Application Note No. 072/2012

Transcription

1 Application Note No. 07/01 Total SO in Wine Distillation Unit K-355: Interlaboratory Test for the Determination of Total SO in Wine

2 1. Introduction An interlaboratory test was carried out involving four different laboratories in Switzerland in order to evaluate the reproducibility and repeatability of the novel BUCHI SO Method [1]. The laboratories were asked to strictly follow the procedure described in the BUCHI Application Note 065/011 'Determination of Total SO in Wine - A Comparison with the OIV SO Method ' []. The task given to the laboratories was to carry out the calibration of the individual distillation units using a stabilized standard solution prepared beforehand and shipped to the participants. A white 'Fendant' wine and a red 'Gamay' wine were provided for the analysis in triplicates. All wine samples were determined within the time frame of March 10 th -15 th 011. Together with the 4 laboratories applying the novel BUCHI SO Method one laboratory tested the samples also according to the official OIV Method for SO determination in wine [3].. Equipment Distillation Unit, preferably K-355 or K-360 SO absorption glass (BUCHI part no ) Titrator capable of doing redox titrations (example: Metrohm Titrino Plus 877) or burette for manual titration Redox electrode (example: Metrohm Pt-Titrode ) or suitable indicator for manual titration Volumetric pipettes Titration vessels, e.g. glass beakers 500 ml 3. Chemicals and Materials Chemicals: Ethanol purum, Fluka (0875) Sodium sulfite, Sigma Aldrich (71989) Citric acid monohydrate, Fluka (7490) Ethanal (acetaldehyde), Fluka (00070) (store in refrigerator) Iodine standard solution 0.05 mol/l, Riedel-deHaën (35090) Na S O 3 standard solution 0.01 mol/l, Riedel-deHaën (3843) Ortho-phosphoric acid (85%), Riedel-deHaën (30417) Methanol, Sigma Aldrich (313) Sulfuric acid 0.5 mol/l, Riedel-deHaën (35354) Starch from potato, Sigma Aldrich (S004) Ethanal solution: Pipette ml ethanal into a volumetric flask of 1 L which contains approx. 100 ml of deionized water. Fill up to the mark with deionized water. Stabilized standard solution of 164 mg SO /L: 10 ml ethanol, 33 mg Na SO 3,.03 g citric acid monohydrate and 30 ml of ethanal solution are placed into a volumetric flask of 1 L and filled up to the mark with deionized water. Provided to participants. 1 % ethanol solution: Pour 10 ml of ethanol into a 1 L volumetric flask and fill up to the mark. Acid mixture: For the preparation of approx. 1 liter mix 500 ml methanol, 400 ml of deionized water and 50 ml o-phosphoric acid 85% in a measuring cylinder of 1 L. For a safe handling please pay attention to all corresponding MSDS! Application Note 07/01 March 01 /17

3 Samples: Fendant white wine test sample Gamay red wine test sample 4. Procedure The determination of SO in wine includes the following steps: Method description Calibration Distillations and Titrations 4.1. Method description The method description shows how the participating laboratories were instructed to perform the experiments and which parameters were expected to be reported Receiving Vessel It is recommended to use the BUCHI SO accessory to collect the distillate (Fig. 1). All participating laboratories were provided with a BUCHI SO accessory. Fig. 1: BUCHI SO accessory consisting of a larger closed vessel and a second one which is open. The two vessels are connected by a duct. Add 5 ml 0.05 mol/i and 30 ml H O to the larger and 30 ml of ethanol to the smaller vessel. Iodine vapors are absorbed almost quantitatively by ethanol. After the distillation the contents of the two vessels are combined in a beaker and the residual iodine is back-titrated Quality Check of Iodine Solution Use iodine solution from the same bottle for the determinations of blanks, standards and samples to avoid a quality check. If a fresh bottle needs to be opened the titers of the two iodine solutions need to be determined and reported Conditioning the Distillation Unit The distillation unit is conditioned by distilling approx. 50 ml of methanol at a steam power of 100% for 5:40 minutes. Application Note 07/01 March 01 3/17

