Aqualab Part 2

1. Discover WATER with AQUALAB ! Part 2 Experimental protocols related to sections of Part 1. By Jean-Jacques BOURGOIS www.recyclabs.org 1

2. 2. pH & Alkalinity Method 1 : pH indicators Set up : see Fig.1. ❶ Pour a volume Vw = 100 mL of water to be tested (sample) into a 250 mL beaker (Erlenmeyer flask or other container). ❷ Add a magnetic stir bar and approximately 20 drops of phenolphthalein (1% methanolic sol.). Observe the color of the solution. ❸ If the solution is colorless, add about 20 drops of bromocresol green (aqueous sol. 0.04%) or methyl orange (aqueous sol. 0.04%) to the beaker. ❹ After rinsing, fill a 50 mL burette with the hydrochloric acid solution (20 mmol.L-1) and zero. ❺ Place the beaker on a magnetic stirrer and under the graduated burette. ❻ Pour in the HCl drop by drop until the sensitive shade of bromocresol green (or methyl orange) is reached: the colour change must be persistent and be accurate to the drop. ❼ Note the volume of acid poured when the coloured indicator has changed colour: we are at equivalence. HCl 20 mmol.L-1 100 mL water sample + indicator Fig.1. 2

3. 2. pH & Alkalinity Remark : It is important to take into account the turning zones of the indicators: pH turning point (Sensitive shade) pH indicator Acid shade Alkaline shade Can be used to assess: TA: KS8,2 Phenolphtalein Colorless 8,2 Rose 9,9 Violet [CO32-] + [OH-] TAC : Methyl Orange (MO) Red 3,1 Orange 4,3 Yellow [CO32-] + [OH-] + [HCO3-] KS4,3 3,6 Green 5,4 » » 4,3 Bromocresol Green (BG) Yellow Blue 3

4. 2. pH & Alkalinity • As the solutions are very dilute, the colors are quite pale and titration should be stopped when the color of the sample turns a persistent orange (MO) or yellow (BG). Approximately the same volumes are obtained. In the case of BG, it is preferable to choose an intermediate value (4.55), the equivalent pH being closer to reality. • More accurate values can be obtained by using a pH titration curve. 4

5. 2. pH & Alkalinity Method 2 : pHmetry ❶ The set-up is identical to that used for pH indicators except that a pH electrode is introduced into the beaker containing the 100 mL of water to be tested (see Fig. 2). ❷ In this method, instead of taking into account only the volume of acid added until the indicator changes colour, a titration curve is created. HCl 20 mmol.L-1 100 mL water sample pH probe Fig.2. 5

6. 2. pH & Alkalinity ❸ This curve can be constructed in two ways: manual or automatic. • Manual: the titrant (HCl 20 mmol.L-1) is added millilitre by millilitre, plotting the pH as a function of the volume of HCl added on a graph. The flow rate is slow down as you near pH 5 (the equivalence point is around pH 4). Titration continues until approximately pH 3. • Automatic: the pH probe is connected to an interface which converts an analog to a digital signal. This interface is connected to a computer equipped with data acquisition software which displays the variation in pH as a function of time (not volume!) in real time. In this case, it is essential to titrate using a regular, continuous burette flow (drop by drop - do not handle the tap during titration!); the stirring must be fairly intense to homogenize the titrated solution. The initial (Vi) and final (Vf) volumes of the titration should also be noted visually. 6

7. 2. pH & Alkalinity Data processing • Titration data is acquired using the SPARKvue® v4.5 software, dedicated to the use of PASCO teaching equipment. This software is used to set up the experiments, process the acquired data and export the data, which can then be studied in greater detail using spreadsheets (Microsoft® Excel, Apple® Numbers, Google Sheets, etc.). The SPARKVue® system exports alphanumeric data in .csv format. To be usable, they must be converted into the appropriate format (e.g. Excel .xslm). In addition to the necessary parameters (in this case time and pH), the Advanced Chemistry Sensor records other non-applicable data. To simplify the spreadsheet, the unused columns should be ignored. • Two options are possible : with or without a connected dropper. First option: without a dropper. Preliminary operation: transformation of time (seconds) into units of volume (mL). Viis the volume shown on the burette at the start of titration at the initial time Tiand Vfis the volume shown at the end of titration at the final time Tf. One second then corresponds to : 1 sec ⇒!"#!$ %"#%$mL 7

