ph Alkalinity of Water

Transcription

1 ph Alkalinity of Water DOC Based on ISO standard :1994 ph-metric Titration 0.4 to 20 mmol/l of Total Alkalinity 1. Introduction Alkalinity of water is its acid-neutralizing capacity. It is the sum of all the titratable bases. For many surface waters, alkalinity values are primarily a function of carbonate, bicarbonate, and hydroxide content. The measured values also may include contributions from borates, phosphates, silicates or other bases if these are present. Alkalinity is determined by end point titration with a strong acid solution. Phenolphthalein alkalinity abbreviated by p is also known as composite alkalinity which corresponds to titratable alkalinity at ph 8.3. Total alkalinity is also known as methyl red (methyl orange) endpoint alkalinity which corresponds to titratable alkalinity at ph 4.5. This application note is based on ISO standard :1994 (Water quality Determination of alkalinity Part 1: Determination of total and composite alkalinity). 2. Principle The phenolphthalein end point alkalinity or composite alkalinity abbreviated by p is the measurement by titration to the phenolphthalein end point (ph 8.3) of that portion of alkalinity arbitrarily attributed to all the hydroxyl c(oh - ) and half the carbonate (CaCO 3 ) content of water. P c(co 3 2- ) c(co 2aq ) + c(oh - ) c(h + ) +c(*) The methyl red (methyl orange) end point alkalinity is an arbitrary measurement of the total alkalinity of water, abbreviated TA, and is obtained by titration to the methyl red (methyl orange) indicator endpoint (ph 4.5) to assess the equivalent hydrogen carbonate (HCO 3 - ), carbonate (CaCO 3 ) and hydroxide (OH - ) concentration of water. TA 2 c(co 3 2- ) + c(hco 3 - ) + c(oh - ) c(h + ) +c(*) Note: In both cases, other buffering substances (*) such as ammonia, borate, phosphate silicate and organic anions may be included in the determination. The number of detected equivalent point depends on the initial ph of the solution. By definition, p is zero for water which has a ph value of 8.3 or less. The current standard uses HCl 0.1 eq/l as a titrant, but another strong acid such as H 2 SO eq/l can also be used. Results are normally expressed as mmol/l (or meq/l) of alkalinity but other units can be used (see Results section). Using an appropriate combined ph electrode with temperature, the alkalinity determination can be linked with a ph and temperature measurement of the sample. 3. Electrode and reagents Combined ph Electrode with Temperature sensor: Intellical PHC805 Titrant: HCl or H 2 SO eq/l solution in deionized water Deionized water 4. Ranges and settings 4.1. Default parameters The sample size and titrant concentration depend on the quality of the water. Using the application note settings described below with the following parameters: V sample = 100 ml Burette volume = 10 ml Titrant concentration = 0.1 eq/l (corresponding to a 0.1 mol/l HCl solution) Page 1 of 10

