Goal of the work: determine by complexometric method:

– sample A– total water hardness, mmol eq/l;

– sample B– mass of calcium and magnesium in the sample, g.

The essence of the work. Ca 2+ and Mg 2+ ions form complexonates, which are stable in an alkaline environment, so they are titrated with a standard EDTA solution in the presence of an ammonia buffer. If you use the indicator eriochrome black T, then in the c.t.t. there is a transition in the color of the solution from lilac (complexes of the indicator with Ca 2+ and Mg 2+) to blue (the free form of the indicator under these conditions).

Hardness of water- This total indicator water quality. It is due to the presence of Ca 2+ and Mg 2+ ions. Total water hardness shows how many millimole equivalents of Ca 2+ and Mg 2+ are in total contained in 1 liter of water.

Me 2+ + H 2 Y 2– = MeY 2– + 2H + Þ f eq (Me 2+) = , f eq (H 2 Y 2–) =

Since the concentration of Ca 2+ and Mg 2+ in water is insignificant, large aliquots of the analyzed water (50.00 or 100.00 ml) are taken for titration using special large-capacity pipettes.

Separate determination of Ca 2+ And Mg 2+ when present together in solution based on titration of a sample with different indicators under different conditions.

First determine total calcium and magnesium content in the sample. For this purpose, an aliquot of the analyzed solution is titrated with complexone III with an indicator eriochrome black T in the environment ammonia buffer. In this case, the following reactions occur:

Ca 2+ + H 2 Y 2– = CaY 2– + 2H + ,

Mg 2+ + H 2 Y 2– = MgY 2– + 2H + Þ

Þ f eq (Ca 2+) = , f eq (Mg 2+) = , f eq (H 2 Y 2–) = .

Then determine the content calcium, titrating the same aliquot with an EDTA solution with an indicator murexide V highly alkaline environment. When adding alkali, magnesium ions camouflage due to precipitation in the form of Mg(OH) 2¯ and do not react with EDTA. Therefore, only calcium ions are titrated:

Ca 2+ + H 2 Y 2– = CaY 2– + 2H +

Under these conditions, the free form of the murexide indicator has a lilac-violet color, and its complex with calcium is brick-red. Content magnesium found in the sample by difference.

Equipment and reagents: burette, graduated cylinder (25 ml), standard EDTA solution, ammonia buffer with pH 9, eriochrome black T indicator mixed with NaCl (1: 100). To analyze sample A additionally : large capacity Mohr pipette (50.00 or 100.00 ml), large capacity conical flasks (250 ml). To analyze sample B additionally : volumetric flask, Mohr pipette, conical flasks, granulated NaOH, murexide mixed with NaCl (1: 100).

Completing of the work

Sample A. Determination of total water hardness. Obtain from laboratory technicians the analyzed solution in conical flask. Pipette an aliquot of 50.00 or 100.00 ml and transfer to another conical flask. Add 20–25 ml of ammonium buffer and an indicator on the tip of a spatula and titrate with a solution of complexone III until the color of the solution changes from lilac to blue.

Based on the titration results, the total water hardness is calculated (mmol-equiv/l):

Draw a conclusion about the characteristics of water using reference data (see &).

Sample B. Separate determination of calcium and magnesium. Receive the analyzed solution from the laboratory technicians into a volumetric flask, adjust to the mark and mix.

For determining total concentration of calcium and magnesium pipette an aliquot of the analyzed solution into a titration flask, add 20–25 ml of ammonia buffer and the eriochrome black T indicator on the tip of the spatula. Titrate with EDTA solution until the lilac color of the solution turns blue. The obtained similar volumes of EDTA are averaged and the average value of the titrant volume is obtained V 1 (Na 2 H 2 Y), which is spent on titrating Ca 2+ and Mg 2+ in total.

For calcium determination pipette the same aliquot of the analyzed solution, add 2-3 NaOH granules to it (to create a highly alkaline environment, check with universal indicator paper!), murexide on the tip of a spatula and titrated with EDTA solution until the brick-red color changes to lilac-violet. The obtained similar volumes of EDTA are averaged and the average value of the titrant volume is obtained V 2 (Na 2 H 2 Y), which is spent on the titration of Ca 2+. From the difference, the volume of titrant used for the titration of Mg 2+ is found:

V 3 (Na 2 H 2 Y) = V 1 (Na 2 H 2 Y) – V 2 (Na 2 H 2 Y).