4 Adjusting Distillation Time Immediately after the conditioning the time needed to fill the first receiving vessel up to the ground joint has to be determined and reported. Add 50 ml of acid mixture to the sample tube Add 50 ml of 1% ethanol solution to sample tube Fill in 35 ml of dist. water to the first receiving vessel Fill in 30 ml of ethanol to the second receiving vessel Set the distillation time to 6:00 minutes Write down time needed to fill the first receiver and stop distillation With the BUCHI SO accessory a typical distillation time would be in a range of 5-6 minutes. For the following distillations set the distillation time to the evaluated time. 4.. Calibration The calibration is based on different volumes of stabilized SO standard solution for which the concentration is given by the weighed amount of mg sodium sulfite (Na SO 3 ) of a purity = 98.4 % dissolved in a 3 L volumetric flask leading to c(so ) stock = 16.0 mg/l. c(so ) stock is calculated according to equation (1). The amounts of SO associated with the corresponding added volumes of standard stock solutions W(SO ) standard is calculated by means of equation (). V T calib is determined by titration and W(SO ) calib calculated by equation (3). Results of the individual laboratories are shown in Tables 1-4. By means of a volumetric pipette add 5 ml of 0.05 mol/l iodine solution into the first receiving vessel of the BUCHI SO absorption glass. Add 30 ml of deionized water to the first receiving vessel and 30 ml of ethanol to the second one. Insert glass stopper connecting the two vessels and make sure the ends of the two plastic tubes are immersed in the liquids. Connect the SO accessory to the outlet of the cooler. In consecutive distillations and titrations pipette 6-7 different volumes in a range of 5 to 50 ml stabilized SO standard solution to the sample tube and add 50 ml of acid mixture. Compensate for the difference between 50 ml and the used volumes of standards by adding 1% (v/v) ethanol solution, e.g. 10 ml standard + 40 ml 1% (v/v) ethanol solution. Attach the sample tube to the distillation unit and distil for the adjusted distillation time. Typically two distillations are sufficient but it is recommended to add distillations if the relative standard deviations are >3.5%. The linear regressions lead to linear equations specific to the distillation units with specific slopes a and intercepts b (Figures - 5) Lab 1 (Automated redox titration with titrator) Table 1: Calibration Data Lab 1 (n = ) V standard [ml] W(SO ) calib [mg] W(SO ) standard [mg] Application Note 07/01 March 01 4/17

5 W(SO ) standard [mgso ] Calibration of SO Determination Using SO -Standard of 16.0 mg/l Lab 1 y = x R² = W(SO ) calib [mgso ] Fig. : Calibration by linear correlation of data obtained at Lab 1. Evaluation of linear equation with slope a = and intercept b = mg SO (specific to K-355 at Lab 1) 4... Lab (Automated redox titration with titrator) Table : Calibration Data Lab (n = ) V standard [ml] W(SO ) calib [mg] W(SO ) standard [mg] Calibration of SO Determination Using SO -Standard of 16.0 mg/l Lab W(SO ) standard [mgso ] y = x R² = W(SO ) calib [mgso ] Fig. 3: Calibration by linear correlation of data obtained at Lab. Evaluation of linear equation with slope a = and intercept b = mg SO (specific to K-355 at Lab). Application Note 07/01 March 01 5/17

6 4..3. Lab 3 (Manual redox titration with starch indicator) Table 3: Calibration Data Lab 3 (n = ) V standard [ml] W(SO ) calib [mg] W(SO ) standard [mg] Calibration of SO Determination Using SO -Standard of 16.0 mg/l Lab 3 W(SO ) standard [mgso ] y = x R² = W(SO ) calib [mgso ] Fig. 4: Calibration by linear correlation of data obtained at Lab 3. Evaluation of linear equation with slope a = and intercept b = mg SO (specific to K-355 at Lab 3) Lab 4 (Manual redox titration with starch indicator) Table 4: Calibration Data Lab 4 (n = ) V standard [ml] W(SO ) calib [mg] W(SO ) standard [mg] Application Note 07/01 March 01 6/17

