GB/T 5534-2024Animal and vegetable fats and oils - Determination of saponification value (English PDF)

1 Scope

The document describes the method for determining the saponification value of animal and vegetable fats and oils. The saponification value is an indicator of the content of free fatty acids and glycerides in an oil or a fatty acid.

The document applies to refined animal and vegetable fats and oils and to crude animal and vegetable oils.

The document does not apply to products containing inorganic acids, unless the inorganic acids can be determined separately.

The saponification value can also be obtained by calculation from the fatty acid data gained by the gas chromatographic analysis shown in Annex B; for such a calculation it has to be made sure that the sample contains no impurities and no thermal degradation products.

3 Terms and definitions

3.1 Saponification value, symbol IS: the number of milligrams of potassium hydroxide needed to saponify one gram of fat or oil.

4 Principle

The test sample is saponified by boiling it under reflux with an excess of potassium hydroxide in ethanol solution, and the excess potassium hydroxide is then titrated with a standard hydrochloric acid titration solution.

5 Reagents

Unless otherwise stated, the reagents used in the document are of analytical grade and the water used is distilled water or demineralised water of equivalent purity.

5.1 Ethanol: volume fraction 95 %.

5.2 Potassium hydroxide in ethanol solution: concentration of potassium hydroxide 0,5 mol/L. The solution is colourless or pale yellow, and a stable colourless solution can be obtained by either of the following routes. Route a): reflux 1 L of the ethanol of 5.1 with 8 g of potassium hydroxide and 5 g of aluminium granules for 1 h, then distil at once; dissolve the required quantity of potassium hydroxide, about 35 g, in the distillate; leave to stand for several days, then pour the clear supernatant liquid off the precipitated potassium carbonate into a brown glass bottle. Route b): add 4 g of aluminium tert butoxide to 1 L of ethanol and leave the mixture to stand for several days; pour off the supernatant liquid and dissolve the required quantity of potassium hydroxide in it; leave to stand for several days, then pour the clear supernatant liquid off the precipitated potassium carbonate into a brown glass bottle.

5.3 Hydrochloric acid standard solution: concentration of hydrochloric acid 0,5 mol/L.

5.4 Alkali blue 6B solution: mass concentration 2,5 g per 100 mL of the ethanol solution of 5.1.

5.5 Phenolphthalein solution: mass concentration 0,1 g per 100 mL of the ethanol solution of 5.1. Phenolphthalein is classed as a carcinogenic, teratogenic or reproductively toxic (CMR) substance and should be used only where no substitute is available.

5.6 Boiling aid.

6 Apparatus

Ordinary laboratory apparatus is used, and in particular the following.

6.1 Conical flask: capacity 250 mL, made of alkali resistant glass, with a ground neck.

6.2 Reflux condenser: with a ground glass joint able to connect to the conical flask of 6.1.

6.3 Heating device: such as a water bath, an electric hotplate or another suitable device; an open flame shall not be used for heating.

6.4 Burette: capacity 50 mL with a smallest graduation of 0,1 mL, or an automatic burette.

6.5 Pipette: capacity 25 mL, or an automatic pipette.

6.6 Analytical balance: smallest scale division 0,000 1 g, weighing accuracy 0,001 g.

7 Sampling

Sampling is not part of what the document specifies; ISO 5555 is recommended.

It matters a great deal that the sample received by the laboratory is representative and has not been damaged or altered during transport or storage.

8 Preparation of test sample

The preparation is carried out as specified in ISO 661.

Where insoluble impurities are present in the test sample, the sample is mixed evenly and then filtered, and this shall be stated in the test report.

9 Procedure

9.1 Test portion: weigh about 2 g of the sample prepared according to clause 8 into the conical flask of 6.1, to the nearest 0,005 g. The weight of 2 g is based on a saponification value, expressed as potassium hydroxide, of 170 mg/g to 200 mg/g; for samples with saponification values in other ranges the test portion should be varied as needed so that about half of the potassium hydroxide in ethanol solution is neutralised. The recommended test portions are given in Table 1.