Based on the obtained volume values V 2 (Na 2 H 2 Y) and V 3 (Na 2 H 2 Y), calculate the mass of calcium and magnesium in the sample (g).

STATE COMMITTEE OF THE RUSSIAN FEDERATION
ENVIRONMENTAL PROTECTION

MOSCOW 1997

(2004 edition)

1. INTRODUCTION

This document establishes a methodology for quantitative chemical analysis natural and purified samples Wastewater to determine the mass concentration of calcium in them in the range from 1.0 to 100 mg/dm 3 by the titrimetric method without diluting and concentrating the sample.

If the mass concentration of calcium in the analyzed sample exceeds the upper limit, it is allowed to dilute the sample with distilled water so that the calcium concentration corresponds to the regulated range.

The determination is hindered by turbidity, color, as well as metal ions: aluminum (> 10 mg/dm 3), iron (> 10 mg/dm 3), copper (> 0.05 mg/dm 3), cobalt and nickel (> 0. 1 mg/dm3), causing a vague color change at the equivalence point. Other cations (lead, cadmium, manganese (II), zinc, strontium, barium) can be partially titrated together with calcium and increase the consumption of Trilon B.

Under analytical conditions, magnesium precipitates in the form of hydroxide and does not interfere with the determination.

Elimination of interfering influences is carried out in accordance with clause 10.

2. PRINCIPLE OF THE METHOD

The titrimetric method for determining the mass concentration of calcium is based on its ability to form a slightly dissociated compound that is stable in an alkaline environment with Trilon B. The end point of the titration is determined by the color change of the indicator (murexide) from pink to red-violet. To increase the clarity of the color transition, it is preferable to use a mixed indicator (murexide + naphthol green B). At the same time, at the end point of titration, the color changes from dirty green to blue.

3. ATTRIBUTED CHARACTERISTICS OF MEASUREMENT ERROR AND ITS COMPONENTS

This technique ensures that analysis results are obtained with an error not exceeding the values ​​given in Table 1.

Measurement range, values ​​of indicators of accuracy, repeatability, reproducibility, correctness

4. MEASURING INSTRUMENTS, AUXILIARY DEVICES, REAGENTS AND MATERIALS

4.1. Measuring instruments

4.2. Assistive devices

Measuring instruments must be verified within the established time limits.

It is allowed to use other, including imported, measuring instruments and auxiliary devices with characteristics no worse than those given in paragraphs. 4.1 and 4.2.

4.3. Reagents and materials

All reagents used for analysis must be of analytical grade. or reagent grade

It is allowed to use reagents manufactured according to other regulatory and technical documentation, including imported ones, with a qualification not lower than analytical grade.

5. SAFETY REQUIREMENTS

5.1. When performing analyses, it is necessary to comply with safety requirements when working with chemical reagents in accordance with GOST 12.1.007.

5.2. Electrical safety when working with electrical installations is ensured in accordance with GOST 12.1.019.

5.3. Organization of occupational safety training for workers is carried out in accordance with GOST 12.0.004.

5.4. The laboratory premises must comply with fire safety requirements in accordance with GOST 12.1.004 and have fire extinguishing equipment in accordance with GOST 12.4.009.

6. REQUIREMENTS FOR OPERATOR QUALIFICATIONS

Measurements can be carried out by an analytical chemist who is proficient in the titrimetric method of analysis.

8. SAMPLE COLLECTION AND STORAGE

8.1. Sampling is carried out in accordance with the requirements of GOST R 51592-2000 “Water. General requirements to sampling."

8.2. Dishes intended for sampling and storing samples are washed with a 1:1 solution of hydrochloric acid and then with distilled water.

8.3. Water samples are collected in glass bottles. When filtering through any filter, the first portions of the filtrate are discarded.

The volume of the sample taken must be at least 300 cm3.

8.4. Samples are not preserved; they are stored at room temperature no more than 6 months.

If during storage a precipitate of calcium carbonate has formed in the sample, immediately before analysis it is dissolved by adding 0.5 - 1 cm 3 of concentrated hydrochloric acid, after pouring the transparent layer above the sediment into a clean, dry flask using a siphon. Then the poured solution and the liquid with the dissolved sediment are combined together and neutralized with a 20% sodium hydroxide solution, adding it drop by drop and monitoring the pH using indicator paper.

8.5. When taking samples, a accompanying document in an approved form, which indicates:

Purpose of analysis, suspected pollutants;

Place, time of selection;

Sample number;

Position, surname of the sample taker, date.

9. PREPARATION FOR MEASUREMENTS

9.1. Preparation of solutions and reagents

9.1.1. Trilon B solution with a concentration of 0.02 mol/dm 3 equivalents.

3.72 g of Trilon B are dissolved in 1 dm 3 of distilled water. The exact concentration of the solution is determined using a standard zinc chloride solution, as described in paragraph 9.2.

The solution is stored in a plastic container for no more than 6 months; its concentration is checked at least once a month.

9.1.2. A solution of zinc chloride with a concentration of 0.02 mol/dm 3 equivalents.

0.35 g of metallic zinc is moistened with a small amount of concentrated hydrochloric acid and immediately washed with distilled water. The zinc is dried in an oven at 105 °C for 1 hour, then cooled and weighed on a laboratory balance with an accuracy of 0.1 mg.

A sample of zinc is placed in a volumetric flask with a capacity of 500 cm 3, into which 10 - 15 cm 3 of distilled water and 1.5 cm 3 of concentrated hydrochloric acid are first added. The zinc is dissolved, after which the volume of the solution is adjusted to the mark on the flask with distilled water.

Calculate the molar concentration of the equivalent of a solution of zinc chloride C zn (1/2 ZnCl 2), mol / dm 3, using the formula:

where a is the weight of metallic zinc, g;

32,69 - molar mass equivalent Zn 2+, g/mol;

V is the volume of the volumetric flask, cm 3.

The zinc chloride solution is stored in a tightly closed glass or polyethylene container for no more than 2 months.

9.1.3. Buffer solution NH 4 Cl+ NH 4 OH.

7.0 g of ammonium chloride is dissolved in a 500 cm 3 volumetric flask in 100 cm 3 of distilled water and 75 cm 3 of concentrated ammonia solution is added. The volume of the solution is adjusted to the mark on the flask with distilled water and mixed thoroughly.

The buffer solution is stored in glass or polyethylene containers for no more than 2 months.

9.1.4. Indicator eriochrome black T.

0.5 g of eriochrome black T is thoroughly ground in a mortar with 50 g of sodium chloride. Used to determine the exact concentration of Trilon B solution.

9.1.5. Murexide indicator.

0.2 g of murexide and 0.5 g of naphthol green B (or 0.2 g of murexide alone) are thoroughly ground in a mortar with 100 g of sodium chloride.

The indicators are stable for 1 year when stored in a dark bottle.

9.1.6. Sodium hydroxide solution, 20%.

20 g of NaOH are dissolved in 80 cm 3 of distilled water.

9.1.7. Sodium hydroxide solution, 8%.

40 g of NaOH are dissolved in 460 cm 3 of distilled water.

9.1.8. Sodium hydroxide solution, 0.4%.

2 g of NaOH are dissolved in 500 cm 3 of distilled water. Sodium hydroxide solutions are stable when stored in tightly closed plastic containers for 2 months.

9.1.9. Sodium sulfide solution.

2 g of sodium sulfide are dissolved in 50 cm 3 of distilled water. Store in a tightly closed plastic container for no more than 7 days.

9.1.10. Sodium diethyl ditocarbamate solution.

5 g of sodium diethyldithiocarbamate are dissolved in 50 cm 3 of distilled water. Store for no more than 14 days.

9.1.11. Hydroxylamine hydrochloride solution.

5 g of hydroxylamine hydrochloride is dissolved in 100 cm 3 of distilled water. Store for no more than 2 months.

9.1.12. Hydrochloric acid solution, 1:3.

200 cm 3 of concentrated hydrochloric acid is mixed with 600 cm 3 of distilled water. Store in a tightly sealed container for no more than 1 year.

9.1.13. Activated carbon.

Preparation activated carbon carried out in accordance with Appendix A.

9.2. Establishing the exact concentration of Trilon B solution

Add 10 cm 3 of zinc chloride solution (clause 9.1.2) to a conical flask with a capacity of 250 cm 3, add distilled water to approximately 100 cm 3, 5 cm 3 of buffer solution and 10 - 15 mg of eriochrome black T indicator. The contents of the conical flask are carefully mix and titrate from a burette with a solution of Trilon B until the color changes from red to blue.

Titration is repeated 2 - 3 times and if there is no discrepancy in the volumes of the Trilon B solution by more than 0.05 cm 3, the average value is taken as the result.

The concentration of Trilon B solution is calculated using the formula:

where C tr is the concentration of Trilon B solution, mol/dm 3 equivalents;

C zn - concentration of zinc chloride solution, mol/dm 3 equivalents;

V tr - volume of Trilon B solution used for titration, cm 3;

V Zn is the volume of zinc chloride solution, cm 3.

10. ELIMINATION OF INTERFERING INFLUENCES

To eliminate the interfering effect of metal cations, masking reagents are added to the sample before titration: 0.5 cm 3 of a solution of sodium sulfide or diethyldithiocarbamate and 0.5 cm 3 of a solution of hydroxylamine hydrochloride.

The determination results may also be distorted in the presence of significant amounts of anions (HCO 3 -, CO 3 2-, PO 4 3-, SiO 3 2-). To reduce their influence, the sample should be titrated immediately after adding alkali.

The interfering influence of suspended and colloidal substances is eliminated by filtering the sample.

If the water sample is noticeably colored due to the presence of substances of natural or anthropogenic origin, it becomes difficult to fix the end point of the titration. In this case, before analysis, the sample should be passed at a speed of 4 - 6 cm 3 /min through a chromatographic column filled with activated carbon (layer height 12 - 15 cm). The first 25 - 30 cm 3 of the sample passing through the column is discarded.

As a rule, colored compounds of anthropogenic origin are sorbed by activated carbon almost completely, while natural compounds (humic substances) are only partially sorbed. If the color of the sample is high and cannot be eliminated due to humic substances, determining the titration end point is greatly facilitated by using a titrated sample of the same water (witness sample) for comparison.

11. TAKE MEASUREMENTS

11.1. Selection of titration conditions

The volume of water sample for calcium determination is selected based on the known value of total hardness or based on the results of evaluative titration.

If the hardness value is less than 4 mmol/dm 3, then 100 cm 3, 4 - 8 mmol/dm 3 - 50 cm 3 and more than 8 mmol/dm 3 - 25 cm 3 water samples are taken for analysis.

For evaluative titration, take 10 cm 3 of water, add 0.2 cm 3 of 8% sodium hydroxide solution, 10 - 15 mg of murexide indicator and titrate with Trilon B solution until the color with the mixed indicator changes from dirty green to blue, and when using only murexide - from pink to red-violet. Based on the volume of Trilon B solution consumed for titration, the corresponding volume of water sample is selected from Table 2.

Depending on the calcium content, the titration is carried out using a microburette or a burette. If, according to the results of the evaluation titration, the volume of Trilon B is less than 0.2 cm 3 or the hardness is less than 0.4 mmol/dm 3 equivalent, use a microburette, otherwise a burette.

11.2. Titration

The required sample volume is measured into a conical flask with a pipette, adjusted, if necessary, to 100 cm 3 with distilled water, 2 cm 3 of 8% sodium hydroxide solution, 0.1 - 0.2 g of murexide indicator are added and titrated with Trilon B solution until the color changes to with a mixed indicator from dirty green to blue, and when using only murexide - from pink to red-violet.

Repeat the titration and, if the discrepancy between parallel titrations does not exceed the values ​​given in Table 3, the average value of the volume of Trilon B is taken as the result. Otherwise, repeat the titration until an acceptable discrepancy in the results is obtained.

Table 3

Acceptable discrepancies between parallel titrations depending on the volume of Trilon B solution

12. PROCESSING OF MEASUREMENT RESULTS

12.1. The mass concentration of calcium in the analyzed water sample is found using the formulas:

or

where X or Xe is the mass concentration of calcium in water, mg/dm 3 or mmol/dm 3, respectively;

C tr - concentration of Trilon B solution, mol/dm 3 equivalents;

V tr - volume of Trilon B solution consumed for sample titration, cm 3;

V is the volume of water sample taken for determination, cm 3;

20.04 - molar mass of Ca 2+ equivalent, g/mol.

If the mass concentration of calcium in the analyzed sample exceeds the upper limit of the range (100 mg/dm 3), the sample is diluted so that the mass concentration of calcium is within the regulated range, and titration is performed in accordance with clause 11.2.

In this case, the mass concentration of calcium in the analyzed water sample X or Xe is found using the formula:

Where X v- mass concentration of calcium in a diluted water sample, mg/dm 3 or mmol/dm 3, respectively;

v is the volume of an aliquot of water sample taken for dilution, cm3;

Vv- volume of water sample after dilution, cm3.

