Water Analysis: Automated Multi-Parameter Determination

Water quality measurement is a critical yet time-consuming process, often involving the use of a wide range of quantitative analytical methods. Types of water quality analysis can vary, ranging from basic field testing for a single analyte up to comprehensive laboratory-based multi-component instrumental analysis.

This article outlines the fully automatized potentiometric determination of six key parameters of water: conductivity, pH, total alkalinity, total hardness, chloride and fluoride content.

Sample Preparation and Procedures

Solution Preparation

1. 0.01 mol/L EDTA:

A total of 37.224 g EDTA was accurately weighed into a 1000 mL volumetric flask. This was dissolved in approximately 500 mL of deionized water before the volume was made up with additional deionized water.

2. 0.01 mol/L AgNO3:          

A total of 16.987 g AgNO3 was accurately weighed into a 1000 mL volumetric flask, dissolve in approximately 200 mL of deionized water before the volume was made up with additional deionized water.

3. 0.01 mol/L ½ H2SO4:

A total of 0.544 mL of 98% H2SO4 was added to a 1000 mL volumetric flask which contained 500 mL of deionized water. This resulted in heat generation, so the sample was allowed to cool down to room temperature before being diluted up to 1000 mL with deionized water.

4. Ammonia Buffer (pH 10):

A total of 111 ml of 25% ammonia was added to a 500 mL volumetric flask which contained 100 mL of deionized water. This volume was made up with water.

5. 0.1% Eriochrome Black T (EBT):

A total of 0.1 g of EBT was dissolved and diluted up to 100 mL in ethanol.

Water Analysis: Automated Multi-Parameter Determination

Image Credit: Mettler Toledo - Titration

6. 0.01 mol/L Zinc Sulphate (ZnSO4):

A total of 20 mL of 0.1 mol/L of ZnSO4 solution (Merck- 1.08879.1000) was diluted to 200 mL using deionized water.

7. 0.01 mol/L Sodium Chloride (NaCl):

A total of 0.5844 g of NaCl was accurately weighed and transferred into a 1000 mL volumetric flask. This was dissolved in 50 mL of deionized water before the volume was made up with additional deionized water.

Titer Determination

0.01 mol/L EDTA2

0.01 mol/L AgNO31

0.01 mol/L ½ H2SO43

Tap Water Analysis

Tap water analysis was performed using the following steps. It was necessary to use two beakers per sample. For each sample, one beaker was prepared with 60 mL of sample before being placed on the InMotion Rack.

A further 50 mL of water sample was prepared and the beakers placed on the InMotion Rack after the series, which includes the 60 mL sample.

The method was commenced, indicating the positions of both the first beaker with 60 mL sample and the first beaker with 50 mL sample. Sensors were rinsed and conditioned prior to each measurement.

Chemistry

Alkalinity

  • H2SO4+2NaOH ➝ Na2SO4 +2H2O
  • H2SO4+2CO32- ➝ 2HCO3- + SO42-
  • H2SO4+2HCO3- ➝ 2CO2 +2H2O + SO2-

Chloride

  • AgNO3 + NaCl ➝ AgCl + NaNO3

Total Hardness

  • Ca2+ + EDTA-H22- ➝ Ca-EDTA2- + 2H+
  • Mg2+ + EDTA-H22- ➝ Mg-EDTA2- + 2H+

Solutions

Titrants

c (EDTA) = 0.01 mol/L

c (AgNO3) = 0.01 mol/L

c ½ (H2SO4) = 0.01 mol/L

Chemicals

PerfectION TISAB III (51340064)