Table 1, headed Test portion, has two columns, estimated saponification value expressed as potassium hydroxide in mg/g and test portion in g, and four rows: 150 to 200 gives 2,2 to 1,8; 200 to 250 gives 1,7 to 1,4; 250 to 300 gives 1,3 to 1,2; above 300 gives 1,1 to 1,0.

9.2.1 Using the pipette of 6.5, add 25,0 mL of the potassium hydroxide in ethanol solution of 5.2 to the test portion, add some boiling aid of 5.6, connect the reflux condenser of 6.2 to the conical flask and place the flask on the heating device of 6.3; bring it slowly to the boil with occasional shaking and keep it boiling for 60 min, or for 2 h in the case of samples with a high melting point and samples that are difficult to saponify.

9.2.2 Add 0,5 mL to 1 mL of the indicator of 5.4 or 5.5 to the hot solution and titrate with the hydrochloric acid standard solution of 5.3 until the colour of the indicator has just disappeared. Where the colour of the solution is dark, phenolphthalein of 5.5 shall not be used as the indicator.

9.3 Blank test: a blank determination is carried out as required by 9.2, without the sample, using 25,0 mL of the potassium hydroxide in ethanol solution of 5.2.

10 Calculation of results

The saponification value IS is calculated by formula (1). The symbols are: V0, the volume of the hydrochloric acid standard solution of 5.3 consumed in the blank determination, in millilitres (mL); V1, the volume of the hydrochloric acid standard solution of 5.3 consumed in the sample determination, in millilitres (mL); c, the actual concentration of the hydrochloric acid standard solution of 5.3, in moles per litre (mol/L); m, the mass of the test portion of 9.1, in grams (g). The formula also carries the factor 56,1.

Where the repeatability requirement of 11.2 is met, the arithmetic mean of two determinations is taken as the result of the determination.

The result is expressed as a whole number, as potassium hydroxide, in milligrams per gram (mg/g).

11 Precision

11.1 Interlaboratory test: in 2000 the German Institute for Standardization (DIN) organised an interlaboratory test in which 22 laboratories took part, see Annex A; two determinations were made on each sample and statistical results were given, evaluated according to ISO 5725-1 and ISO 5725-2, the statistical results being shown in Table A.1.

11.2 Repeatability: the absolute difference between two test results obtained in the same laboratory by the same operator using the same equipment, by the same test method, on the same test object, independently and within a short interval of time, shall exceed the repeatability limit r shown in Table A.1 in not more than 5 % of cases.

11.3 Reproducibility: the absolute difference between two test results obtained in different laboratories by different operators using different equipment, by the same test method, on the same test object and independently, shall exceed the reproducibility limit R shown in Table A.1 in not more than 5 % of cases.

12 Test report

The test report shall state: a) all the relevant information needed to identify the test sample; b) the sampling method, where it is known; c) whether the test sample needed to be filtered; d) the test method document used, for example GB/T 5534-2024; e) a statement of the indicator used, 5.4 or 5.5; f) any operating detail not specified in the document, or regarded as optional, together with all details that may affect the result; g) the determination result, and where a repeatability test was carried out, the final result.

A Annex A (informative) Results of the interlaboratory test

An international joint test was carried out by 22 laboratories in 8 countries on 5 samples: A, coconut oil; B, palm oil; C, rapeseed oil; D, medium chain triglyceride (MCT) oil; E, a mixture of 60 % of A with 40 % of D.

In 2000 the German Institute for Standardization (DIN) organised the joint test and the results obtained were subjected to a statistical analysis of data precision according to ISO 5725-2; the results are given in Table A.1.

