SIST EN 717-1:2005
(Main)Wood-based panels - Determination of formaldehyde release - Part 1: Formaldehyde emission by the chamber method
Wood-based panels - Determination of formaldehyde release - Part 1: Formaldehyde emission by the chamber method
This European Standard specifies a chamber method with three options of test chambers for the determination of the formaldehyde emission from wood-based panels in terms of the steady-state concentration in a climate chamber under defined conditions, which relate to typical conditions in real-life. This chamber method can also be applied to the estimation of formaldehyde concentrations under various conditions in practice, by the use of mathematical models.
Holzwerkstoffe - Bestimmung der Formaldehydabgabe - Teil 1: Formaldehydabgabe nach der Prüfkammer-Methode
Diese Europäische Norm legt ein Verfahren für die Bestimmung der Formaldehydabgabe von Holzwerkstoffen fest mittels der Ausgleichskonzentration in einer Klimakammer unter definierten, praxisnahen Bedingungen. Als Optionen werden drei Prüfkammertypen beschrieben. Diese Prüfkammermethode erlaubt auch die Schätzung von Formaldehydkonzentrationen unter verschiedenen Praxisbedingungen mittels mathematischer Modelle.
Panneaux a base de bois - Détermination du dégagement de formaldéhyde - Partie 1 : Emission de for-maldéhyde par la méthode a la chambre
La présente Norme européenne spécifie une méthode a la chambre avec trois options de chambre d'essai pour la détermination de l'émission de formaldéhyde des panneaux a base de bois a une concentration a l'état stable dans une chambre climatisée dans des conditions définies qui se rapportent a des conditions moyennes représentatives de la réalité courante. Cette méthode a la chambre peut aussi etre appliquée pour l'estimation des concentrations de formaldéhyde dans différentes conditions de la pratique, ceci par utilisation de modeles mathématiques.
Lesne plošče - Ugotavljanje sproščanja formaldehida - 1. del: Sproščanje formaldehida po komorni metodi
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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.RãþDQMHHolzwerkstoffe - Bestimmung der Formaldehydabgabe - Teil 1: Formaldehydabgabe nach der Prüfkammer-MethodePanneaux a base de bois - Détermination du dégagement de formaldéhyde - Partie 1 : Emission de for-maldéhyde par la méthode a la chambreWood-based panels - Determination of formaldehyde release - Part 1: Formaldehyde emission by the chamber method79.060.01Wood-based panels in generalICS:Ta slovenski standard je istoveten z:EN 717-1:2004SIST EN 717-1:2005en01-januar-2005SIST EN 717-1:2005SLOVENSKI
STANDARDSIST ENV 717-1:20001DGRPHãþD
SIST EN 717-1:2005
EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 717-1October 2004ICS 79.060.01Supersedes ENV 717-1:1998
English versionWood-based panels - Determination of formaldehyde release -Part 1: Formaldehyde emission by the chamber methodPanneaux à base de bois - Détermination du dégagementde formaldéhyde - Partie 1 : Emission de for-maldéhydepar la méthode à la chambreHolzwerkstoffe - Bestimmung der Formaldehydabgabe -Teil 1: Formaldehydabgabe nach der Prüfkammer-MethodeThis European Standard was approved by CEN on 16 August 2004.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2004 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 717-1:2004: ESIST EN 717-1:2005
EN 717-1:2004 (E) 2 Contents Page Foreword.3 Introduction.4 1 Scope.5 2 Normative references.5 3 Terms and definitions.5 4 Principle.6 5 Reagents.6 5.1 General.6 5.2 Acetylacetone solution.6 5.3 Ammonium acetate solution.6 5.4 Formaldehyde standard solution.6 6 Apparatus.7 6.1 Test chamber.7 6.2 Air sampling system.8 6.3 Equipment for chemical analysis.8 6.4 Equipment for verification of air exchange rate.9 7 Test pieces.9 8 Procedure.9 8.1 Test conditions.9 8.2 Verification of test conditions.9 8.3 Chamber preparation.10 8.4 Preparation of test pieces.11 8.5 Loading and starting procedure.11 8.6 Air sampling and analysis.11 8.7 Test duration.12 9 Determination of formaldehyde emission.12 9.1 General.12 9.2 Principle.12 9.3 Procedure.12 9.4 Calibration curve.12 9.5 Calculation of the amount of absorbed formaldehyde.13 9.6 Calculation of the formaldehyde emission.14 10 Determination of test result.14 11 Expression of results.14 12 Test report.14 Annex A (normative)