2 4.2. Working ranges In accordance with the norm ISO , the previous configuration with 0.1 eq/l of titrant HCl or H 2 SO 4, 10 ml-burette is done for a Total Alkalinity between 0.4 mmol/l (20 mg/l CaCO 3 ) corresponding to 0.4 ml of titrant 0.1 eq/l and 20 mmol/l (1000 mg/l CaCO 3 ) corresponding to 20 ml of titrant 0.1 eq/l. For the best accuracy and reproducibility, the result range is between 3.5 meq/l or 175 mg/l CaCO 3 for 35% of the cylinder 10 ml-burette capacity and 10 meq/l or 500 mg/l CaCO 3 for the cylinder 10 ml-burette capacity. With the same conditions, the "experimental" limit corresponding to a titrant volume of 0.5 ml is 0.5 meq/l or 25 mg/l CaCO 3. For low alkalinity, below 0.5 mmol/l or 25 mg/l CaCO 3 (corresponding to 0.5 ml of titrant 0.1 eq/l), it is recommended to use a low alkalinity method with 0.02 eq/l titrant and 200 ml for sample volume, using the calculation above. For high alkalinity, between 10 mmol/l (500 mg/l CaCO 3 ) and 20 mmol/l (1000 mg/l CaCO 3 ) it is recommended to use smaller sample volumes (less than 50 ml) with the same titrant 0.1 eq/l Settings Note: In the following tables, an x in the or column indicates that the parameter can be edited in that respective. Refer also to the User Manual Sample Default parameters: Setting Name x x Water s Amount x x 100 [ml] Settings for leveling (Method: leveling) If the option of an external pump is present on the device, the leveling method allows for the leveling of the sample at the beginning of the titration. Default parameters: Setting Active x x No s Time x x 30 [s] The time of the leveling has to be adapted depending on the amount of sample and the leveling tube Settings for ph measurement (Method: phi) By default the initial measurement of ph is done with the parameters described below: Setting Active x x Yes Max. stability time x 120 [s] Stability criterion x [ph/min] Stirring speed x x 25 [%] Settings for alkalinity measurement The alkalinities are measured using a 10-mL burette, an incremental addition and an End Point detection method with the parameters described below: Titrant settings used for the calculation: Setting Name x x HCl Titer x x 0.1 [eq/l] Page 2 of 10

3 Note: The Real concentration value is updated after each automatic or manual calibration of titrant. Detection settings: Results settings: R x Setting Active x x Yes Stirring speed x x 25 [%] Measured parameter Predose x x 0 [ml] Max. vol. stop point x 20 [ml] Ordinate stop point x 3.4 [ph] Stop on last EQP x Yes Delay x x 0 [s] Stability criterion x [mph/min] Min. increment size x 0.1 [ml] Max. increment size x 1.0 [ml] Equivalence point #1 x 8.3 [ph] Equivalence point #2 x 4.5 [ph] Result x name x x p ph R1 R2 R3 R4 Total Alkalinity p (1) Total Alkalinity (2) R x max. resolution x 3 decimals 3 decimals 1 decimal 1 decimal R x hide x x Yes Yes No No R x limit check x x Yes Yes Yes Yes R x min x x 0 mmol/l 0.4 mmol/l 0 mg/l as 20 mg/l as CaCO 3 CaCO 3 R x max x x 10 mmol/l 20 mmol/l 500 mg/l as 1000 mg/l as CaCO 3 CaCO 3 R x unit x mmol/l mmol/l mg/l as (1) CaCO 3 (2) mg/l as CaCO 3 R x EQP index x 1 2 N/A N/A R x equation x N/A N/A R3 = R2*FX (1) R3 = R2*FX (2) R x user value x x N/A N/A 50 (1) 50 (2) R x sample x 1 equivalent R x titrant 1 if HCl is used x equivalent 2 if H 2 SO 4 is used (1) (2) R3 (Phenolphthalein alkalinity): in the default settings, p is expressed in mg/l as CaCO 3 with the User Value set at 50. R4 (Total alkalinity): in the default settings, Total Alkalinity is expressed in mg/l as CaCO 3 with the User Value set at 50. For results R1 and R2, only the Name and the Rx max. resolution can be modified. R1 and R2 have to be expressed in mmol/l in order to have correct results for R3 and R4 as results of the equation. For results R3 and R4, R x max. resolution, R x min, R x max, R x unit and R x user value can be modified to express the results in alternative units using the R x user value from mmol/l (see Alternative units on page 5). Note: When the titrant is H 2 SO 4 some coefficients must be adjusted: Set the Name of the titrant and the Titrant equivalents value to 2 (refer to Mode in the User Manual). Page 3 of 10