7 10 Calibration of SO Determination Using SO -Standard of 16.0 mg/l Lab 4 W(SO ) standard [mgso ] y = 1.137x R² = W(SO ) calib [mgso ] Fig. 5: Calibration by linear correlation of data obtained at Lab 4. Evaluation of linear equation with slope a = and intercept b = mg SO (specific to K-355 at Lab 4). The squared Pearson coefficients R for the individual calibrations demonstrate a good correlation found in the linear regressions evaluated for each of the participating laboratories. The variations of the intercepts b, with positive and negative values, represent offsets in SO results which are specific to individual distillation units. In order to allow comparing results of different distillation units and different laboratories it is mandatory to apply the complete linear correlation equation and not omit the intercept b for any reason Distillations and Titrations Blanks Start with the triplicate blank determination immediately after the conditioning or adjustment. Pipette 5 ml of 0.05 mol/l iodine solution into the first receiving vessel Add 30 ml of dist. water to the first receiving vessel Add 30 ml of ethanol to the second receiving vessel Insert glass stopper connecting the two vessels and make sure the ends of the two plastic tubes are immersed into the liquids Connect the SO accessory to the outlet of the cooler Add 50 ml of acid mixture to the sample tube Pour 50 ml of 1 % ethanol solution into the sample tube Attach sample tube to distillation unit Distil for the time set in the adjustment step Combine the contents of the two receiving vessels in a 500 ml beaker and carefully rinse the stopper and the two vessels three times with deionized water. Acidify the combined distillates with ml of 0.5 mol/l sulfuric acid Titrate with 0.01 mol/l thiosulfate standard solution Report volumes of titrant used for blanks. Typical titrant consumptions for the blanks are in a range of ml and the standard deviation should not exceed sd > 0.7 ml. Application Note 07/01 March 01 7/17

8 4.3.. Standards Continue with the determinations of the standards immediately after the blank determinations. Pipette 5 ml of 0.05 mol/l iodine solution into the first receiving vessel Add 30 ml of dist. water to the first receiving vessel Add 30 ml of ethanol to the second receiving vessel Insert glass stopper connecting the two vessels and make sure the ends of the two plastic tubes are immersed into the liquids Connect the SO accessory to the outlet of the cooler Add 50 ml of acid mixture to the sample tube In 6 consecutive duplicate distillations add Standard 1: 5 ml stabilized SO standard + 45 ml 1% ethanol Standard : 10 ml stabilized SO standard + 40 ml 1% ethanol Standard 3: 15 ml stabilized SO standard + 35 ml 1% ethanol Standard 4: 0 ml stabilized SO standard + 30 ml 1% ethanol Standard 5: 30 ml stabilized SO standard + 0 ml 1% ethanol Standard 6: 50 ml stabilized SO standard Attach the sample tube to the distillation unit Distil for the time set in the adjustment step Combine the contents of the two receiving vessels in a 500 ml beaker and carefully rinse the stopper and the two vessels three times with deionized water. Acidify the combined distillates with ml of 0.5 mol/l sulfuric acid Titrate with 0.01 mol/l thiosulfate standard solution Report volumes of titrant used for standards. Typically two distillations are sufficient but it is recommended to carry out one more distillation if the relative standard deviation of the titrant volumes is > 1.5% Wine samples Continue with the determinations of the wine samples immediately after the determinations of the standards. If distillations were interrupted the experiments can be continued after conditioning the distillation unit by distilling methanol again. Pipette 5 ml of 0.05 mol/l iodine solution into the first receiving vessel Add 30 ml of dist. water to the first receiving vessel Add 30 ml of ethanol to the second receiving vessel Insert glass stopper connecting the two vessels and make sure the ends of the two plastic tubes are immersed into the liquids Connect the SO accessory to the outlet of the cooler Add 50 ml of acid mixture to the sample tube Add 50 ml of wine to the sample tube Attach the sample tube Distil for the time set in the adjustment step Combine the contents of the two receiving vessels in a 500 ml beaker and carefully rinse the stopper and the two vessels three times with deionized water. Acidify the combined distillates with ml of 0.5 mol/l sulfuric acid Titrate with 0.01 mol/l thiosulfate standard solution Report volumes of titrant used for wine samples Application Note 07/01 March 01 8/17