8. 2. pH & Alkalinity • Here is a screenshot showing the real-time titration curve of 100 mL of Evian water with HCl 20 mmol.L-1: The acquisition frequency is 1 Hz (one pH plot per second). In this case : Vi= 0.1 mL and Vf= 32.3 mL. Ti= 0 sec and Tf= 302 sec. 1 sec therefore corresponds to : 32.2/302 = 0.10662 mL (Fig.3.) Fig. 3. 8

9. 2. pH & Alkalinity • Second option: using a dropper First step: dropper calibration. To obtain a titration curve representing the pH directly as a function of the volume of titrant added, the volume of drops counted by the device must be determined precisely. This operation, although necessary, can sometimes be time-consuming... To comply with the definition of KS4.3, the equivalent volume at pH 4.3 is noted, i.e. for : tap water: 16.21 mL Evian water: 28.9 mL river water: 21.78 mL sea water: 12.68 mL Fig.4. Titration curve of Evian water by HCl 20 mmol.L-1 9

10. 2. pH & Alkalinity • Precise determination of the equivalence point. In Excel, insert a new column next to the time and enter the corresponding values for the volumes of titrant added. It is now possible to create a graph of a titration curve: pH = f(V). The equivalence point (Veq; pHeq) is at the inflection point of this curve. To determine the exact coordinates of this point, we use the derivative method. The derivative function is constructed as follows: for each point i on the titration curve, we calculate (spreadsheet) : '() '*= ()!"#+()!$# *!"#+*!$# 10

11. 2. pH & Alkalinity We obtain a scatter plot (derivative curve) and the value of the volume Veqand the pHeqare at the minimum of this curve (case of titration of a weak base by a strong acid). For Evian water, the equivalence point therefore has the coordinates (29.9; 4.02), see Fig.5.. • Note: • Often, the inflection point is not exactly at pH 4.3 (here pH 4.02). However, this has little effect on the volume of titrant (here 0.39 mL, or 1.35%) and gives practically the same results. The plot often appears irregular and fluctuations appearing outside the inflection point zone can be neglected. Smoothing with a moving average can reduce irregularities (Excel) and improve the appearance of the graph. • Fig.5. Scatter plot of the derivative of the curve pH = f(V) 11

12. 3. Water Hardness : Calcium and Magnesium Experimental protocol • Preparation of the solutions : Titrant : Na2EDTA 10 mmol.L-1 Ammonia buffer pH ≈ 10 (500 mL): mix 250 mL of a 1 mol.L-1aqueous ammonia solution (NH3) and 250 mL of a 1 mol.L-1 ammonium chloride (NH4Cl) solution Indicator :1% Eriochrome Black T (EBT) in 5-10% ethyl alcohol Solution to be titrated : 100 mL water sample Na2EDTA 10 mmol.L-1 u After rinsing the burette with the Na2EDTA solution, fill to the mark and zero. v Fill a 250 mL beaker (or other container) with the sample and heat to 40°C. w Add 5 mL of buffer solution and 5 drops of EBT. x Place the beaker on a magnetic stirrer and insert a magnetic bar. y Start titration until the color changes from violet-pink to blue. Stop the addition of Na2EDTA and note the volume. The first step is often not very accurate. ❻ Carry out a second accurate titration; the titrant should be added drop by drop at the level of the color change zone. Note the volume and carry out the calculations. 100 mL water + 5 mL buffer + 5 drops EBT 12

