How to test acid?

1. The presence of Cl⁻ can be shown by forming white ppt. with acidified AgNO₃.


2. The presence of SO₄²⁻ can be shown by forming white ppt. with acidified BaCl₂.

3. The easiest way is brown ring method. You can add solution of FeSO4 (fresh) and then concentrated H₂SO4 droply. If the brown ring occurs, it indicates the anion of NO₃⁻. The reaction is:
FeSO₄ + NO₃⁻ + 5H₂O --> [Fe(H₂O)5NO]SO₄ (brown)

Directions for using the Acidimeter or Acid testing Kit (depending on which setup you are using)

If using the Acidimeter fill the glass reservoir with .1N NaOH (Sodium Hydroxide Solution)
If using Acidimeter fill the graduated column with the NaOH by applying pressure to the rubber pump. It will go to zero with any excess draining back into the reservoir
or
if using 'Acid Testing Kit' fill the provided syringe with 3 ml of .1N NaOH.
Now using a syringe or a pipette, measure 9 ml of milk
Pour this sample into a test tube or a beaker (preferably held against white surface)
Add 5 drops of phenolphthalein to the milk sample and stir gently
Open the tap (stopcock) or use syringe to let the NaOH run drop by drop into the sample while stirring gently
Continue until the appearance of a light pink coloring which persists for 10-20 seconds
Look on the column or syringe to see how many ml of NaOH were used
.. multiply this by .1 (move decimal point 1place left) for the actual TA%

Example: If the amount of NaOH used is 1.6ml then the titrateable acidity is .16 %

Some books may refer to Dornic acid degrees ... This is simply done by moving your column reading one place to the right ... i.e. graduation is 1.6 = 16 Dornic degrees.


Note: The life of Sodium Hydroxide Solution : approximately 1 year in a sealed bottle

Acidic solutions give bad egg smell with sodium sulphide solution. They also form colorless bubbles when sodium carbonate/hydrogencarbonate is added.

original way
blue litmus paper

5.1 酸值測定
5.1.1 原理
中和1g樣品中游離的脂肪酸所需要的氫氧化鉀毫克數。
5.1.2 試劑和試液
a. 95％乙醇;
b. 氫氧化鉀標準溶液〔c(KOH)=0.05mol/L〕:稱取3g氫氧化鉀溶於1000mL蒸餾水中,靜置一周,取上層清液搖勻,參照 GB 601標定;
c.酚酞指示液:1g/L乙醇溶液;
d.中性乙醇:以酚酞為指示劑,用氫氧化鉀標準溶液(5.1.2b)將95％乙醇(5.1.2a)調至微紅色。
5.1.3 試驗步驟
稱取3g樣品(准至0.000 2g)於錐形瓶中,加50mL中性乙醇(5.1.2d),在水浴上加熱溶解後放至室溫,加1滴酚酞指示劑(5.1.2c),用氫氧化鉀標準溶液(5.1.2b)滴至微紅色,10s不褪色為終點。
5.1.4 結果計算
酸值X1(mgKOH/g)按式(1)計算: