How To Measure Calcium In Water

Calcium ions and magnesium ions are common inorganic salt in water, which are also the main components of water hardness. For example, the white precipitate produced when water is boiling is caused by water’s calcium and magnesium ions. Therefore, we should measure the calcium ion in water for many industries, such as water in the boiler and textile dyeing industries.

There are four common methods to determine the content of calcium and magnesium in water:

• EDTA complexometric titration
• Potassium permanganate titration
• UV / Vis spectrophotometry
• Atomic absorption spectrometry

Although EDTA complexometric titration and UV or Vis spectrophotometry are simple operations and low cost, it is difficult to eliminate the interference of other ions in water, such as iron and manganese ions, which affects the accuracy of the final test results. Atomic absorption spectrophotometry is widely used to determine calcium and magnesium ions in water due to simple operation, high sensitivity, etc.

Determination of total calcium and magnesium in water by  EDTA titration method

Range

It is suitable to determine the total amount of calcium and magnesium in groundwater and surface water. It is unsuitable for water with high salt content, such as seawater. The minimum concentration determined by this method is 0.05mmol/l.

Principle

Under the condition of PH10, calcium and magnesium ions are complexometric titrated with EDTA solution. Chromium Black T is used as an indicator to produce a purplish-red or purple solution with calcium and magnesium.

In a titration, free calcium and magnesium ions first react with EDTA, and calcium and magnesium ions complexed with indicator then react with EDTA. When reaching the endpoint, the color of the solution changes from purple to sky blue.

Reagent

Analytical pure reagents and distilled water, or water with equivalent purity

Buffer solution (pH = 10).

Weigh 1.25g of disodium magnesium EDTA (c10h12n2o8na2mg) and 16.9g of ammonium chloride (NH4Cl) and dissolve them in 143ml of concentrated ammonia (NH3 • H2O) and dilute them to 250ml with water.

If there is no EDTA disodium magnesium, we can take the first 16.9g of ammonium chloride and dissolve it in 143ml ammonia water. In addition, dissolve 0.78g magnesium sulfate (MgSO4 • 7H2O) and 1.179g EDTA disodium dihydrate (c10h12n2o8na2 • 2H2O) in 50ml water, add 2ml prepared ammonium chloride, ammonia solution, and about 0.2g Chromium Black T indicator dry powder.

At this time, the solution should be purplish red. In the case of sky blue, add a tiny amount of magnesium sulfate to make it purplish red. Add disodium EDTA solution drop by drop until the solution changes from purplish red to sky blue.

Combine the two solutions and add distilled water to make it constant to 250ml. If the solution turns purple after the combination, the reagent blank shall be subtracted from the calculation result.

Disodium EDTA standard solution: ≈ 10mmol / L.

Preparation

Dry a part of EDTA disodium dihydrate at 80 ℃ for two hours, put it into a dryer to cool to room temperature, weigh 3.725g and dissolve it in water, put it in a volumetric flask to 1000ml, put it in a polyethylene bottle, and check its concentration regularly.

Calibration

Use standard calcium solution to calibrate EDTA disodium solution. Dilute 20.0ml of standard calcium solution to 50ml.

Calculation

C1 ＝c2V2/V1

C2 — concentration of standard calcium solution, mmol / L;

V2 — the volume of standard calcium solution, ml;

V1 — the volume of disodium EDTA solution consumed in calibration, ml.

Calcium standard solution: 10mmol / L.

Determination of calcium in tap water by Flame Atomic Absorption Spectrometry

Principle

In the case of a sharp line light source, the absorption of resonance line by ground-state atomic vapor conforms to Lambert Beer’s law

A=lg(Io/I)=KLNo

A: Absorbance

Lo: incident light intensity

I: transmitted light intensity

K: absorption coefficient

l: optical path

No: the density of ground-state atoms.

When the sample is atomized and the absolute temperature of the flame is lower than 3000K, it can be considered that the number of ground-state atoms in the atomic vapor is very close to the total number of atoms. Under fixed experimental conditions, the total number of atoms is directly proportional to the concentration C of the measured elements in the sample, So:

A=KC .

Sensitivity S = (c) × 0.004)/A

Instruments and reagents

1. Instruments:

Atomic absorption spectrophotometer, gas compressor, calcium hollow cathode lamp, and acetylene cylinder.Six pieces of 50 ml volumetric flasks; One 10 ml, 5 ml, and 2 ml pipettes, a beaker

2. Reagent: calcium standard solution (1000 μ g / ml).

Steps

1. Preparation of calcium standard solution

Dilute the standard calcium solution into a standard working solution of 100 μ g / ml calcium.

2. Preparation of calcium series standard solutions

Take five pieces of 50 ml volumetric flasks, orderly add 0.50, 1.00, 2.00, 3.00, and 4.00 ml standard working solution of 100 μ g / ml calcium, dilute to the scale with distilled water, and shake well.

3. Preparation of unknown sample solution

Take 10.00 ml tap water into a 50 ml volumetric flask, dilute it with high-purity water to the scale, and shake well.