Test chambers.17 Annex B (normative)
Determination of air exchange rate.24 Annex C (normative)
Determination of the steady-state emission value.26 Annex D (informative)
Analytical procedure for the fluorimetric determination of formaldehyde
content.29 Bibliography.31
SIST EN 717-1:2005
EN 717-1:2004 (E) 3 Foreword This document (EN 717-1:2004) has been prepared by Technical Committee CEN/TC 112 “Wood-based panels”, the secretariat of which is held by DIN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by April 2005, and conflicting national standards shall be withdrawn at the latest by April 2005. This document supersedes ENV 717-1:1998. Compared to ENV 717-1:1998 the following modifications have been made. a) Conditions for a reduced test period have been specified in Clause 10. b) The requirements for the air-tightness of the test chamber in 8.2.2 have been changed. This European Standard is one of a series, which specifies methods for determining formaldehyde potential in or formaldehyde release from wood-based panels. The other standards of this series are: EN 120, Wood based panels — Determination of formaldehyde content — Extraction method called the perforator method. EN 717-2, Wood-based panels — Determination of formaldehyde release — Part 2: Formaldehyde release by the gas analysis method. EN 717-3, Wood-based panels — Determination of formaldehyde release — Part 3: Formaldehyde release by the flask method. This European Standard is based on CEN report CR 213 “Particleboards — Determination of Formaldehyde Emis-sion under Specified Conditions” and COST Project 613: Indoor Air Quality and its Impact on Man, Report No. 2: “Formaldehyde emission from wood-based materials: Guideline for the determination of steady state concentra-tions in test chambers”. This document includes a Bibliography. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following coun-tries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Esto-nia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. SIST EN 717-1:2005
EN 717-1:2004 (E) 4 Introduction Each testing institute is responsible for the quality of the test procedure. To ensure the reproducibility of the test
results, it is recommended to participate in a round-robin test for calibration at least once a year. SIST EN 717-1:2005
EN 717-1:2004 (E) 5 1 Scope This European Standard specifies a chamber method with three options of test chambers for the determination of the formaldehyde emission from wood-based panels in terms of the steady-state concentration in a climate cham-ber under defined conditions, which relate to typical conditions in real-life. This chamber method can also be ap-plied to the estimation of formaldehyde concentrations under various conditions in practice, by the use of mathe-matical models. This standard can also be used for the testing of formaldehyde emissions of products other than wood-based
panels. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 326-1, Wood-based panels — Sampling, cutting and inspection — Part 1: Sampling and cutting of test pieces and expression of test results. 3 Terms and definitions For the purposes of this European Standard, the following terms and definitions apply. 3.1 volume of the chamber total air volume of the unloaded chamber, including recirculating ventilation ducts, expressed in cubic metres (m3) 3.2 loading factor ratio of the total surface area of the test piece, excluding the area of the edges, to the volume of the chamber, ex-pressed in square metres per cubic metre (m2/m3) 3.3 air exchange rate quotient of air volume passing through the chamber per hour (m3/h) and the chamber volume (m3) 3.4 air velocity velocity of the air near the surface of test pieces in the chamber in metres per second (m/s) 3.5 steady-state state when the formaldehyde emission of the wood-based panels is quasi constant under the test conditions, this means that the formaldehyde concentration in the chamber remains constant NOTE In practice, a true steady-state is not achievable because formaldehyde is emitted irreversibly. This standard defines a steady-state condition for the purpose of the test (see Clause 10). SIST EN 717-1:2005