12.2. For the result of the analysis X avg take the arithmetic mean of two parallel determinations X 1 and X 2:

for which the following condition is satisfied:

Where r- repeatability limit, the values ​​of which are given in Table 4.

If condition (1) is not met, methods can be used to verify the acceptability of the results of parallel determinations and establish the final result in accordance with section 5 of GOST R ISO 5725-6.

Table 4

Repeatability limit values ​​at probability P = 0.95

The discrepancy between the analytical results obtained in two laboratories should not exceed the reproducibility limit. If this condition is met, both analysis results are acceptable, and their arithmetic mean can be used as the final value. The reproducibility limit values ​​are given in Table 5.

Table 5

Reproducibility limit values ​​at probability P = 0.95

If the reproducibility limit is exceeded, methods for assessing the acceptability of analysis results can be used in accordance with section 5 of GOST R ISO 5725-6.

13. REGISTRATION OF ANALYSIS RESULTS

Analysis result X avg in documents providing for its use, it can be presented in the form:

X avg± ?, P = 0.95,

Where? - indicator of the accuracy of the technique.

Meaning? calculated by the formula:

Meaning? shown in Table 1.

If the water sample was diluted due to the calcium mass concentration exceeding the upper limit of the range, the value? selected from Table 1 for the mass concentration of calcium in a diluted water sample X v .

It is acceptable to present the result of the analysis in documents issued by the laboratory in the form:

X avg±? l, P = 0.95,

given that? l< ?,

Where X avg- the result of the analysis obtained in accordance with the instructions in the methodology;

± ? l- the value of the error characteristic of the analysis results, established during the implementation of the technique in the laboratory, and ensured by monitoring the stability of the analysis results.

The numerical values ​​of the measurement result must end with a digit of the same digit as the values ​​of the error characteristic.

Note. When presenting the analysis result in documents issued by the laboratory, indicate:

Number of results of parallel determinations used to calculate the result of the analysis;

Method for determining the result of the analysis (arithmetic mean or median of the results of parallel determinations).

14. QUALITY CONTROL OF ANALYSIS RESULTS WHEN IMPLEMENTING THE METHOD IN THE LABORATORY

Quality control of analysis results when implementing the method in the laboratory includes:

Operational control of the analysis procedure (based on the assessment of the error in the implementation of a separate control procedure);

Monitoring the stability of analysis results (based on monitoring the stability of standard deviation of repeatability, standard deviation of intra-laboratory precision, error).

14.1. Algorithm for operational control of the analysis procedure using the additive method

K k with control standard TO.

K k calculated using the formula.

Where X" cf- the result of the analysis of the mass concentration of calcium in a sample with a known additive - the arithmetic mean of two results of parallel determinations, the discrepancy between which satisfies condition (1) of section 12.2;

X avg- the result of the analysis of the mass concentration of calcium in the original sample - the arithmetic mean of two results of parallel determinations, the discrepancy between which satisfies condition (1) of section 12.2;

S d - amount of additive.

Control standard TO calculated by the formula:

Where - the values ​​of the error characteristic of the analysis results, established in the laboratory when implementing the method, corresponding to the mass concentration of calcium in the sample with a known additive and in the original sample, respectively.

Note.

If condition (2) is not met, the control procedure is repeated. If condition (2) is not met again, the reasons leading to unsatisfactory results are determined and measures are taken to eliminate them.

14.2. Algorithm for operational control of the analysis procedure using samples for control

Operational control of the analysis procedure is carried out by comparing the result of a separate control procedure K k with control standard TO.

Result of the control procedure K k calculated by the formula:

Where From Wed- the result of the analysis of the mass concentration of calcium in the control sample - the arithmetic mean of two results of parallel determinations, the discrepancy between which satisfies condition (1) of section 12.2;

WITH- certified value of the control sample.

Control standard TO calculated by the formula:

where ±? l - characteristic of the error of the analysis results corresponding to the certified value of the control sample.

Note. It is permissible to characterize the error of analysis results when introducing a technique in a laboratory based on the expression: ? l = 0.84 · ?, with subsequent clarification as information accumulates in the process of monitoring the stability of the analysis results.

The analysis procedure is considered satisfactory if the following conditions are met:

If condition (3) is not met, the control procedure is repeated. If condition (3) is not met again, the reasons leading to unsatisfactory results are clarified and measures are taken to eliminate them.

The frequency of operational control of the analysis procedure, as well as the implemented procedures for monitoring the stability of analysis results, are regulated in the Laboratory Quality Manual.