4 4.4. Recommendations for modifications of settings Values of End points The values p ph 8.3 and TA ph 4.5 can be modified in the titration program depending on the sample: End point ph Test condition Phenolphthalein Total Alkalinity Alkalinity Alkalinity, 30 mg CaCO 3 /L Alkalinity, 150 mg CaCO 3 /L Alkalinity, 500 mg CaCO 3 /L Silicates, phosphates known or suspected Routine or automated analyses Industrial waste or complex system Modification of parameters To reduce the time of titration, a predose in volume (by default volume at 0 ml) prior to starting the titration followed by a delay of at least 5 seconds can be set. Ensure that the predose does not overshoot the end point by completely titrating the sample. 5. Procedure 5.1. Sample analysis Sampling This standard can be used with natural, drinking and wastewaters with TA between 0.4 and 20 mmol/l. The sample must not be filtered, diluted, concentrated or altered in any way. Avoid excessive agitation and prolonged exposure to air. Samples should be analyzed as soon as possible after collection but can be stored at least 24 hours by cooling to 4 C or below. Warm the sample to room temperature before analyzing Sample preparation Pipette 100 ml of water into the 150 ml-plastic beaker Analysis step Dip the electrode and delivery tip in the sample. Press Start. 6. Results 6.1. Displayed Results At the end of the titration the following results are available: 1. Value of phi 2. Value of p in mg/l as CaCO 3 (volume of titrant in ml to reach defined ph, 8.3 ph by default) 3. Value of Total Alkalinity in mg/l as CaCO 3 (volume in ml to reach defined ph, 4.5 ph by default) Note: Results in mmol/l are calculated but, by default, are hidden in the result screen. However, they are available in the Data Log (refer to the User Manual for additional information) Results calculation of Alkalinity in meq/l The results are normally expressed in mmoles/l (or meq/l) of alkalinity as described by the following relation: V(titr) C(titr) 1000 R = V(smp) V(titr) = Total volume of titrant in ml, delivered to reach the end point (e.g. ph 4.5) C(titr) = Titrant concentration in eq/l (currently 0.1 eq/l) V(smp) = Volume of the sample (currently 100 ml) With these parameters, 1 ml of 0.1 eq/l of strong acid represents 0.1 meq or mmol of alkalinity. Page 4 of 10

5 Alternative units Depending on the country, many other units can be used for the results. Factors for the conversion from millimoles per liter are given in the table below (column R x user value): R x user value Comments mmol/l CaCO CO 2-3 has 2 alkalinity functions per molecule mg/l CaCO 3 50 MW of CaCO 3 is g/mol with 2alkaline functions per molecule mg/l HCO 3 61 MW of HCO - 3 is 61 g/mol with 1 alkaline function per molecule Clark degree 3.50 German degree 2.80 French degree 5.0 U.S. degree Examples of ph-alkalinity determination The results described below are indicative and obtained for a given water type in optimized conditions respecting Good Laboratory Practices. These indicative values are sample-dependent, electrode-dependent and operating cell dependent ph Alkalinity determination of mineral water (phi < 8.3) Results for 5 determinations in milliequivalents/l: Sample: 100 ml of mineral water. Nominal concentration: 296 mg/l as CaCO 3 Settings: see values by default Number of determinations: 10 Temperature of analysis: room temperature Mean value of Total Alkalinity: mmol/l Standard deviation: mmol/l Relative standard deviation: 0.02% Curve ph versus volume of titrant: 7.2. ph Alkalinity determination of water with phi > 8.3 and high alkalinity Results for 10 determinations in milliequivalents/l: Sample: 100 ml of Na 2 CO 3 solution Nominal concentration: 250 mg/l as CaCO 3 Settings: see values by default Number of determinations: 10 Temperature of analysis: room temperature Mean value of Total Alkalinity: mmol/l Standard deviation: mmol/l Relative standard deviation: 0.26% Curve ph versus volume of titrant: 8. Bibliography Water quality - Determination of total and composite alkalinity, International standard ISO (1994) EPA method number Standard Methods For the Examination of Water and Wastewater, 22nd edition, 2012, 2-34 part 2320 Page 5 of 10

