SOP: Determination of Equilibrium Relative Humidity by Wink's Weight Equilibrium Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

A product is in equilibrium with its environment when the rate of moisture loss from the product to the surrounding atmosphere equals the rate of moisture gained from the atmosphere. The relative humidity at which the substance will neither gain nor lose moisture under given temperature conditions is the equilibrium relative humidity (ERH). In Wink’s weight equilibrium method, samples of the product are brought to actual weight equilibrium when exposed to different relative humidities, and the equilibrium moisture content at each humidity gives the moisture–equilibrium curves, from which the initial, critical and danger points are read. Products having an ERH below 50% are usually considered hygroscopic; those above 50%, hygroemissive.

Apparatus required

• Crystallizing dish (12.5 × 6.5 cm) as the main body of the test unit; grind the top edge on a flat glass plate smeared with carborundum paste to remove irregularities; apply silicone grease to the ground edge for a good seal with the glass cover plate
• Petri dish (9.3 × 1.8 cm), supported on a bracket made from 0.6 cm aluminium sheet (8.1 cm long, 1.25 cm wide, bent to fit the dish snugly); the bracket is fastened to the threaded end of a 0.32 cm diameter brass rod (8.1 cm overall) with two 2-56 brass nuts; the rod’s top end is bent into a flat hook for suspension from the balance pan hook; a brass cone (lower half angle ≈ 30°) is soldered 2.5 cm from the bottom end of the rod
• Square plate-glass cover (13.75 × 13.75 cm) with a 0.47 cm diameter hole drilled 0.6 cm off-centre (necessary because of the space limitations in the analytical balance), with no chipping where the hole is drilled
• Small desiccators as an alternative (Petri dish supported on a glass tripod or porcelain plate); or the humidity control unit shown in the book’s Fig. 16.7
• Single-pan semi-automatic analytical balance

Sample preparation

Note the tare weight; weigh exactly 5–10 g lots of the powder and spread uniformly on Petri dishes; determine the moisture content of the sample by drying in a vacuum oven at 70°C. Pour approximately 40 ml (or more if a desiccator is used) of saturated salt solution into each crystallizing dish; add salt in excess to ensure saturation. For desorption isotherms, have mounds of the excess salt exposed above the level of the solution.

Reference data — ERH of saturated salt solutions

The following ERH values (%, at 22.8°C / 30°C / 37.8°C) are given for the saturated salt solutions used:

• Ammonium phosphate, NH₄H₂PO₄ — 92.9 / 92.0 / 91.1
• Potassium chromate, K₂CrO₄ — 86.5 / 86.3 / 85.6
• Ammonium sulphate, (NH₄)₂SO₄ — 80.1 / 79.6 / 79.1
• Sodium chloride, NaCl — 75.6 / 75.2 / 75.1 (the 22.8°C figure is garbled in the OCR as “isis” and is carried from the book’s usual value with a note)
• Sodium acetate, NaC₂H₃O₂ — 74.8 / 71.4 / 67.7
• Sodium nitrite, NaNO₂ — 64.8 / 63.3 / 61.8
• Sodium bromide, NaBr — 58.5 / 56.3 / 53.7
• Sodium dichromate, Na₂Cr₂O₇ — 54.1 / 52.0 / 50.0
• Magnesium nitrate, Mg(NO₃)₂ — 53.5 / 51.4 / 49.0
• Potassium nitrite, KNO₂ — 48.6 / 47.2 / 45.9
• Calcium nitrate, Ca(NO₃)₂ — 51.8 / 46.6 / 38.9
• Potassium thiocyanate, KCNS — 46.6 / 43.7 / 41.1
• Potassium carbonate, K₂CO₃ — 43.9 / 43.5 / 43.4
• Chromium trioxide, CrO₃ — 39.2 / 40.0 / 40.2
• Magnesium chloride, MgCl₂ — 32.9 / 32.4 / 31.9
• Potassium acetate, KC₂H₃O₂ — 22.9 / 22.0 / 20.4
• Lithium chloride, LiCl — 11.1 / 11.1 / 11.1 (the 30°C figure is garbled in the OCR as “t.2” and is carried as 11.1 from the bracketing 11.1 values — verify against the printed book)

Note: the printed chemical formulae in the table are heavily garbled by OCR (e.g., “(NH,).SO,”); the formulae above follow the named salts; the numerical values are legible in the source.

Reference data — H₂SO₄ solutions for specific relative humidities at 25°C

(Relative humidity %, specific gravity, normality): 0 / 1.835 / —; 5 / 1.608 / 22.0; 10 / 1.552 / 19.5; 15 / 1.512 / 18.0; 20 / 1.478 / 16.8; 25 / 1.450 / 15.8; 30 / 1.425 / 14.8; 35 / 1.400 / 13.9; 40 / 1.377 / 13.1; 45 / 1.355 / 12.3; 50 / 1.335 / 11.5; 55 / 1.314 / 10.8; 60 / 1.293 / 10.0; 65 / 1.271 / 9.2; 70 / 1.248 / 8.3; 75 / 1.224 / 7.4; 80 / 1.197 / 6.3; 85 / 1.165 / 5.2; 90 / 1.127 / 3.9; 95 / 1.076 / 2.3; 100 / 1.000 / 0.0.

Procedure — Weight equilibrium method

Expose the dishes to different relative humidities ranging from 0 to 98% inside the crystallizing dishes or desiccators containing H₂SO₄ solutions of different strengths at 25°C, or saturated salt solutions of definite relative humidities at 25°C and 37°C

↓

Determine the gain or loss in weight of the sample at the end of 1, 2, 3, 4, 5 and 6 hr and thereafter at intervals of 24 hr up to 360 hr, to establish the moisture equilibrium of the powder at 25°C; at 37°C, weigh the dishes at fortnightly intervals up to 75–120 days, observing changes in moisture uptake and adverse changes in the powder (caking, discolouration, mould growth), which show the critical point and the storage characteristics

↓

Note the weight of the powder when there is no further loss or gain

Calculation

• Equilibrium moisture content (%) = ((S − X) / S) × 100

Where,

• S = weight of sample after equilibration (the initial weight before equilibration should be the same as A)
• X = dry matter content in the sample taken for determination of ERH = A − B
• A = weight of sample taken
• B = moisture content

Interpretation

Draw the moisture equilibrium curves and mark on the curve: (i) the initial point (J) — the moisture content and the ERH of the product as prepared; (ii) the critical point (C) — the stage at which the product just becomes lumpy; (iii) the danger point (D) — a point 5% lower in RH than the critical point. The portion of the curve between D and C is the safety range (SR); the portion between J and D is the safety margin (SM), a guide in packaging the material. The package adopted should not permit the product to reach the danger point, so that there is a safe margin of error.

Notes

• The book’s worked example (mango custard powder): initial moisture 2.0% (13.0% RH); critical point at 4.4% moisture (32% RH), beyond which caking and discolouration are observed; danger point at 4.2% moisture and 27% RH. The product is hygroscopic.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Wink, W.A., Ind. Eng. Chem. Anal. Edn., 18, 251 (1946) — cited in the book.

Landrock, A.H. & B.E. Proctor, Mod. Packag., 24, (6), 123 (1951) — cited in the book.