⚗️ The sum of calcium and magnesium (total hardness) in water and waste water — complexometric titration with EDTA against eriochrome black T

Physicochemistry PN-C-04554-4

Overview

The method is a classic of water analysis: the EDTA first binds the free calcium and magnesium ions, and at the equivalence point it takes them from the complex with the indicator, releasing the indicator and changing the colour of the solution. ISO 6059:1984 describes it thus: "Complexometric titration of calcium and magnesium with an aqueous solution of the disodium salt of EDTA at a pH value of 10. Mordant black 11, which forms a claret or violet colour in the presence of calcium and magnesium ions, is used as the indicator". The results are given in units of molar concentration; if the content of calcium has been determined separately, the mass concentration of magnesium can be calculated.

Scope: ISO 6059 provides the method for groundwater, surface water and drinking water, expressly noting that it is not intended for waste water or for waters of high salinity (for example sea water); the lowest determinable concentration is 0.05 mmol/l. The Polish standard PN-C-04554-4:1999 extends the application to waste water, which can be seen in its very title. A handbook of water analysis gives for the national version the range „powyżej 5 mg/dm³ twardości wyrażanej jako CaCO₃" [above 5 mg/dm³ of hardness expressed as CaCO₃] and a procedure on 100 cm³ of sample with an ammonia buffer of pH 10 and eriochrome black T ground with sodium chloride.

In the same sample the two elements can be separated: the calcium is titrated against murexide at a pH close to 13 (the colour changes from red-pink to violet, after the addition of sodium hydroxide), and then, after decolourisation with hydrochloric acid and the addition of ammonia water — the magnesium against eriochrome black T at a pH of about 10. This route corresponds to PN-ISO 6058 (calcium) and makes it possible to calculate the magnesium from the difference.

Practical limitations: samples with a large amount of suspended matter are filtered through a 0.45 µm filter as soon as possible after sampling, reckoning with the fact that part of the calcium and magnesium remains on the filter. Samples preserved with acid have to be neutralised with a calculated amount of sodium hydroxide 2 mol/l and the dilution taken into account in the calculations. Portions with a sum of calcium and magnesium above 3.6 mmol/l are diluted and the dilution factor is recorded. The titration is carried out quickly at the beginning and slowly towards the end, and the end point is the disappearance of the last red tinge — a further drop of EDTA no longer changes the colour.

Method principle

To 50.0 ml of the solution tested, 4 ml of buffer solution are added (67.5 g of ammonium chloride dissolved in 570 ml of 25 % ammonia with the addition of 5.0 g of the disodium-magnesium salt of EDTA, made up to 1000 ml; 10 ml of this solution diluted to 100 ml must give a pH of 10 ± 0.1, otherwise the solution is discarded) and 3 drops of the indicator — eriochrome black T (mordant black 11). The solution takes on a red-violet colour at pH 10.0 ± 0.1. It is titrated immediately with a standardised solution of EDTA c(Na₂EDTA) = 10 mmol/l (3.725 g of the disodium salt of EDTA dihydrate dried for 2 h at 80 °C per 1 litre) with continuous stirring — quickly at the beginning, slowly towards the end — until the last red tinge disappears and the blue colour is stable. The EDTA solution is standardised against a standard solution of calcium carbonate c(CaCO₃) = 10 mmol/l (1.001 g of calcium carbonate dried for 2 h at 150 °C, dissolved with drops of hydrochloric acid 4 mol/l, boiled to remove the carbon dioxide and made up to 1000 ml; 1 ml corresponds to 0.4008 mg of calcium). In the national version 100 cm³ of sample are used, 1 cm³ of ammonia buffer and a pinch of eriochrome black T ground in a mortar with sodium chloride, and the result is converted into German degrees or mg CaCO₃/l.

Applications

• Drinking water and tap water — total hardness as a technological and consumer parameter
• Boiler, cooling and process waters — control of softening and of the risk of scale
• Waste water (the national version PN-C-04554-4) — the balance of calcium and magnesium, control of precipitation
• Calculation of magnesium from the difference where the calcium has been determined separately against murexide (PN-ISO 6058)
• Laboratories without instrumental apparatus — only a burette, a conical flask and the reagents are needed

Key parameters

Standard

Standard number

PN-C-04554-4:1999

Title (PL)

Woda i ścieki — Badania twardości — Oznaczanie sumarycznej zawartości wapnia i magnezu w ściekach metodą miareczkową z EDTA oraz obliczanie zawartości magnezu w wodzie i ściekach

Title (EN)

Water quality — Determination of the sum of calcium and magnesium — EDTA titrimetric method

Step-by-step procedure

1. Preparation of the test portion

   The samples do not require pretreatment. With a high content of suspended matter filter them through a 0.45 µm filter as soon as possible after sampling (note: the filtration may remove part of the calcium and magnesium). Dilute portions with a sum of Ca+Mg above 3.6 mmol/l and record the dilution factor F. Neutralise samples preserved with acid with a calculated amount of NaOH 2 mol/l and take the dilution into account in the calculations.

2. Standardisation of the titrant

   Standardise the EDTA solution against the standard solution of calcium carbonate: dilute 20.0 ml of the standard to 50 ml and titrate by the same procedure as the sample. Calculate the concentration of the EDTA in mmol/l.

3. Measuring out and buffering

   Transfer with a pipette 50.0 ml of the solution tested into a 250 ml conical flask, add 4 ml of the buffer solution and 3 drops of eriochrome black T. The colour should be red-violet and the pH 10.0 ± 0.1. (The national version: 100 cm³ of sample, 1 cm³ of buffer and a pinch of the indicator ground with NaCl.)

4. Titration

   Titrate immediately with the EDTA solution from the burette with continuous stirring — quickly at the beginning, slowly towards the end. The end point: the disappearance of the last red tinge; a further drop of EDTA no longer changes the colour.

   ⏱ Time: 2–4 min

5. Determination of calcium separately (optional)

   To 100 cm³ of a fresh portion add murexide and 2 cm³ of NaOH 2.5 mol/l (pH about 13), titrate with EDTA until the colour changes from red-pink to violet. Then decolourise the same sample with 7 cm³ of HCl (1+1), add 7 cm³ of 25 % ammonia water and eriochrome black T, and complete the titration of the magnesium to a blue colour.

6. Calculation

   Calculate the sum of the concentrations of calcium and magnesium in mmol/l from the volume of EDTA used, its titre and the volume of the test portion, taking the dilution factor into account. Where necessary convert into mg CaCO₃/l or German degrees, and where the calcium is known — calculate the mass concentration of magnesium.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• 25 % ammonia and the buffer solution — work under a fume hood, protect the eyes and the respiratory tract; the vapours are highly irritant.
• Hydrochloric acid 4 mol/l and sodium hydroxide 2.5 mol/l — corrosive substances, eye and hand protection is required.
• The buffer solution has a limited stability — check the pH before every series, discard it when out of date (a false end point).
• The indicators (eriochrome black T, murexide) stain the skin and the glassware; wash the glassware immediately after the titration.
• Collect waste containing EDTA and ammonia separately — do not pour it into the drains without neutralisation.

Frequently asked questions