6 9. Appendices: Electrode and Titrant calibrations To increase accuracy of the measurement of ph-alkalinity of the sample, it is recommended to perform a calibration of the electrode and titrant every 7 days. Always perform a calibration for a new electrode and/or titrant Calibration of the electrode Note: In the following tables, an x in the or column indicates that the parameter can be edited in that respective. Refer also to the User Manual Electrode and reagents Combined ph Electrode with Temperature sensor: Intellical PHC805 Titrant: HCl or H 2 SO eq/l solution in deionized water IUPAC Series ph standards (500 ml): ph 4.005, ph and ph Electrode calibration Calibrate the electrode with ph 4.005, and ph IUPAC Series ph standards Electrode calibration settings By default the calibration of the electrode is done with the parameters described below: Setting Calibration frequency x x 7 [Day(s)] Stability criterion x [ph/min] Max. stability time x 300 [s] Stirring speed x x 25 [%] Buffer set x x IUPAC 1.68, 4.01, 7.00, 10.01, Results of electrode calibration At the end of the series of buffers, the results are displayed. The values given below are indicative results, obtained in optimized conditions respecting Good Laboratory Practices. These indicative values are sampledependent, electrode-dependent and operating cell dependent. Electrode PHC805 Slope at 25 C 99.2% Slope mv/ph Offset 10.5 mv 9.2. Calibration of the titrant by a primary standard: Borax Note: In the following tables, an x in the or column indicates that the parameter can be edited in that respective. Refer also to the User Manual Principle The strong acid (HCl or H 2 SO 4 ) used as titrant has to be calibrated with Borax which reacts with H + ions according to: B 4 O H + + H 2 O -> 4HBO Electrode and reagents Combined ph Electrode with Temperature sensor: Intellical PHC805 Titrant: HCl or H 2 SO eq/l solution in deionized water Standard solution: 95 mg of Borax (Na 2 B 4 O 7.10H 2 O) Deionized water Page 6 of 10

7 Default parameters The sample size and titrant concentration depend on the quality of the water. Use the application note settings described below with the following parameters: V sample = 100 ml Burette volume = 10 ml Titrant concentration = 0.1 eq/l (corresponding to a 0.1 mol/l HCl solution) Titrant calibration settings The sample size of the standard solution is set at 95 mg, completed to 100 ml with deionized water. The calibration is done using a 10-mL burette, a dynamic incremental addition and an Inflexion Point as the detection method with the parameters described below: Detection settings: Standard: Setting Stirring speed x x 25 [%] Predose x x 2 [ml] Max. vol. stop point x 8 [ml] Ordinate stop point x 2 [ph] Stop on last EQP x Yes Delay x x 5 [s] Stability criterion x [mph/min] Min. increment size x 0.02 [ml] Max. increment size x 0.8 [ml] EQP 1 min ordinate x 4.5 [ph] EQP 1 max ordinate x 6.5 [ph] Max. resolution x 4 decimals R1 limit check x Yes Min. titer x [eq/l] Max. titer x [eq/l] Setting Standard name x Borax Standard amount x 95 [mg] Min. amount x 90 [mg] Max. amount x 100 [mg] Concentration x x 99.5 [%] x [mg] Resolution x 1 decimal Standard equivalents x 2 x 1 if HCl is used Titrant equivalents 2 if H 2 SO 4 is used Molar weight x Note: When the titrant is H 2 SO 4 some coefficients must be adjusted: Set the Name of the titrant and the Titrant equivalents value to 2 (refer to Mode in the User Manual). Page 7 of 10

