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Method for determining the concentration of BARIUM (Ba++)
in Aqueous Solutions

Electrode Specifications      Links to publications on the WWW     Go to Ion Selective Electrodes page

NOTE: The barium electrode is interfered with by several ions which are constituents of the most commonly used ISABs so there is no recommended ISAB for this ion and the best results will be obtained using the Standard Addition method (or Sample Addition for samples with >100 ppm Ba). Nevertheless, samples with low ionic stength which do not require ISAB can be analysed satisfactorily by the more common Direct Potentiometry method as described in General Operating Instructions.

Furthermore, best results are obtained by measuring in still (not stirred) solutions using the following procedure:

1) After removing the electrodes from a previous solution, rinse with a jet of deionised water and dry gently with a paper towel.

2) After immersion, manually swirl beaker for about 30 seconds - or until mV are stable ± 1mV.

3) Leave to stand still and watch the mV reading fall until stable or until it starts to rise again - this normally takes less than 2 mins for 10 ppm and above - upto 5 mins for less than 1 ppm.

4) Take mV reading at lowest point


Apparatus Required:

Ion-Selective Electrode for Barium ion (ELIT 8081 PVC membrane)

Reference electrode: single junction silver chloride (ELIT 001n)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm Ba as BaCl2

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.

Calibration

Before use, the electrodes must be Pre-conditioned by soaking in 1000 ppm Ba for at least 5 mins if the electrodes are in regular use or preferably overnight or even longer on first use or after a prolonged rest. If the sample concentration is completely unknown then an initial calibration must be made by measuring say 1, 10, 100, 1000 ppm standards and finding the slope in the conventional manner. This can then be used to measure a few representative samples by Direct Potentiometry. Once the concentration range expected for the samples is known then a more precise calibration can be made under the current operating conditions and in the appropriate range by making up a standard with approximately the same concentration as the samples and analysing it by the Standard Addition method, then adjusting the slope and recalculating using the ELIT software until the calculated concentration is correct. If samples have a wide range of concentrations, over several orders of magnitude, then it may be necessary to use different standards for each decade of concentration to ensure the most accurate slope determination.

Standard Selection

If you have the ELIT ion analyser software then simply enter the electrode slope and expected sample concentration and volume into the Standard Addition (or Sample Addition) software to calculate an appropriate standard concentration and volume. Alternatively, follow the instructions in the Basic Principle section of the Standard Addition page of the nico2000.net website.
Make up the calculated standard concentration by serial dilution of the 1000ppm standard.

Sample Measurement

1) Pipette the pre-determined sample volume (usually 50 or 100ml) into a beaker, immerse the electrodes, manually swirl the solution then leave to stand still and watch as the voltage falls to a stable value or starts to rise again. Record the stable voltage or the lowest value before it starts to rise again.

2) Add the calculated volume of standard and swirl or stir well until a roughly stable reading indicates that two solutions have completely mixed. Then wait with still solution (switch off stirrer, if used) for a new stable voltage or reversal of drift (when the electrodes have re-equilibrated) and record the second reading.

The software will then calculate the sample concentration and give an estimate of the quality of the determination - if the sample concentration is very different from the expected value then you will be prompted to make a second measurement using a more appropriate volume or concentration of standard.

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Technical Specifications
for the Barium Ion-Selective Electrode (ELIT 8081)

Click here to download a printer-friendly (pdf) Specification Sheet.

Introduction
The Barium Ion-Selective Electrode has a solid-state PVC polymer matrix membrane. The electrode is designed for the detection of barium ions (Ba+2 ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Barium Ion is a divalent cation.
One mole of ( Ba+2 ) is 137.327 grams; 1000 ppm is 0.007 M
Dissolve 1.779g Barium Chloride di-hydrate (BaCl2.2H2O) in 1 litre deionised water.

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25°C = < 2.5 MOhm

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000 ppm Ba+2 as BaCl2
(But see General Operating Instructions)
Preconditioning time : at least 5 minutes
Optimum pH range : pH 3 to pH 10
Temperature range : 0 to 50°C
Recommended ISAB : NONE - For samples with high Ionic Strength, use Standard Addition method or Sample Addition for samples with more than 100 ppm Ba.
Recommended reference electrode : Single junction AgCl (ELIT 001)
Electrode slope at 25°C : 21±3 mV/decade
Concentration range : 0.5 to 13,700 ppm (4x10-6 to 1x10-1 Molar)
Response time : < 10 seconds
Defined as time to complete 90% of the change in potential after immersion in the new solution.
Time for stable reading after immersion : < 1 to > 5 minutes
Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.
Potential drift (in 1000 ppm) : < 3 mV/day (8 hours)
Measured at constant temperature and with ISE and Reference Electrode continually immersed.

Analytical Note: Best measured in still (un-stirred) solutions.

Interference:
The following ions cause interference to the Barium measurement (selectivity coefficients (SC) in brackets): Strontium (0.09), Potassium (0.02), Sodium (0.02), Magnesium (0.006), Ammonium (0.003), Calcium (0.002), Lithium (0.002).

The SC is the approximate apparent increase in the measured concentration caused by 1 unit of the interferent.  Thus the likely effect of any interfering ion (% increase) can be calculated as follows: 

       ((expected concentration) x (SC) / (expected Ba concentration)) x 100.

Strontium has the highest interference but is unlikely to be present in significant concentrations in most samples. Any Potassium or Sodium ions present will cause a significant positive error if they have concentrations of greater than ten times that of the Barium.  Magnesium can be tolerated up to about twenty times the Barium, and Calcium & Lithium up to about fifty times.

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