EN 717-1:2004 (E) 6 3.6 emission value the steady-state formaldehyde concentration in the chamber, obtained under constant temperature, relative humid-ity, loading factor and air exchange rate, expressed by mass to volume in milligrams formaldehyde per cubic metre air (mg/m3) NOTE At 23 °C and 1 013 hPa, the following relationship exists for formaldehyde:
1 ppm (parts per million) = 1,24 mg/m3
1 mg/m³ = 0,81 ppm (parts per million) 4 Principle Test pieces of known surface area, are placed in a chamber, in which the temperature, relative humidity, air velo-city and exchange rate are controlled at defined values. Formaldehyde emitted from the test pieces mixes with the air in the chamber. The air in the chamber is sampled periodically. The formaldehyde concentration is determined by drawing air from the chamber through gas washing bottles containing water, which absorbs the formaldehyde. The formaldehyde concentration in the water is determined. The concentration of formaldehyde in the chamber at-mosphere is calculated from the concentration in the water in the gas washing bottles and the volume of the sam-pled air. It is expressed in milligrams per cubic metre (mg/m3). Sampling is periodically continued until the formal-dehyde concentration in the chamber has reached a steady-state. NOTE The influences of temperature, relative humidity, loading factor and air exchange rate on the formaldehyde concen-tration in the chamber atmosphere can be described by formulas (e.g. Andersen formula). An interrelation between the structure of the test pieces, especially of their surfaces and the air velocity is also apparent but cannot be exactly described by a formula. 5 Reagents 5.1 General Reagents and water of recognised analytical purity shall be used for the analysis. 5.2 Acetylacetone solution 4 ml acetylacetone are added to a 1 000 ml volumetric flask and made up to the mark with water. 5.3 Ammonium acetate solution 200 g ammonium acetate are dissolved in water in a 1 000 ml volumetric flask and made up to the mark. NOTE Commercially prepared solutions may be used. 5.4 Formaldehyde standard solution Standard iodine solution c(l2) = 0,05 mol/l Standard sodium thiosulphate solution c(Na2S2O3) = 0,1 mol/l Standard sodium hydroxide solution c(NaOH) = 1 mol/l Standard sulphuric acid solution c(H2SO4) = 1 mol/l SIST EN 717-1:2005
EN 717-1:2004 (E) 7 6 Apparatus 6.1 Test chamber 6.1.1 General This standard applies to different test chambers for formaldehyde emission testing which are described in Annex A. General specifications and requirements which apply to all types of test chambers included in this standard are given in 6.1.2 to 6.1.8. 6.1.2 Test chamber materials Materials used for the inner walls and ducts of test chambers shall have a smooth surface, which, prior to testing, can be effectively cleaned with water. The surface shall be as inert and non-absorptive as possible to formalde-hyde. NOTE Proven materials are stainless steel or aluminium (sandblasted or polished), glass and some types of plastics (PVC, PMMA). 6.1.3 Air-tightness of the chamber The test chamber shall be air-tight in order to avoid uncontrolled air exchange. The criteria of air-tightness are given in 8.2.2. 6.1.4 Air circulation in the test chamber The test chamber shall contain facilities (such as fan systems) capable of maintaining: intensive air mixing in the chamber; an air velocity of 0,1 m/s to 0,3 m/s at the surface of the test pieces (see 8.2.5). 6.1.5 Air exchange facilities The test chamber shall contain air inlet and/or outlet facilities capable of regulating the air flow and thus the rate of air exchange (replacement of chamber air by clean, conditioned air) with an error limit of 5 % at an air exchange rate of 1/h. Precautions shall be taken to ensure that the clean air inlet and the air circulation system are adequately placed to ensure sufficient mixing and that ambient air cannot enter into the air outlet, even during sampling. 6.1.6 Clean air supply of the test chamber Equipment capable of providing clean air with a maximum formaldehyde content of 0,006 mg/m3 (0,005 ppm). SIST EN 717-1:2005
