one after the other.

The multimeter converts automatically phi-measurements into pH-values at 25°c by computa­tion. The pH-probe has a very thin glass electrode that must always be kept wet. It is stored in a 3-molar potassium chloride (KCI) buffer solution of pH=7 and must be calibrated for two pH- values with the expected measurements in the middle. Some devices give oxygen concentra­tions not only in mg/L but also relate the measurement to the saturation value at the current temperature and give the oxygen (saturation index) also as %. We got the following results:

2.2 Titration for alkalinity

Alkalinity is the capacity of an aqueous solution (of our water sample) to neutralize an acid. Alkalinity depends mainly on carbonates, i.e. on the concentration of bicarbonate (HC03') and C03 2־ ions. Their concentration depends on the carbon dioxide (C02) concentration. Since C02 evaporates fast, also the alkalinity must be measured fast. This is done by titration with 1.6 normal sulfuric acid (1.6 equ./L = 0.8 mol/L of H2S04 ): 100 ml of fresh water are taken into a glass bulb/flask. A Bromide-Cresol-Green-Methyl-pH-Indicator is added to detect the color change at pH 4.3. The sampled water is a little bit basic with a pH>7. Adding acid lowers the pH. The amount of acid added until color changes from green to pink is proportional to alka­linity. Always mixing all ingredients by moving the flask around we added slowly the acid by turning the wheel of a micro titrator (Hach Digital-Titrator) releasing small drops of acid.

The titrator counts the drops of acid. In our case (100 ml water, 1.6-normal acid, certain drop size) the number of drops is equal to the alkalinity in mg СаСОз per liter. We had to assemble titrator, acid cartridge and delivery tube, to remove air and to set the counter to zero before first use:

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Fig. 8: Titrator with acid cartridge and delivery tube; picture from manufacturer.

2.3 Results from field measurements

Multimeter measurements and titration of the surface water at six locations at Darmbach deli­vered the following results:

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Fig. 9: Field measurements from six stations at Darmbach on May 11 (6 = Post-Woog, 1 = spring).

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