9 5. Calculation The participating laboratories did not have to carry out the calculations but delivered the following results: 1. Start and end dates of the analyses. Distillation Unit / Accessory 3. Quality check of iodine solution if bottle of iodine needs to be replaced in process 4. Conditioning of distillation unit (date and time) 5. Adjusting distillation time (date and time) 6. Blanks (three determinations, standard deviation should not exceed 0.7 ml, date and time). Report consumption of thiosulfate standard. 7. Standards (two determinations, relative standard deviation should not exceed 1.5%, date and time). 5, 10, 15, 0, 30, 50 ml of standard pipetted into sample tube for SO determination. Report consumption of thiosulfate standard. 8. Wine samples (3 determinations, relative standard deviation should not exceed 1.5%). Report consumption of thiosulfate standard. Based on the data delivered by the participating laboratories the results were calculated by the BUCHI laboratory using the following equations in an Excel template specially designed for the data evaluation in iodine-thiosulfate back-titrations. Equations c(so ) stock m = standard p(na SO ) M(SO ) M( Na SO ) V 3 3 stock (1) W(SO ) standard c(so) stock Vstandard = () 1000 T T T (Vblank -Vcalib) c M(SO) W(SO) calib = (3) z W(SO ) sample T T T (Vblank -Vsample) c M(SO) = (4) z c(so ) W(SO ) 1000 sample sample = (5) Vsample W(SO) = a W(SO) b (6) sample + c(so ) W(SO = (7) V ) sample 1000 c(so ) stock : Concentration of stabilized SO standard [mg SO /L] m standard : Weight of Na SO 3 in standard solution [g] p(na SO 3 ) : Purity of sodium sulfite i [%] M(SO ) : Molar mass SO = [g/mol] M(Na SO 3 ) : Molar mass Na SO 3 = [g/mol] V stock : Volume of standard stock solution [L] W(SO ) standard : Weight of SO calculated from V standard [mg SO ] V standard : Volume of stabilized SO standard [ml] W(SO ) calib : Determined weight of SO in standard calibration [mg SO ] Application Note 07/01 March 01 9/17

10 V T blank V T calib c T : Consumption of thiosulfate solution for blank [ml] : Consumption of thiosulfate solution for standard [ml] : Concentration of thiosulfate standard [mol/l] z : Redox valency of thiosulfate = W(SO ) sample : Determined weight of SO [mg SO ] V T sample : 0.01 mol/l thiosulfate consumption for sample [ml] c(so ) sample : Determined SO concentration in sample (by BUCHI SO method) [mg SO /L] V sample : Sample volume [ml] c(so ) OIV : Determined SO concentration in sample (by DTNB SO method) [mg SO /L] W(SO ) : Weight of SO [mg SO ] a : Slope of linear regression b : Intercept of linear regression [mg SO ] c(so ) : SO concentration of sample [mg SO /L] 6. Results 6.1. Data Evaluation The results of the total SO contents of the wine samples are given for triplicate determinations as carried out by means of the BUCHI Template 'Titrations V 3.01.XLT' which allow to take the individual 'slopes a' and the 'intercepts b' into account ii. The results of three SO determinations together with standard deviations are presented in Tables 5-1. For every lab the results include results of total SO determinations of a white Fendant wine of 18.4 (sd = 0.8) mg SO /L and of a red Gamay wine of 71.9 (sd = 0.9) mg SO /L determined in triplicate by means of the OIV SO Method [3] (see Table 13 and Table 14) Lab 1 (Automated redox titration with titrator) Table 5: Results of Total SO Determinations for Fendant at Lab 1 (n = 3). Application Note 07/01 March 01 10/17

11 Table 6: Results of Total SO Determinations for Gamay at Lab 1 (n = 3) Lab (Automated redox titration with titrator) Table 7: Results of Total SO Determinations for Fendant at Lab (n = 3). Application Note 07/01 March 01 11/17