Appendix A

(required)

Preparation of activated carbon

A portion of activated carbon sufficient to fill the column is placed in a conical flask, 100 - 150 cm 3 of a 4 mol/dm 3 hydrochloric acid solution is added and boiled for 2 - 3 hours. If the acid solution becomes colored, repeat the operation until it remains colorless. The coal is washed with distilled water until the reaction is neutral on universal indicator paper, 100 - 150 cm 3 of a 1 mol/dm 3 sodium hydroxide solution is added and left for 8 - 10 hours. If color appears, the operation is repeated.

Purified coal is washed with distilled water until a neutral reaction. Store in a bottle with distilled water for up to 6 months.

To fill the column, shake the flask and transfer the coal along with water into the column, excess water is drained through the tap. The height of the coal layer should be 12 - 15 cm. Before passing the sample, water is removed from the column.

After passing each water sample, the carbon in the column is regenerated by washing with a 0.4% sodium hydroxide solution until the color disappears, then with distilled water until a neutral reaction.

FEDERAL AGENCY FOR TECHNICAL REGULATION AND METROLOGY

State Scientific Metrological Center

FSUE "Ural Research Institute of Metrology"

CERTIFICATE

on certification of measurement techniques

№ 223.1.01.02.92/2008

FEDERAL AGENCY FOR EDUCATION STATE POLAR ACADEMY

Department of Geoecology

Laboratory reports

Discipline: “Chemistry of Natural Environments”

Completed by: Novikov R., 681gr.

Checked by: Zykova A.A.

Saint Petersburg

Laboratory work No. 1 Determination of total water hardness

Water hardness is a property of natural water that depends on the presence of mainly dissolved calcium and magnesium salts in it. Hardness caused by the presence of calcium salts is called calcium, depending on the magnesium content – magnesium. The total content of these salts in water is called general hardness. Total hardness is usually determined by complexometric titration. Depending on the use of a particular indicator, either the total hardness is found (indicator - eriochrome-black), or successively calcium (indicator - murexide) and magnesium (indicator - eriochrome-black) or by difference. The method is based on titration of a water sample with a solution of disodium salt of ethylenediaminetetraacetic acid - complexone III or trilon-B. Water hardness is expressed by the number of milligrams - equivalents (mmol-eq/l) of calcium and magnesium in one liter of water.

Instruments and reagents:

10 ml burette, 250 ml conical flask, pipettes, test samples (control task and tap water), erychrome-black indicator (mixture of indicator with dry NaCl), ammonium chloride buffer solution, Trilon-B solution.

Progress (when analyzingtap water):

Pour 100 ml of tap water into a 250 ml conical flask using a graduated cylinder. Add 5 ml of buffer mixture and a little dry indicator eriochrome black

With continuous stirring, titrate with Trilon-B solution until the color changes from red-violet to blue.

For measurement accuracy, titrate three times (error no more than 0.01)

For calculation : F (mmol-eq/l) = Cn trilon ∙V trilon ∙1000/V water,

where Cn is the normality of the Trilon-B solution

V trilon – volume of complexone solution used for sample titration

V water – volume of water sample taken for determination.

Measurement results:

Test solution (tap water)

F = 2.52 (mmol-eq/l)

Conclusion: The total hardness is 2.52 mmol-eq/l. Thus, tap water belongs to the class soft waters

Laboratory work No. 2 Determination of calcium and magnesium content in natural waters

Goal of the work - Separate determination of calcium and magnesium content in natural waters by complexometric titration.

After determining the total hardness of water, characterized by the total number of millimole equivalents of calcium and magnesium in 1 liter of water, the calcium content is determined separately by titrating the sample with Trilon-B in the presence of an indicator (murexide). Then, from the difference between the total hardness and the calcium content, the magnesium content is calculated.

Work progress (when analyzing tap water):

Measure 100 ml of the test sample (tap water) into a 250 ml conical flask with a measuring cylinder, add 5 ml of 2N NaOH solution and add a small amount of murexide indicator to the tip of a spatula.

Titrate with a solution of Trilon-B (C = 0.01 N) until the pink color changes to blue-violet, which does not discolor within thirty seconds.)

Titration is repeated 3 times and the average result is taken from the obtained values.