8 Results: At the end of the titrant calibration the titer is calculated and expressed in eq/l. The saved value will be used for the following Alkalinity titrations Procedure Setting Titer 0.1 [eq/l] A 0.1 eq/l Borax solution contains 0.05 mol/l (or 1/20 mol/l) of Borax. To calibrate the acidic titrant with the 10 ml-burette: 1. Prepare a 100-mL sample of 0.1 eq/l Borax solution by accurately weighing about 95 mg of Borax. 2. Complete to 100 ml with deionized water. 3. Dip the electrode and delivery tip in the solution. 4. Press Start Results of the titrant calibration The result is expressed as eq/l concentration of the titrant and based on the following formula: Where: [2 x m(sdt)x P% ] C(titr) = [M(sdt)x V(titr)] m(sdt) = Weighed mass of Borax in mg, about 95 mg weighed accurately M(sdt) = Molar weight of Borax, g/mol P % = Purity of the weighted powder, 99.5 % V(titr) = Total volume of titrant delivered to reach the equivalent point, in ml The calibration result can be accepted if 5 determinations give a result with a relative standard deviation of less than 0.5% Examples of titrant calibration Results for 10 determinations in milliequivalents/l The results described below are indicative and obtained for a given water type in optimized conditions respecting Good Laboratory Practices. These indicatives values are sample-dependent, electrode-dependent and operating cell dependent. Sample: 95 mg of Borax with purity 99.5% completed to 100 ml of distilled water Settings: see values by default Number of determinations: 5 Temperature of analysis: room temperature Titer of titrant: eq/l SD: eq/l RSD: 0.09 % Curve ph versus volume of titrant: Page 8 of 10

9 9.3. Calibration of the titrant by a secondary standard: NaOH Note: In the following tables, an x in the or column indicates that the parameter can be edited in that respective. Refer also to the User Manual Principle In some cases sodium hydroxide (NaOH) is used to calibrate HCl or H 2 SO 4 titrant despite the fact that NaOH is not suitable as a primary standard because it readily absorbs moisture (H 2 O) and carbon dioxide (CO 2 ) from the atmosphere. This classical reaction takes place: NaOH + H + --> H 2 O + Na Settings Detection settings: The limits of the inflexion points have to be adapted to the reaction. Standard settings for NaOH: Mode Mode Setting EQP 1 min ordinate x 6.5 [ph] EQP 1 max ordinate x 10.5 [ph] Mode Mode Setting Standard name x NaOH Standard amount x 100 [ml] Min. amount x 95 [ml] Max. amount x 105 [ml] Concentration x [%] x ml Resolution x 4 decimals Standard equivalents x 1 x 1 if HCl is used Titrant equivalents 2 if H 2 SO 4 is used Molar weight x [g/mol] Procedure To calibrate the acidic titrant with the 10 ml-burette: 1. Accurately prepare a 100 ml sample of eq/l NaOH freshly prepared solution. 2. Dip the electrode and delivery tip in the solution. 3. Press Start Results of the titrant calibration The result is expressed as eq/l concentration of the titrant and based on the following formula: Where: [ V(sdt) x C(sdt) ] C(titr) = V(titr) V(sdt) = Volume of NaOH solution in ml, ml C(sdt) = Concentration of NaOH solution in ml, eq/l V(titr) = Total volume of titrant delivered to reach the equivalent point, in ml The calibration result can be accepted if 5 determinations give a result with a relative standard deviation of less than 0.5%. Page 9 of 10

10 Examples of titrant calibration Results for 5 determinations in milliequivalents/l The results described below are indicative and obtained for a given water type in optimized conditions respecting Good Laboratory Practices. These indicatives values are sample-dependent, electrode-dependent and operating cell dependent. Titrant: H 2 SO 4 Sample: 100 ml of NaOH eq/l Settings: see values by default Number of determinations: 5 Temperature of analysis: room temperature Titer of titrant (Average): eq/l SD: eq/l RSD: % Curve ph versus volume of titrant: Note: For this curve, the calibration is stopped after a delivery of 8 ml of titrant and not after the last equivalent point is found Quality of the NaOH solution Over time, a slow degradation of the sodium hydroxide solution occurs which leads to a bad titrant calibration. As the solution absorbs carbon dioxide from the atmosphere, carbonate species appear and a second inflexion appears into the curve at low ph (between 5 and 4 ph). When such a profile is observed, the sodium solution should not be used for titrant calibration. Page 10 of 10