12 Table 8: Results of Total SO Determinations for Gamay at Lab (n = 3) Lab 3 (Manual redox titration with starch indicator) Table 9: Results of Total SO Determinations for Fendant at Lab 3 (n = 3). Application Note 07/01 March 01 1/17

13 Table 10: Results of Total SO Determinations for Gamay at Lab 3 (n = 3) Lab 4 (Manual redox titration with starch indicator) Table 11: Results of Total SO Determinations for Fendant at Lab 4 (n = 3). Application Note 07/01 March 01 13/17

14 Table 1: Results of Total SO Determinations for Gamay at Lab 4 (n = 3). 6.. Interpretations of the data evaluation In Table 13 and Table 14 the results of the total SO determinations by means of the BUCHI SO Method are compared to the respective total SO contents determined by means of the OIV SO Method. The determination of the total SO content of the stabilized standard solution by means of the OIV SO Method yielded c(so ) OIV = mg SO /L (n = ) which is lower than the effective SO content c(so ) stock = mg SO /L as calculated by equation (1). Consequently the results of the wines determined by the BUCHI SO Method are higher than the ones found by the OIV SO Method iii. Table 13: Comparison of SO contents in white Fendant wines determined at Labs 1 4 with the results obtained by means of the OIV SO Method and calculation of averages and standard deviations in intra- (r) and inter-laboratory (R) surveys. The largest value (bold) of the respective standard deviations s r and S R were used in the estimations of the repeatability r and the reproducibility R summarized in Table 15. Fendant (50 ml sample) OIV Lab 1 Lab Lab 3 Lab 4 Av. Lab 1-4 c(so ) [mg/l] Standard Deviation S R [mg/l] Average r Standard Deviation s r Application Note 07/01 March 01 14/17

15 Table 14: Comparison of SO contents in red Gamay wines determined at Labs 1 4 with the results obtained by means of the OIV SO Method and calculation of averages and standard deviations in intra- (r) and inter-laboratory (R) surveys. The largest value (bold) of the respective standard deviations s r and S were used in the estimations of the repeatability (r) and the reproducibility (R) summarized in Table 15. Gamay (50 ml sample) OIV Lab 1 Lab Lab 3 Lab 4 Av. Lab 1-4 c(so ) [mg/l] Standard Deviation S R [mg/l] Average r Standard Deviation s r The repeatability (r) and the reproducibility (R) of the OIV SO Method are defined and described in the literature [3]. In Table 15 the repeatability (r) and the reproducibility (R) for samples > 50 mg SO /L (0 ml wine) and for samples < 50 mg SO /L (50 ml wine) are given (green) and compared to r and R calculated accordingly for the results given in Tables 13 and 14 for Lab 1 4 with 50 ml wine samples. The largest S r 1.5 and 1.1 mg SO /L of 50 ml wine samples lead to repeatability limits (r) of 4.1 for Fendant and 3.1 mg SO /L for Gamay wine. The largest S R.3 and.7 mg SO /L of 50 ml wine samples lead to reproducibility limits (R) of 6.5 and 7.5 mg SO /L for Fendant and Gamay wines. Table 15: Comparison of repeatability (r) and reproducibility (R) for BUCHI SO Method (yellow) and OIV SO Method (green). The two wines contained >50 mg SO/l and 50 ml wine samples were used. Sample Identification OIV-MA-INT-04 >50 mgso /l OIV-MA-INT-04 <50 mgso /l Fendant Repeatability standard deviation (S r )[mgso /l] Repeatability limit (r)(.8 x S r )[mgso /l] Reproducibility standard deviation (S R )[mgso /l].3.7 Reproducibility limit (R) (.8xS R )[mgso /l] Gamay In Figures 6 and 7 the experimental results given in Tables 13 and 14 for Fendant and Gamay wines are displayed in bar charts comparing OIV SO results with the ones obtained from the participating laboratories. The error indicators given in Figures 6 and 7 refer to the repeatability (r) calculated for the standard deviations s r as given in Tables 13 and 14. For wine samples containing total SO at levels lying within the calibration range the results are repeatable in the specified laboratories within the error range r = 4.1 mg SO /L for Fendant and 3.1 mg SO /L for Gamay and reproducible over all participating laboratories within the error range R = 6.5 mg SO /L for Fendant and 7.5 mg SO /L for Gamay. 140 Fendant c(so ) [mg/l] OIV Lab 1 Lab Lab 3 Lab 4 Fig. 6: Comparison of SO contents in white Fendant wine found by applying the novel BUCHI SO Method at 4 different Swiss labs (yellow) with the result obtained by using the OIV SO Method (green). The graphical representation corresponds to the data given in Table 13. The small and connected error indicators ( ) refer to the respective repeatability (r) and the separated indicators ( ) relate to the reproducibilities (R) of 9 mg SO /L for OIV and 6.4 mg SO /L for Lab 1 4. Application Note 07/01 March 01 15/17