For calculation: The calcium hardness value of the sample is calculated using the equation:

F (Ca +2) = (C trilon ∙V trilon /V research sample)∙ 1000 (mmol-eq/l)

F Mg = F total. – FCa (mmol-eq/l)

Measurement results:

Tap water

F (Ca +2 water supply) =(0.05*3.47)/100*1000=1.74(mmol-eq/l)

F (Mg +2add.water) =2.52-1.74=0.78 (mmol-equiv/l)

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Hydrochemistry

Methodology for determining calcium and magnesium ions in natural waters (determining total water hardness)

D.Yu. Kovalev

1. Brief theory

2. Preparation of solutions

3. Taking measurements

3.1 Titrimetric method

3.2 Ion exchange chromatography method

1. Brief theory

Chemical analysis of natural and drinking water shows that any water is not a pure substance with the formula H 2 O, but a mixture large quantity substances.

Numerous analyzes of natural waters have shown that among large number components dissolved in them, 90% of the salt content consists of carbonates, bicarbonates, chlorides and sulfates of calcium, magnesium and sodium. O.A. Alekin proposed a classification of natural waters based on the results of their chemical analysis. Based on the predominant anion, waters are divided into three classes: carbonate (hydrocarbonate), chloride and sulfate. Based on the predominant cation, waters are divided into three groups: calcium, magnesium and sodium.

Natural waters constantly contain calcium and magnesium ions, which ensure water hardness. The source of their entry into water is the dissolution of gypsum, limestone and dolomites that are part of the rocks. In sanitary and hygienic terms, calcium and magnesium ions do not pose a great danger, but excessive hardness of water makes it unsuitable for domestic purposes, because the resulting scale damages the heating elements of electrical water heating systems. Optimal water hardness is up to 7 mEq/l.

Two methods are used to determine calcium and magnesium ions:

1. titrimetric

2. ion exchange chromatography method

1. The most accurate and widespread method for determining total hardness is complexometric, based on the formation of strong intra-complex compounds with Trilon B by Ca 2+ and Mg 2+ ions. Eriochrome black is used as an indicator when determining total hardness. Depending on the overall hardness, the concentration of the Trilon B working solution and the volume of the water sample may be different.

To determine calcium in natural waters, the trilonometric method with the indicator murexide is mainly used.

2. Preparation of solutions

Trilon B solution with a concentration of 0.02 mol/dm 3 equivalents.

Weight 3.72 g. Trilon B is dissolved in 1 dm 3 of distilled water. The exact concentration is determined using a standard zinc chloride solution. The solution is stored in a plastic container, its concentration is checked at least once a month.

A solution of zinc chloride with a concentration of 0.02 mol/dm 3 equivalents.

Weigh on technical scales about 0.35 g of metallic zinc, moisten it with a small amount of concentrated hydrochloric acid and immediately wash with distilled water. The zinc is dried in an oven at 105 for 1 hour, then cooled and weighed on an analytical balance.

A sample of zinc is placed in a volumetric flask with a capacity of 500 cm 3, into which 10-15 cm 3 of distilled water and 1.5 cm 3 of concentrated hydrochloric acid are first added. Zinc is dissolved. After dissolving the zinc, the volume of the solution is adjusted to the mark on the flask with distilled water. Calculate the molar concentration of the equivalent of a solution of zinc chloride C Zn (1/2 ZnCl 2), mol/dm 3, using the formula:

where m is the weight of metallic zinc, g; 32.69 - molar mass of Zn 2+ equivalent, g/mol; V is the volume of the volumetric flask, cm 3.

Buffer solution NH 4 Cl + NH 4 OH.

7.0 g of ammonium chloride is dissolved in a 500 cm 3 volumetric flask in 100 cm 3 of distilled water and 75 cm 3 of concentrated ammonia solution is added. The volume of the solution is adjusted to the mark with distilled water and mixed thoroughly. The buffer solution is stored in glass or polyethylene containers for no more than 2 months. Sodium hydroxide, 2 mol/dm3.

40 g of sodium hydroxide are dissolved in a 500 cm 3 volumetric flask and the solution is brought to the mark with distilled water.

Indicator eriochrome black T.

Grind 0.25 g of eriochrome black T with 50 g of sodium chloride in a mortar.

Murexide indicator.

Grind 0.5 g of murexide with 100 g of sodium chloride. Water solution It's better not to cook, because... Murexide is unstable in solution.

Sodium sulfide solution, 4%.

2 g of sodium sulfide are dissolved in 50 cm 3 of distilled water. Store in a tightly sealed plastic container for no more than a week.