Table A.1, headed Statistical analysis of the test results, is laid out with one parameter column and five sample columns, in the printed order rapeseed oil, palm oil, coconut oil, 60 % coconut oil plus 40 % medium chain triglyceride oil, and medium chain triglyceride oil. The parameter rows are the number of participating laboratories N, the number of laboratories retained after outliers were removed n, the number of independent test results on each sample from all laboratories z, the mean saponification value expressed as potassium hydroxide in mg/g, the repeatability standard deviation sr, the repeatability coefficient of variation CVr in %, the repeatability limit r, the reproducibility standard deviation SR, the reproducibility coefficient of variation CVR in %, and the reproducibility limit R.

The digitiser's watermark lies across the upper left part of this table, so a number of the cells in the first four parameter rows and in three of the five sample columns cannot be read with certainty; those figures are therefore not reproduced here. Only the layout of the table and the quantities it fixes are given above.

B Annex B (informative) Calculation of the saponification value from fatty acid composition data

B.1 General: the formulae of B.3 to B.6 give a mathematical way of calculating the saponification value of a fat or oil and of its constituent acylglycerols from a fatty acid composition given as fatty acids, fatty acid methyl esters or other fatty acid esters. The formulae given are therefore better suited to use on a computer, and B.7 gives a worked example that can be followed by hand. The determination of fatty acid methyl esters should be carried out according to ISO 12966-2, ISO 12966-3 and ISO 12966-4.

B.2 Symbols: the list covers the carbon number of the i-th fatty acid or fatty acid ester CF(i); the carbon number of the i-th triacylglycerol CT(i); the index i for a particular fatty acid, fatty acid ester or triacylglycerol; the calculated saponification value ISC; the integer constant k of the fatty acid derivative; the mean relative molecular mass of all the fatty acids in the test sample; the relative molecular mass of CH2, 14,026 7; the relative molecular mass of H2, 2,015 9; the relative molecular mass of HCOOH, 46,025 5; the relative molecular mass of the i-th fatty acid or fatty acid ester MF(i); the relative molecular mass of the i-th triacylglycerol MT(i); the number of double bonds in the i-th fatty acid or fatty acid ester; the number of double bonds in the i-th triacylglycerol; the mass fraction of the i-th fatty acid or fatty acid ester; the mass fraction of the i-th triacylglycerol; the mass fraction of unsaponifiable matter; the mole fraction of the i-th fatty acid or fatty acid ester; and the mole fraction of the i-th triacylglycerol.

B.3 Calculation of the relative molecular mass of a fatty acid or fatty acid ester: the constant k takes the value 0 for the acid, 1 for the methyl ester, 2 for the ethyl ester, 3 for the propyl ester and 4 for the butyl ester; a relative molecular mass given by the user for the acid may also be used.

B.7 Worked example: the sample used has the following methyl ester composition, expressed as mass fractions: 16:0, 10,6 %; 18:0, 4,2 %; 18:1, 22,7 %; 18:2, 54,5 %; 18:3, 8,0 %. Some of the relative molecular masses obtained with k equal to 1 are palmitic acid 270,45; stearic acid 298,51; oleic acid 296,49; linoleic acid 294,47; linolenic acid 292,46. The relative molecular mass of a triacylglycerol is the sum of the masses of the three fatty acid methyl esters less the mass of four hydrogen atoms, 4,031 8, which gives 807,32 for the 16:0 triacylglycerol, 891,50 for 18:0, 885,44 for 18:1, 879,38 for 18:2 and 873,35 for 18:3. The mean relative molecular mass of all the triacylglycerols works out at 873,14, and the calculated saponification value at 193. A note to the worked example states that the result is expressed as a positive number.

B.8 Correction for free fatty acids, partial glycerides and unsaponifiable matter: for free fatty acids and partial glycerides the appropriate relative molecular mass and saponification value are calculated with the formulae of B.3 to B.6, and the total relative molecular mass and the saponification reaction are calculated from the mass balance of the given proportions of free fatty acid and glycerol; the saponification value is then corrected for the mass fraction of unsaponifiable matter.