16 Gamay 90.0 c(so ) [mg/l] OIV Lab 1 Lab Lab 3 Lab 4 Fig 7: Comparison of SO contents in red Gamay wine found by applying the novel BUCHI SO Method at 4 different Swiss labs (yellow) with the result obtained by using the OIV SO Method (green). The graphical representation corresponds to the data given in Table 14. The small and connected error indicators ( ) refer to the respective repeatability (r) and the separated indicators ( ) relate to the reproducibilities (R) of 9 mg SO /L for OIV and 7.5 mg SO /L for Lab 1 4. Results obtained by the OIV SO Method (green bars) and the ones found by means of the BUCHI SO Method are comparable with one another within the overlapping error limits specified for the two methods according to Table Comparison to Standard Methods The International Organization of Vine and Wine (Organization Internationale de la Vigne et du Vin = OIV) has published the 'Compendium of International Methods of Wine and Must Analysis' which provides standard procedures for various constituents in wine including the widely accepted standard for the analysis of Total SO in wine [3]. In Table 16 the OIV SO Method is compared to the BUCHI SO Method. Table 16: Comparison of the OIV SO Method to the BUCHI SO Method Deviation OIV SO Method BUCHI SO Method SO entrainment N -stream Steam Acidification phosphoric acid Methanol / H O / phosphoric acid Distillation time 15 min 5-7 min Solution in receiver H O Aqueous I -solution Titration type Acid-base Redox back titration Titrant NaOH Thiosulfate Titrated analyte H SO 4 I 8. Conclusion The new BUCHI SO Method for the determination of total SO in wine reproduces the SO result obtained by means of the OIV SO Method which to date is the widely accepted standard method and shows comparable repeatability (r) and reproducibility (R) parameters. Application Note 07/01 March 01 16/17

17 9. Acknowledgement The author would like to express his gratitude to Mr. Kay Pfluger for his greatly appreciated support in carrying out total SO determinations by the OIV SO Method at the Labs of SCHENK SA in Rolle. 10. References [1] Verfahren und Vorrichtung zur SO -Gehalts-bestimmung in Getränken und Lebensmitteln, European Patent EP [] BUCHI Application Note 065/011 Determination of total SO in Wine - A Comparison with the OIV SO Method [3] Compendium of International Methods of Wine and Must Analysis, Edition 011, Volume, OIV-MA-AS33-04A and OIV-MA-AS33-04B. Operation Manual Distillation Unit K-355 i Purity of 98.4 % according to Certificate of Analysis is from Sigma-Aldrich for sodium sulfite lot BCBC304 determined by redox titration. ii Excel template `Titrations.xls` is available on request from BUCHI Headquarters in Flawil Switzerland. Explanations to template as used in Tables 6-1: Recovery Calculation is not used. Linear corrections for SO is applied using the calibration data given in Fig. -5. The three individual results of the total SO determinations carried out by means of the OIV SO Method are shown in the captions of the table of results for sample 1-3. iii A linear relationship between results obtained by the OIV SO Method and the ones obtained by the BUCHI SO Method can be established. Application Note 07/01 March 01 17/17