Hydroxylamine hydrochloride solution.

5 g of hydroxylamine hydrochloride is dissolved in 100 cm 3 of distilled water. Store for no more than 2 months.

Establishing the exact concentration of Trilon B solution.

Add 10 cm 3 of zinc chloride solution to a conical flask with a capacity of 250 cm 3, add distilled water to approximately 100 cm 3, 5 cm 3 of a buffer solution and 10-15 mg of the indicator eriochrome black T. The contents of the conical flask are thoroughly mixed and titrated from a burette with a Trilon solution B until the color changes from red to blue. The concentration of Trilon B solution is calculated using the formula:

3. Taking measurements

3.1 Titrimetric method

Determination of calcium and magnesium ions

Elimination of Interfering Ions

To eliminate the interfering influence of iron, zinc, copper and tin cations, add 0.5 ml of sodium sulfide solution to the sample.

To eliminate the interfering effect of manganese, add 0.5 ml of hydroxylamine hydrochloric acid solution to the sample.

Progress of analysis

v Evaluative titration

Before analyzing a water sample with an unknown hardness value, an evaluative titration is carried out. To do this, take 10 cm 3 of water, add 0.5 cm 3 of a buffer solution, an indicator (eriochrome black T) and titrate until the color changes from red to blue. Based on the amount of Trilon B consumed, the corresponding volume of water sample is selected from Table 1.

ion exchange chromatography water magnesium

v Determination of the amount of calcium and magnesium

To a sample of the required volume (see Evaluative titration) 100 cm 3 add 5 cm 3 buffer, indicator (eriochrome black T) on a spatula. Titrate immediately with stirring until the color changes from wine red to blue.

v Determination of calcium

To a sample of the required volume (see Evaluative titration) 100 cm 3 add 2 cm 3 NaOH (2N) and an indicator (murexide) on a spatula. Titrate until the color changes from red to purple. The color of the solution should be compared with the color of the titrated solution.

where Str is the molar concentration of the equivalent of Trilon B, mol/dm 3 ; V"tr is the volume of Trilon B used for titration with murexide, cm 3 (see Determination of calcium); 20.04 is the equivalent mass of Ca 2+; Vsample is the volume of the sample taken for analysis, cm 3.

where Str is the molar concentration of the equivalent of Trilon B, mol/dm 3 ; V tr - volume of Trilon B used for titration with eriochrome black T, cm 3 (see Determination of the amount of calcium and magnesium); V"tr - volume of Trilon B used for titration with murexide, cm 3 (see Determination of calcium); 12.15 - equivalent mass of Mg 2+; Vsample - volume of sample taken for analysis, cm 3.

v Determination of total water hardness

The total hardness is found using the formula:

where C tr is the molar concentration of the equivalent of Trilon B, mol/dm 3 ; Vtr is the volume of Trilon B solution used for sample titration, cm 3 ; Vsample - sample volume taken for analysis, cm3.

Additive method. To determine this method, an additive equal to 50-150% (preferably 100%) of water hardness is introduced into the sample (see Determination of total water hardness) GSO 8206-2002.

Then the total hardness of the water with the additive is calculated.

a. Results of measurements obtained under reproducibility conditions for sample 1.

Sample 1: lake Average, s. Ozernoe, 85 km from the coast, date: 10/1/13, time: 16.55, t = +3.

The exact concentration of Trilon B has been established: Trilon C = 0.002226 (mol/dm 3). When performing an evaluative titration, the volume of the required sample corresponds to 100 (ml).

a. Measurement results using the sample addition method 1. Total water hardness: .

Additive volume:

Total water hardness with additive: .

Additive check:

3.2 Ion exchange chromatography method

The eluent is methanesulfonic acid. The method is based on the chromatographic separation of cations due to their different mobility during migration through an ion chromatography column, followed by recording the electrical conductivity of the eluate.

The chromatograph is prepared for operation in accordance with the operating manual (instructions) so that when measuring calibration solutions, separation of the peaks of the analyzed cations is achieved with a separation coefficient of at least 1 (Appendix B to GOST R 51392).

Injection of the sample into the chromatograph and further measurements of the electrical conductivity of the eluate are carried out in accordance with the operating manual (instructions). In the resulting chromatograms, the cations contained in the sample are identified by the retention time of the peaks, and the peak areas of each cation are determined. Based on the obtained calibration characteristics, the concentration of each cation in the sample is determined.

Results of the ion exchange chromatography method

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