1. Application

This applies to the test method of determining residual starch in green sand.

2. Definition

The residual starch is the starch and similar substances remained in green sand.

3. Chemicals and apparatus:

3.1 Chemicals

1. 2.5% chloric acid Special class concentrated chloric acid (HCl) of 58ml is dissolved in pure water of 1000ml.
2. 0.9% sodium hydroxide solution Special class sodium hydroxide (NaOH) of 9g is dissolved in pure water of 1000ml.
3. 80% phenol Special class phenol (C₆H₅OH) of 9g is mixed with pure water of 20ml.
4. Concentrated sulfuric acid Special class concentrated sulfuric acid (H₂SO₄).

3.2 Apparatus

A photoelectric photometer or a photoelectric colorimetric analyzer is used.

3.3 Balance

A balance of the precision of 0.01g is used.

3.4 Inspection

The precision should be occasionally checked to assure the precision of the instruments used in this method.

4. Testing procedure

4.1 Preparation of sample solution

A sample is sufficiently dried at 105±5 degree C and cooled in a desiccator. 5g of the sample is put in a conical flask of 300ml capacity and 2.5% chloric acid of 50ml¹⁾ is added. A cooling pipe²⁾ is attached to the flask and the solution is boiled for 30 minutes on an electric heater³⁾ to hydrolyze starch. The solution after cooling is poured in a measuring flask of 200ml capacity. 0.9% sodium hydroxide solution of 150ml is added for neutralization, making the total volume to be 200ml. The solution is stirred and filtered by a No5C filter paper⁴⁾ to obtain a sample solution.

Note¹⁾: A measuring flask of 100ml may be used.
Note²⁾: An air cooler may be fitted to the flask opening for preventing evaporation.
Note³⁾: Heating should be controlled so as to maintain very weak boiling. A hot plate with a temperature controller is convenient.
Note⁴⁾: By waiting 10 minutes after stirring, filtering becomes easy, because bentonite agglomerates and sinks during the waiting time.

4.2 Operation

1. Solution is sampled into a test tube of inner diameter of 6 to 20mm by using a measuring pipette of 2ml size. The amount of the sample is determined depending on the predicted amount of starch as shown in the table below.

   Predicted amount of residual starch, %	0~0.5	0.5~1.0	1.0~2.0
   Sample size, ml	2.0	1.0	0.5

2. One drop of 80% phenol solution⁵⁾ (0.05ml) is added to the sample. Concentrated sulfuric acid6) of 5ml is poured in the test tube in less than 10 seconds in a manner so as to directly hit the sample solution surface, causing generation of heat and color.
   Note⁵⁾): The designated amount should be added (one drop of a 5ml whole pipette is about 0.05 ml).
   Note⁶⁾: The reaction of this test depends on the heat generated at dilution of the sulfuric acid. Therefore, the speed of addition of the acid should be fast and constant. It is recommended to attach a pipetter to the pipette for pushing out the acid. Sucking and pushing out of acid by using operator’s mouth should be avoided for safety.
3. After leaving for one hour, light absorption at 470μm wave length is measured using a photoelectric photometer or a photoelectric colorimetric analyzer. A reference solution is prepared from pure water in the same manner as that for the sample.

4.3 Preparing calibration curve

1. A reference solution is prepared by the same manner as that of the sample solution from starch 0.2g, bentonite 0.5g, and silica sand 4.5g after drying at 105±5 degree C.
2. Preparation of reference solution of different concentrations.

   Reference solution, ml	5	10	15	20	25
   Pure water, ml	195	190	185	180	175
   Total volume, ml	200	200	200	200	200
   Amount of starch, mg	5	10	15	20	25

3. A reference solution of 2ml is taken and tested in the manner in 4.2 for measuring light absorption for preparing a reference curve.

4.4 Calculation of residual starch

The amount of starch is obtained from the reference curve, and the residual starch in green sand is calculated by the following equation.

\[A = { T \over C } \times 100 \]

where

\[ \left\{ \begin{array}{l} A &= residual \ starch \ (mg) \\ T &= amount \ of \ starch \ obtained \ from \ the reference \ curve \ (mg) \\ C &= coefficient, \ the \ value \ of \ which \ is \ as \ below, \ depending \ on \ the \ sample \ size \ selected \ according \ to \ 4.2.\\ & sample \ size \ 2ml \cdots 50 \quad \quad sample \ size 1ml \cdots 25 \quad \quad sample \ size \ 0.5ml \cdots 12.5. \end{array} \right. \]

5. Expression

The residual starch is expressed in %.

6. Record

The result is calculated to the second decimal place. Test is repeated two times or more and averaged using values within 5% deviation.