EN 717-1:2004 (E) 8 6.1.7 Temperature and relative humidity regulating systems Equipment capable of maintaining the temperature and the relative humidity in the test chamber within the following limits: Temperature: (23 ± 0,5) °C Relative humidity: (45 ± 3) % 6.1.8 Equipment for monitoring of test conditions Measuring equipment and recording facilities capable of continuous or frequent monitoring of the specified test conditions with an error limit as follows: Temperature: 0,1 °C Relative humidity: 1 % Air exchange rate: 0,03/h Air velocity: 0,05 m/s 6.2 Air sampling system 6.2.1 General Figure 1 shows the principle of a sampling system for the determination of the formaldehyde concentration in the chamber air. The sampling tube shall be placed either in the air outlet, or inside the chamber, close to the air outlet. NOTE Other sampling systems may be used, provided it can be shown that they give equivalent results. 6.2.2 Equipment 6.2.2.1 General The air sampling system consists of the following components which are given in Figure 1. The numbers in brack-ets refer to the numbers in Figure 1: 6.2.2.2 Sampling tube (1) 6.2.2.3 Two 100 ml gas washing bottles, containing water, for absorption and subsequent determination of formaldehyde (2) 6.2.2.4 Silica absorber for drying the air (3) 6.2.2.5 Gas flow valve (4) 6.2.2.6 Gas sampling pump (5) 6.2.2.7 Gas flow meter (6) 6.2.2.8 Gas meter (including a thermometer) for measuring the volume of air (7) 6.2.2.9 Air pressure meter (8) 6.3 Equipment for chemical analysis 6.3.1 Spectrophotometer, suitable for use with cells with a path-length of at least 50 mm and capable of measur-ing absorbance at 412 nm SIST EN 717-1:2005
EN 717-1:2004 (E) 9 6.3.2 Water bath, capable of maintaining a temperature of (40 ± 1) °C 6.3.3 Six volumetric flasks, 100 ml (calibrated at 20 °C) 6.3.4 Two volumetric flasks, 1 000 ml (calibrated at 20 °C) 6.3.5 Bulb pipettes, 5 ml, 10 ml, 15 ml, 20 ml, 25 ml, 50 ml, and 100 ml (calibrated at 20 °C) 6.3.6 Microburette 6.3.7 At least three flasks, 50 ml, with stoppers 6.3.8 Balance, scale interval 0,001 g 6.4 Equipment for verification of air exchange rate 6.4.1 Compressed-gas cylinder with tracer-gas 6.4.2 Detector for continuous monitoring of tracer-gas 6.4.3 Recorder NOTE Dinitrogen monoxide (N2O) with infrared (IR) detection has proved to be suitable. 7 Test pieces Sample the test pieces according to the general principles of EN 326-1 and cut them to a size which corresponds to a total loading rate of 1 m²/m³ (see Annex A). Wrap the test pieces hermetically immediately after cutting and leave them wrapped until the start of the test. 8 Procedure 8.1 Test conditions The following conditions shall be maintained in the chamber throughout the test: Temperature (23 ± 0,5) °C; Relative humidity (45 ± 3) %; Loading factor (1,0 ± 0,02) m²/m³; Air exchange rate (1,0 ± 0,05)/h; Air velocity at the surface of the test pieces (see 8.2.5) (0,1 to 0,3) m/s. 8.2 Verification of test conditions 8.2.1 Clean air supply of the test chamber When determined in accordance with Clause 9, the formaldehyde content of the air supplied to the chamber shall not exceed 0,006 mg/m3 (0,005 ppm). 8.2.2 Air-tightness of the test chamber In order to avoid uncontrolled air exchange by intrusion of ambient air, the test chamber shall be operated at a slight over-pressure. SIST EN 717-1:2005
EN 717-1:2004 (E) 10 Air-tightness shall be checked regularly, either by pressure drop measurements or by comparison of simultaneous measurement of flow rates at the inlet and the outlet ports, or by measuring tracer gas dilution. The test chamber is considered sufficiently air-tight if at least one of the following requirements is fulfilled: the air leakage is less than 10− 2 × chamber volume per minute at an overpressure of 1 000 Pa; the inlet and outlet air flow differ by less than 2 %; the tracer gas dilution is less than 0,05/h. 8.2.3 Temperature and relative humidity control system The temperature shall be controlled either by placing the test chamber