Sulfuric acid, 98%

Silver Nitrate

0.1% EBT in ethanol

EDTA

Compounds

Chloride, M = 35.55 g/mol, z = 1

Calcium carbonate, M = 100.09 g/mol, z = 1

Standards

0.01 mol/L ZnSO4 = 5.0 mL2

NaCl, primary standard (0.01 mol/L) = 5.0 mL1

Tris (hydroxymethyl) aminomethane TRIS = 0.05 g – 0.1 g3

Instruments and Accessories

  • LabX™ Software 2018
  • InMotion Flex Autosampler (30094120)
  • Spatula
  • 1 x conductivity board (51109840)
  • InLab 731Conductivity probe (51344020)
  • 3 x Burette DV1005 -5 mL (51107500)
  • 3 x SP280 peristaltic pump (30094237)
  • Pump/Stirrer option for InMotion (30094234)
  • 4 x Dosing unit (51109030)
  • 2 x Compact Stirrer (51109150)
  • Temperature Sensor (51300164)
  • Titration Excellence T9 (30252676)
  • perfectION Combination F- ISE (51344815)
  • DGi115-SC, pH combination sensor (51109502)
  • 1 x Burette DV1020 -20 mL (51107502)
  • Titration beakers PP 100 mL (101974)
  • DMi141-SC (51109530)
  • Dosing tube adapter 4 to 1(51108356)
  • DP5 Phototrode (51109300)
  • 1-SD660 membrane pump (30094165)
  • 2 x analog board (51109818)
  • XPE205 Analytical balance (30087653)

Results

Titer Determination

  1. H2SO4 = 0.977503
  2. EDTA = 0.990462
  3. AgNO3 = 0.987581

Multi-Parameter

Source: Mettler Toledo - Titration

  R1: Conductivity
(µS/cm)
R2: Fluoride
content (ppm)
R3: pH R4: Alkalinity
(mmol/L)
R6: Chloride
content (ppm)
R8: Total
hardness (mg/L)
1 118.060 3.7470 9.766 0.3608 19.2983 51.6404
2 119.380 3.6630 9.459 0.3572 19.2316 50.3333
3 119.270 3.6380 9.745 0.3567 19.9108 50.7516
4 119.300 3.5900 9.366 0.3631 18.9608 49.6106
5 119.760 3.6170 9.424 0.3620 19.9009 48.9440
6 119.620 3.5330 9.642 0.3612 19.2557 50.2492
Mean 119.232 3.6313 9.567 0.3602 19.4264 50.2548
s 0.605 0.072 0.173 0.003 0.390 0.928
srel 0.508% 1.986% 1.807% 0.727% 2.007% 1.847%

 

Remarks

The conductivity probe should be situated 1 cm below the perfectION F-. Conductivity is measured using the InMotion Tower’s ‘Conductivity’ position, meaning that only the conductivity probe could be dipped, therefore avoiding any contamination from the perfectION F- electrolyte.

Both the DGi115-SC and perfectION F- require calibration prior to analysis - segmented for ISE calibration and 3 point linear for pH.

Each electrode should be carefully cleaned following each analysis. Ensuring that sensors are rinsed and conditioned is vital to achieve precise, accurate results.

Before use, the output signal of the DP5 was adjusted at 1000 mV. This was accomplished by running a manual measurement on the sensor in deionized water before turning the small knob on the housing to stabilize it for 20 minutes.

Maintaining a low stirring speed helped avoid the formation of bubbles during titration. This is important because the presence of bubbles would disturb the photometric indication.

Measurement is performed using the following steps:

  1. Conductivity is measured in every sample in the series containing 60 mL of sample. This is done using the ‘Conductivity’ measuring position of the InMotion tower T/1B.
  2. The InMotion tower is lowered before 5 mL of TISAB is automatically added. Fluoride determination is done via direct measurement using the combined PerfectION F- ISE.
  3. Measurements of the second series are executed on the InMotion Tower T/1A using 50 mL of sample. pH is measured using the DGi115-SC electrode, while total alkalinity is determined by titration with ½ H2SO4 (0.01 mol/L).
  4. The same sample is then titrated using AgNO3 (0.01 mol/L) in order to determine the chloride content. This is done using the DMi141-SC electrode.
  5. 10 mL of buffer (pH 10) plus 1 mL of 0.1% EBT indicator is then added to the same sample before total hardness is determined using EDTA (0.01 mol/L) as titrant. A DP5 Phototrode is also used.

It is also possible to adapt the flowchart to accommodate specific customer needs; for example, the pumps employed in the current scheme may be replaced with a liquid handler, TV6 valve or a reversible pump (SPR 200).

A liquid handler may also be utilized to dispense a range of solvents in the event of any shortage of pumps or dosing units. It should be noted that the precipitate formed must be filtered and categorized as special waste. This solution must therefore be neutralized prior to final disposal.

References

  1. Application Note M006_2012
  2. Application Note M007_2012
  3. Application Note M011_2012
  4. Application Brochure no. 37, Water Analysis

 

 

This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Titration.

For more information on this source, please visit Mettler Toledo - Titration.

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