13. Step 1 : calibrating line preparation 4. Sulfates Stock solution: 10-3 mole.L-1 SO4-- Molecular mass (g.mol-1) 1 mg.l-1 = 10-5 mol.L-1 Stock solution: 100 mg.L-1 SO4-- 0,246 g.L-1 To beprepared Experimental set-up • The essay will be performed by turbidimetry (PASCO equipment) using a 6-point calibration line. The opposite table describes the various stages in the preparation of the reaction media leading to the establishment of the line used to evaluate the sulfate concentration of the unknown solution. Any other soluble sulfate can be used for calibration (e.g. Na2SO4or K2SO4). MgSO4.7H2O [SO4--] 246,47 96,06 mg.L-1 96,06 Step 2 : preparation of the reagents Remark : For 500 mL reagent, mix: H2O Ethanol 96% Glycerol HCl (6 M) KCl or NaCl BaCl2 If these substances contain traces of sulfates, a slight cloudiness may already have appeared when the reagent was prepared. If this is the case, allow the reagent to stand for at least 24 hours before measuring. Do not shake ! 300 mL 100 mL 50 mL 30 mL 70 g 20 g This is often the case when using household products. Step 3: preparation of reaction media In 100 mL beakers (or other containers of identical size) mix : Note: • The absorbance values obtained are highly dependent on the preparation of the solutions. Absorbance can be measured at any wavelength (here 567 nm). Absorbance is not measured due to a colored solution (electronic transition), but due to a loss of light intensity caused by turbidity. [SO4--] mg.L-1 N° of beaker Distilled water (mL) Stock sol. (mL) Reagent (mL) 0 1 2 3 4 5 6 50 45 40 35 30 25 0 5 5 5 5 5 5 5 10 20 30 40 50 10 15 20 25 7... Samples of unknown concentration ? 50 mL 5 Step 4: after homogenizing , spectrophotometer reading at 567 nm (1 cm cuvettes) 13

14. Step 1 : Preparation of the nitrate standard solution 5. Nitrates MM g.mol-1 Stock sol. : 100 mg.L-1NO3- 163 mg.L-1to prepare KNO3 101,1 NO3- 100 mg.L-1 62 Step 2: preparation of reaction media phase 1 In beakers (or other identical glass containers) of 100 mL introduce : [NO3-]mg.L-1 0 5 10 15 20 25 N° beakers 1 2 3 4 5 6 7… Samples of unknown concentration Distilled water (mL) 10 9,5 9 8,5 8 7,5 Stock sol (mL) 0 0,5 1 1,5 2 2,5 Na salicylate 0,5 % (mL) 1 1 1 1 1 1 • Experimental setup The assay is carried out by spectrophotometry (PASCO equipment) using a 6-point calibration line. The opposite table shows the various stages in the preparation of the reaction media leading to the establishment of the line used to assess the nitrate concentration of the unknown solution. 10 mL 1 ? Step 3: Evaporate these solutions to dryness in a microwave oven set to low power to avoid boiling! Step 4: preparation of reaction media phase 2 After complete evaporation, take up the cooled residue by adding to each container, in order : 1 Þ 2 Þ 3 Þ Þ Étape 5 Staining reagent (mL) 15 Add very gradually H2SO498% (mL) H2O dist (mL) Spectrophotometer reading at 425 nm after homogenisation and cooling (1 cm cuvette). 2 15 Add very gradually Leave to react for 10 minutes, stirring gently Caution! Risk of splashing due to sudden rise in temperature! Staining reagent : 40 % NaOH and 6 % Na & K tartrate in aqueous solution (distilled water). 14

15. 7. Seawater salinity Principle of the essays • 1) Volumetry This method is based on Dittmar's law. The halide content (mainly chlorides and bromides) is measured and expressed as the mass (g) of chlorine equivalent to the total mass of halogens present in 1 kg of sea water (SW); this value is called the chlorinity. This is multiplied by a factor of 1.80655, which represents the ratio of the average salinity (S = 35) to the quantity of chlorine present in SW, which is 19.37394 g.kg-1 . Principle of the Mohr method : This is a argentometric titration. We have a solution of chloride ions (halides) of unknown concentration. Silver nitrate (AgNO3) is added to this solution. For each quantity of silver ion (Ag+) added, a precipitate of silver chloride (AgCl) is formed: Ag+(l) + Cl-(l)à AgCl(s)¯ The initially transparent solution becomes cloudy with a white opalescence and then opaque. Equivalence is reached at stoichiometric equilibrium: n(Ag+) = n(Cl-). As there is no way of visualising this equivalence, as the white cloudiness present since the beginning of the titration no longer changes, we use a coloured indicator of the presence of excess Ag+ ions. We use potassium chromate (K2CrO4) in aqueous solution, which forms a brick-red precipitate of silver chromate (Ag2CrO4) as soon as the Ag+ ions are in excess. Important note: Some silver salts are very sensitive to light; their solutions should be kept in brown glass bottles or wrapped in aluminium foil, otherwise they will gradually darken. In addition, contact of AgNO3 with the skin causes indelible black spots. It is strongly advised to handle silver nitrate with gloves and goggles! 15