within a location controlled to the appropri-ate temperature, or by controlling the temperature within the chamber. In the latter case, the chamber walls shall be insulated effectively to avoid condensation of moisture on their inner surface. Control of relative humidity shall be made either by external humidity control of the clean air supply, or internal hu-midity control of the air within the chamber. In the latter case, precautions shall be taken to avoid condensation, or spray of water, inside the chamber. Temperature and relative humidity shall be monitored either continuously, or frequently, and independently of the air conditioning system. Sensors shall be placed in a representative position inside the chamber. After loading the chamber, any initial deviations of temperature and relative humidity due to ambient air and uncon-ditioned test pieces shall be recorded. 8.2.4 Air exchange The clean and conditioned air supply to the chamber shall either be monitored continuously, or frequently. Suitable methods are specified in Annex A. The air exchange rate shall not vary by more than 0,05 exchanges of air per hour. The effective air exchange rate shall be regularly checked, by using either a calibrated gas meter, or the tracer gas procedure described in Annex B. 8.2.5 Air velocity in the chamber Prior to testing, the air velocity in the test chamber loaded with test pieces shall be set to a value between 0,1 m/s and 0,3 m/s, measured at representative positions not more than 20 mm from the surface of the test pieces. NOTE Hot wire or film anemometers calibrated in the range of 0 m/s to 0,5 m/s are suitable for air velocity measurement.
The positions of the measuring points depend upon the volume of chamber and the type of air flow. Air velocity shall be measured at a minimum of 4 positions in large chambers (≥ 12 m3) and at a minimum of 2 positions in small chambers (1 m³ or 0,225 m³). 8.2.6 Performance of the chamber The performance of the test chamber can be tested by a procedure described by [7]. 8.3 Chamber preparation Set the chamber to the conditions given in 8.1. The determination (see 8.6) of the formaldehyde concentration in the empty chamber (“background-level”) shall be carried out not less than 1 h after establishing the test conditions according to 8.1. SIST EN 717-1:2005
EN 717-1:2004 (E) 11 If the formaldehyde concentration in the chamber atmosphere is 0,006 mg/m³ or lower, the chamber can be loaded with the test pieces. If the formaldehyde concentration is higher than 0,006 mg/m3, the chamber shall be purged by running empty to reduce the formaldehyde concentration to the accepted background level of 0,006 mg/m3. If necessary, dust and other particles from the bottom shall be removed and the walls and other interior surfaces of the chamber shall be cleaned before starting the air cleaning procedure. 8.4 Preparation of test pieces 8.4.1 General Unwrap the test pieces, seal the edges if necessary and place the test pieces in the chamber. The ratio of the length of open (unsealed) edges U related to the surface area A shall be U/A = 1,5 m/m2. NOTE As a result of the constant ratio U/A = 1,5 m/m2 the percentage of open edges area related to the surface area de-pends on the thickness of the test piece, as shown in the following examples: Panel thickness Ratio of the open edge area to total surface area 10 mm 19 mm 32 mm 1,5 % 2,8 % 4,8 %
8.4.2 Large chambers (see A.1) Edge sealing shall not be done for the 1 m × 2 m test pieces tested in large chambers (≥ 12 m3). The perimeter, i. e the length of open (unsealed) edges U for 1 m × 2 m pieces is 6 m and the surface area A is 4 m2. Thus the ratio of U/A is 1,5 m/m2. 8.4.3 Small chambers (see A.2 and A.3) In order to obtain the same ratio of U/A = 1,5 m/m2 for smaller test pieces in small chambers, partial edge sealing is necessary. This sealing should be done using self-adhesive aluminium tape. NOTE The length of edges to be sealed is given in A.2.3 for 1 m3 chambers and in A.3.3 for 0,225 m3 chambers. 