Science Background chemistry chemical analysis Sulfate content in water



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Determination of sulfate content (titration)

Science Background


CHEMISTRY

Chemical analysis


Sulfate content in water


The sulfate ion is of major significance. It is relatively abundant in nature, as it is the third most abundant ionic species in seawater, after only Na+ and Cl-. It is also widely used or produced in the chemical industry, either as a reactant or as a waste product.

Many methods for analysis of the sulfate content of various kinds of water are based on the formation of insoluble barium sulfate. By mixing a water sample of unknown sulfate concentration with a solution of barium chloride, a precipitate is formed:

Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

Some methods (gravimetric analysis, turbidity measurement) are based on measuring the total amount of precipitate that is formed when adding an excess of barium. By for example weighing (in gravimetric analysis) the amount of barium sulfate that is formed, the concentration of sulfate may be calculated. This method, however, is prone to errors (Garcia & Schultz, 2016)

As an alternative, a conductivity titration may be performed by slowly adding barium chloride solution to an unknown water sample. Conductivity will slowly decrease, as the formation of a precipitate reduces the amount of free ionic species. After all the sulfate as precipitated, the addition of more barium chloride solution will result in an increase in conductivity. This way, the titration curve will show a minimum which corresponds to the equivalence point.

Source: Garcia, J., & Schultz, L. D. (2016). Determination of Sulfate by Conductometric Titration: An Undergraduate Laboratory Experiment. Journal of Chemical Education, 93(5), 910-914. doi:10.1021/acs.jchemed.5b00941


Conductivity


The Conductivity sensor measures the ability to conduct electricity in water solutions. When salts and other inorganic chemicals dissolve in water, they break apart into electrically charged ions. Ions increase the water’s ability to conduct electrical current. Common ions in water that conduct electrical current include sodium, chloride, calcium, and magnesium. Organic compounds, such as sugars, oils, and alcohols, do not form ions.

The principle by which the sensor measures conductivity is simple - two graphite plates (cells) are placed in the sample, a potential is applied across the plates and the current is measured. The Conductivity sensor actually measures the conductance of the solution (the inverse of the resistivity R), which is determined from the voltage and current values according to Ohm's law (G = 1/R = I/V).

The specific cell constant (K) of the conductivity electrode is used to determine the conductivity (C). The conductivity is the cell conductance multiplied by the cell constant, C=G*K. The electrode separation distance divided by the electrode area determines the cell constant. The supplied electrode has a nominal cell constant K of 1.0 cm-1.

The SI unit of conductance is Siemens (S). Since S is a very large unit, conductance of aqueous samples is commonly measured in S, and conductivity in S/cm. Some typical conductivity ranges of hydrous solutions are:



Sample

Conductivity (μS/cm)

Pure water

0.055

Distilled water

0.5

Deionized water

0.1 – 10

Rain water

20 - 100

Drinking water

50 - 200

Tap water

100 - 1500

River water

250 - 800

Brackish water

1000 - 8000

KCI 0.01 M

1410

MgSO4

5810

KCI 0.1 M

12900

Ocean water

53000

H2S04

82600

KCI 1.0 M

112000

Titration


Titration is an analytical method in which a standard solution with the known concentration is used to determine the concentration of another solution.

During titration the standard solution (also known as titrant) is slowly added to the solution of unknown concentration by means of a burette. The endpoint of the reaction can be observed by the colour change when using an indicator (for example phenolphthalein) or detected by pH measurement. At the end point an amount of standard solution has been added that just completely reacts with the solution titrated.

The moles of standard solution can be calculated by multiplying the volume of standard solution used by its molarity.

nstandard solution = Vstandard solution * cstandard solution

The moles in the titrated solution of unknown concentration are then found using the coefficient in the chemical equation. Then, dividing the moles of the titrated solution by the volume of that solution gives the concentration of the titrated solution.

ctitrated solution = ntitrated solution / Vtitrated solution.


Step motor burette


The step motor burette (titrator) is an actuator with which a liquid can be added to a solution automatically and relatively accurately. That way, a certain volume of a liquid can be added at constant speed, allowing for the creation of a graph in which you can plot pH versus volume added. This might also be achieved by using a drop counter and photo gate, but the step motor burette is much more convenient.

More information about the function of the step motor burette, connecting it to an interface or tips about possible experiments can be found in the actuator’s manual.








Determination of sulfate content (titration) – Science background


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