4. Measurement of absorbance

According to the steps of flame atomic absorption spectrophotometer, turn on the computer’s power supply, atomic absorption spectrophotometer, and air compressor in sequence. Turn on the auxiliary gas switch and adjust the acetylene output pressure to 0.09mpa.

The working conditions of the regulating instrument :

acetylene flow 2.0 L / min, wavelength 422.7 nm, spectral passband 0.5 nm, lamp current 6 Ma.Ignite after air leakage inspection and measure the absorbance of the prepared solution.

Data processing

The working curve is drawn with the absorbance of the standard series of calcium solutions as the ordinate and the concentration as the abscissa, and the linear regression equation is obtained. By substituting the absorbance of the unknown sample solution into the regression equation, we can get the calcium content in the tap water sample to calculate the calcium content in the tap water.

Determination of calcium content in tap water by potassium permanganate Titration

The principle:

Using the reaction of some metal ions (such as alkaline earth metal, Pb 2 +, Cd 2 +) and C2O42 – to produce insoluble oxalate precipitation. We can determine the content indirectly by the potassium permanganate method. Firstly, all Ca2 + is precipitated into CaC2O4, dissolved in dilute H2SO4 after filtration and washing.

Reaction equation:

Ca2 + + C2O4 2 – = CaC2O4 ↓ CaC2O4 + H2SO4 = CaSO4 + h2c2o45h2c2o4 + 2mno42 – + 6h + = 2mn2 + + 10co2 ↑ + 8H2O

Under acidic conditions, the reaction of calibrating KMnO4 solution with Na2C2O4 :

2mno4 – + 5 C2O42 – + 16h + = 2mn2 + + 10co2 + 8H2O.

When titrating, use the purple-red of MnO4 – to indicate the endpoint.

Calculation:

acetylene flow 2.0 L / min, wavelength 422.7 nm, spectral passband 0.5 nm, lamp current 6 Ma.Ignite after air leakage inspection and measure the absorbance of the prepared solution.

Data processing

The working curve is drawn with the absorbance of the standard series of calcium solutions as the ordinate and the concentration as the abscissa, and get the linear regression equation. By substituting the absorbance of the unknown sample solution into the regression equation, we can get the content of calcium in the tap water sample ,so as to calculate the content of calcium in the tap water.

Determination of calcium content in tap water by potassium permanganate Titration

The principle:

Using the reaction of some metal ions (such as alkaline earth metal, Pb 2 +, Cd 2 +) and C2O42 – to produce insoluble oxalate precipitation. We can determine the content indirectly by potassium permanganate method. Firstly, all Ca2 + is precipitated into CaC2O4, which is dissolved in dilute H2SO4 after filtration and washing.

Reaction equation:

Ca2 + + C2O4 2 – = CaC2O4 ↓ CaC2O4 + H2SO4 = CaSO4 + h2c2o45h2c2o4 + 2mno42 – + 6h + = 2mn2 + + 10co2 ↑ + 8H2O

Under acidic conditions, the reaction of calibrating KMnO4 solution with Na2C2O4 :

2mno4 – + 5 C2O42 – + 16h + = 2mn2 + + 10co2 + 8H2O.

When titrating, use the purple red of MnO4 – to indicate the end point.

Calculation:

Reagent Instrument

Reagent: KMnO4 (s) analytical reagent; Na2C2O4 (s) analytical pure; H2SO4 solution (1 mol / L), oxalamine ((NH4) 2c2o4) 5 g / L,ammonia (10%),HCl (1:1), methyl orange (1 g / L), nitric acid (2 mol / L), silver nitrate (0.1 mol / L)

Instruments:

tray balance, analytical balance, beaker with 250ml, water bath pot, funnel, measuring cylinder with 10ml, 50ml, acid burette with 50ml, bottle washing, iron stand and glass rod.

Step:

1) Accurately transfer 200ml tap water, place them in 100ml beakers, add an appropriate amount of distilled water, cover the Petri dish, slowly add 2 ~ 5ml 6mol / LHCI solution, and heat to increase its dissolution.

2) Add 2 ~ 3 drops of methyl orange to the solution, and then add 7mol/l of ammonia to neutralize the solution from red to yellow. Add about 50 ml of 0.05mol/l (NH4) 2 C2O4 drop by drop during a hot water bath for 30 min.

3) Filter after cooling, wash the precipitate in the beaker several times, then transfer it into the funnel, and continue to wash the precipitate until there is no Cl – (take the washing solution in a small test tube and check with AgNO3 in HNO3 medium until there is no white precipitate).

4) Lay the filter paper with precipitate on the inner wall of the original beaker, wash the precipitate from the filter paper into the beaker with 50 ml of 1 mol • L – 1 H2SO4, wash it twice with a washing bottle, add distilled water to make the total volume of about 100 ml, heat it to 70 ~ 80 ℃, titrate with 0.02 mol / L KMnO4 standard solution until the solution is light red, and then stir the filter paper into the solution.

If the solution fades, continue the titration, The endpoint is that the light red does not disappear within 30 s. Calculate the calcium content and its relative average deviation.