8.5 Loading and starting procedure Place the test pieces in the test chamber. They shall be vertical and approximately in the centre of the chamber, with their surfaces parallel to the direction of the air flow, and separated by not less than 200 mm (see Figures A.1 to A.5 and A.7). The first air sampling shall be made not less than 3 h after loading the chamber and starting the test procedure. 8.6 Air sampling and analysis Add at least 25 ml of water to each of the two gas washing bottles and connect them to the apparatus (see 6.2 and Figure 1). Sample the air from the chamber periodically by passing a minimum of 120 l, at a rate of approximately 2 l/min, through the gas washing bottles. Pipette 10 ml of each of the absorption solutions into a 50 ml flask and add 10 ml acetylacetone solution (see 5.2) and 10 ml of ammonium acetate solution (see 5.3). Stopper the flasks and determine the formaldehyde content of the solutions according to Clause 9. NOTE The volume of air to be sampled depends on its formaldehyde concentration. With photometric determination the procedure described above is suitable for concentrations higher than 0,005 mg/m3. For determination of lower concentrations, the volume of the sampled air should be increased and/or the volume of the air sample solution reduced. The sensitivity of the analysis can also be increased by using a fluorimetric determination of the reaction product (diacetyldihydrolutidine) instead of a photometric determination.
SIST EN 717-1:2005
EN 717-1:2004 (E) 12 8.7 Test duration The sampling procedure is repeated twice a day until enough data are available to calculate the steady-state (see Annex C). The time interval between the two samplings shall be more than 3 h. 9 Determination of formaldehyde emission 9.1 General The formaldehyde content of the aqueous solutions from each sampling period shall be determined photometrically by the acetylacetone method. NOTE 1 To enhance the sensitivity of the analysis, the formaldehyde content can also be determined fluorimetrically (proce-dure see Annex D). NOTE 2 The determination according to ISO 16000-3 (DNPH) can also be used. 9.2 Principle The determination is based on the Hantzsch reaction in which formaldehyde reacts with ammonium ions and acety-lacetone to yield diacetyldihydrolutidine (DDL) (see Figure 2). DDL has an absorption maximum at 412 nm. The re-action is specific to formaldehyde. 9.3 Procedure Heat the stoppered flasks (see 8.6) for 15 min in a water bath at (40 ± 1) °C. The solutions are then stored at room temperature for 1 h, protected against the influence of light. The absorbance of the solutions is determined at a wavelength of 412 nm against water, using a spectrophotometer with a cell of 50 mm path-length (see 6.3.1). A blank value is determined in parallel, using a solution made up of 10 ml water, 10 ml acetylacetone solution (see 5.2) and 10 ml ammonium acetate solution (see 5.3) and is taken into consideration when calculating the formal-dehyde content of the sample solution. NOTE Cells with greater path-length can be used, but will give different calibration curves and in this case a modified evaluation procedure is necessary. 9.4 Calibration curve 9.4.1 General The calibration curve (see Figure 3) is produced from a standard formaldehyde solution, the concentration of which has been determined by iodometric titration. This calibration curve shall be checked at least once a week. 9.4.2 Formaldehyde standard solution Reagents: Standard iodine solution c(I2) = 0,05 mol/l Standard sodium thiosulphate solution c(Na2S2O3) = 0,1 mol/l Standard sodium hydroxide solution c(NaOH) = 1 mol/l Standard sulphuric acid solution c(H2SO4) = 1 mol/l The above solutions shall be standardized before use starch solution 1 % by mass SIST EN 717-1:2005
EN 717-1:2004 (E) 13 Dilute about 1 g formaldehyde solution (concentration 35 % to 40 %) with water in a 1 000 ml volumetric flask and make up to the mark. Determine the exact formaldehyde concentration as follows: Mix 20 ml of the formaldehyde standard solution with 25 ml iodine solution and 10 ml sodium hydroxide solution. After 15 min standing protected from light add 15 ml of sulphuric acid solution. Tritrate back the excess iodine with the sodium thiosulphate solution
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