16. 7. Seawater salinity Principle of the essays • 1) - Procedure The set-up is that of a direct titration. Preparation of the solutions: Titrant solution : aqueous solution of AgNO3 5.10-2 mol.L-1 . Indicator solution: 5% aqueous K2CrO4 solution. Solution to be titrated: undiluted SW sample. Volumetry (Mohr) AgNO350 mmol.L-1 ❶ After rinsing the burette with the AgNO3 solution, fill to the mark and adjust the zero. ❷ Into a 250 mL beaker introduce 1 mL of undiluted SW, add approximately 20 mL of distilled water and 5 mL of the K2CrO4 solution. wPlace the beaker on a magnetic stirrer and insert a magnetic bar . xStart titration until the colour changes from yellow to red. Stop the addition of AgNO3 and note the equivalent volume VE. The first procedure is often not very accurate. yCarry out a second, more precise titration; the titrant should be added drop by drop at the turning point. Note: the turning point does not occur suddenly but appears as an orange colour near the equivalence point. 1 mL SW + 20 mL distilled water + 5 mL K2CrO4 5 % indicator 16

17. 7. Seawater salinity 2) In this case, a conductivity sensor is used instead of a coloured precipitation indicator. It measures changes in the conductivity of the titrated solution in real time as AgNO3 is added. This operation can be carried out without prior calibration of the probe, as variations in conductivity are observed rather than absolute values. - Procedure (PASCO hard and software) The set-up is that of a direct titration. Preparation of the solutions: Titrant solution : aqueous solution of AgNO3 50 mmol.L-1 . Solution to be titrated: undiluted SW sample. Conductometric titration AgNO350 mmol.L-1 ❶ After rinsing the burette with the AgNO3 solution, fill to the mark and adjust the zero. ❷ Into a 250 mL beaker introduce 1 mL of undiluted SW sample and pour approximately 175 mL of distilled water . wPlace the beaker on a magnetic stirrer and insert a bar magnet. xPlace the conductivity probe in the beaker so that the measuring cell is completely immersed. Carefully remove any air bubbles that could impede the flow of ions and start stirring. Allow the system to stabilize ; the value displayed will be that of SW diluted 175x ! yStart the titration by AgNO3 50 mmol.L-1. 1 mL SW + 175 mL distilled water 17

18. 7. Seawater salinity • Explanation of the conductivity evolution during titration 1) Conductivity decreases as AgCl precipitates: Cl- ions are eliminated (along with Ag+ions) and are replaced by nitrate ions (NO3-). As the molar ionic conductivity of this ion (7.142 mS.m2.mol-1) is lower than that of the chloride ion (7.631 mS.m2.mol-1), the current flow is decreased between the cell electrodes. 2) When the equivalence point is reached (nAg+= nCl-), conductivity increases rapidly because AgNO3is no longer consumed. 3) The conductivity graph σ= f (VAg+added) shows two straight lines with different slopes. Equivalence can be identified by the change in slope of the 2 lines. Their intersection gives the equivalent volume VE(see opposite figure). In this case VE= 12.01 mL. 18

19. 7. Seawater salinity • Calculation of salinity : At equivalence, the quantity of total halides expressed in mol.L-1of Cl-initially present in the SW sample in the beaker and the quantity of silver ions (Ag+) added are in stoichiometric proportions. Let : CAg+ the AgNO3concentration of the titrating solution = 50 mmol.L-1 VEthe equivalent volume (in mL) Vwthe volume of seawater sample Cwthe concentration of halides in the sample expressed in mol.L-1of Cl- We therefore have : CAg+× VE= Vw× Cw, hence "!= We need to express chlorinity in g.kg-1. As the atomic mass of chlorine is 35.45 g.mole-1, the concentration becomes: (Cw× 35.45) g.L-1. The density (ρ) of the SW sample at 20°C is measured beforehand, so the chlorinity is : (Cw× 35.45 × ρ-1) g.kg-1 Since salinity is equal to chlorinity × 1.80655, the final result becomes : "!"# #$×V$ S = Cw× 35.45 × ρ-1× 1.80655 g.kg-1. 19