Lecture 5: Cations

Ammonia- Calcium- Calcium and Magnesium (Hardness)- Sodium

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Environmental Engineering DepartmentEnvironmental Measurements (EENV 4244)

1. Ammonia

Introduction:

Direct determination of ammonia can be carried out for drinking water, clean surface water and good quality nitrified wastewater effluent.

In other instances, and where interference and present and greater precision is necessary, a preliminary distillation step is required.

Apply the direct nesslerization method to domestic wastewater only when errors of 1-2 mg/l are acceptable.

Use this method only after it has been proven that it yields results comparable to those obtained after distillation.

The Nessler method is sensitive to 20 µg NH3-N /L. under optimum conditions and may be used for up to 5 mg NH3-N/L.

Turbidity, color and substances precipitated by OH-, such as Ca and Mg interfere and may be removed by preliminary distillation or, less satisfactorily, by precipitation with ZnSO4 and alkali.

Interferences:

Glycine, urea, glutamine acid, cyanates, acetamide, hydrazine and some amines can interfere with the method.

Residual chlorine should be removed by sample pretreatment, destroy residual chlorine immediately after sample collection to prevent its reaction with ammonia.

2. Calcium; EDTA titrimetric method

Principle

When EDTA is added to water containing both Calcium and Magnesium, it combines first with Calcium

Calcium can be determined directly with EDTA, when the pH is made sufficiently high that the Magnesium is largely precipitated as Mg(OH)2 and indicator is used that combines with Calcium only.

Interference

Orthophosphate precipitates Calcium at the pH of the test. Alkalinity is excess of 300 mg/l may cause an indistinct end point in hard water.

3. Calcium and Magnesium (Hardness), EDTA titrimetric method:

Principle

If Eriochrome Black T is added to a water sample containing Ca and Mg ions at a pH 10 ± 0.1, the solution become wine red. If EDTA is added, the Ca and Mg will complexed, and when all of the Ca and Mg has been complexed the solution turns from rose red to blue, making the end-point of the titration.

Mg ions must be present to yield satisfactory end-point. To insure this, a small amount of complexometrically neutral Mg-EDTA is added to the buffer; this automatically introduces sufficient Mg. A limit of 5 minutes is set for the duration of the titration to minimize the tendency toward CaCO3 precipitation.

Interference

Some metal ions interfere by causing fading or indistinct end-points or by consumption of EDTA. This interference can be reduced by the addition of certain inhibitors before titration. With heavy metal or polyphosphate concentrations below those indicated in the table, use inhibitor I or II.

When the higher concentration of heavy metals are present, determine Ca and Mg with the atomic adsorption technique.

Max. interference conc. mg/l

Interfering substance Inhibitor I Inhibitor II

Al 20 20

Cd 20

Co Over 20 0.3

Cu Over 30 20

Fe Over 30 5

Pb 20

Mn 1

Ni Over 20 0.3

Zn 200

Polyphosphate 10

Maximum concentrations of interferences permissible with various inhibitors values based on using 25 ml sample diluted to 50 ml.

Suspended or colloidal organic matter also may interfere with the end-point.

Eliminate this interference by evaporating the sample to dryness on a steam bath and heating at 550 0C until the organic matter is completely oxidized.

Normally 15- 20 minutes is long enough to oxidize 200 mg of solids.

Dissolve the residue in 20 ml 1 M HCl, neutralize to pH 7 with 1 M NaOH and make up to 50 ml with H2O .

Cool to room temperature and continue according to the procedure.

4. Sodium; flame emission photometry

Storage of samples

Do not store sample in soft-glass bottles because of the possibility of contamination from leaching of the Glass. Use acid washed polyethylene or borosilicate glass bottles. Adjust sample to pH˂ 2 with HNO3 this will dissolve potassium salts and reduce adsorption on vessel walls.

Principle

Trace amount of Na can be determined in a direct reading type of flame photometer at a wavelength of 589 nm and a slit width of 0.7 nm. Minimum detectable concentration is approximately 0.1 mg/l.