Experiment 17 Determination of the Content of Borax

Experiment 17 Determination of the content of Borax

Principles

Borax (sodium borate Na2B4O7·10H2O) is soluble in water and produces Na+ and B4O72- ions. According to Brönsted definition B4O72- is a proton acceptor. It takes protons from water molecules and thus is a base. Therefore, the quantity of borax can be determined through acid-base titration.

When standard hydrochloric acid solution is used to titrate borax solution, the following reaction takes place:

B4O72- + 2H+ + 5H2O = 4H3BO3

or

Na2B4O7·10H2O + 2HCl = 2NaCl + 4H3BO3 + 5HH2O

According to the equations above, at stoichiometric point, we have the following equations:

n(HCl) = n(Na2B4O7·10H2O)

ω(Na2B4O7·10H2O) =

Where:c(HCl) is the molarity (mol·L-1) of the standard HCl solution consumed in the titration, V(HCl) is the volume of the standard HCl solution consumed (mL), Wsample is the mass of the borax used in each titration (g), M (Na2B4O7·10H2O) is the molar mass of the borax (301.37).

At the endpoint the pH of the solution is 5.1 when standard hydrochloric acid solution is used to titrate borax solution. Methyl red can be chosen as the indicator.

Apparatus and Reagents

Apparatus: analytic balance, acidic burette (50mL), volumetric pipet (20mL), conical flask(250mL×2), volumetric flask (250mL,100mL), beaker(100mL), burette holder, weighing bottle, washing bottle

Reagents: Borax sample (Na2B4O7·10H2O), standard HCl solution, 0.1％methyl red indicator

Procedure

1.Weigh out approximately from weighing bottle about2.0 g Na2B4O7·10H2O (±0.0001g precisely) with analytical balance. Put the sample in a 100mL beaker. Add 40mL distilled water. Heat the solution when stirring with a glass rod until all borax has dissolved. Then transfer the solution to a 100mL volumetric flask. Wash out the beaker several times with a small amount of distilled water and add the washings to the original solution. Add water to the calibration mark and shake the flask sufficiently to homogenize the solution.

2. Rinse a clean volumetric pipet three times with a small amount of borax solution. Measure out 20.00 mL the above borax solution by volumetric pipet and pour it into 250mL conical flask. Wash the inner wall of the conical flask down with a little distilled water from a washing bottle. Add 2 drops of the 0.1% methyl red indicator and the solution becomes yellow.

3. Titrate with the standard hydrochloric acid solution until the solution in the flask turns pink. It is the endpoint of the titration. Record the volume of the standard hydrochloric acid solution used in the titration. Repeat the titration process 2～3 times until the relative error of the analytical results is 0.2% or lower.

4. Calculate the content of the borax using the above equation

Recording and treating data

Table 1 Determination of Quantity of Borax Contained

Questions:

1. When titrating borax with standard HCl solution, which indicator do you think is better, methyl red or methyl orange? Why?

Experiment 16 Acid-Base Titration Analysis

Purposes

1. Learn the principle and procedures of acid-base titration.

2. Learn titration operation and how to determine the end point of the titration.

3. Learn how to prepare standard solutions and how to analyze acidic and basic substances.

Principles

Acid-base titration is also known as neutralization titration. It is a titration analysis based on proton transfer. It can be used to determine the concentration of acids and bases, and can also be used to determine the content of substances that react with an acid or a base. When an acid or an acidic sample is analyzed, the solution of a strong base with known concentration is used to react with it. The content of the acidic compound is calculated based on the amount of the base used. When a base or a basic compound is analyzed, the solution of a strong acid with known concentration is used to react with it. The content of the base or basic substance is calculated by the amount of the acid used.

The titration analysis usually involves three steps: preparing the standard solutions, standardizing the concentration of the standard solutions, and determining the content of the samples.

In a typical acid-base analysis, hydrochloric acid or sodium hydroxide solutions is used as the standard solution. Due to the facts that hydrochloric acid is volatile and sodium hydroxide absorbs moisture and carbon dioxide from the air, their standard solutions cannot be prepared directly. A solution with approximately the concentration required is made first. Its accurate concentration is then standardized with a primary standard substance. Its concentration can also be determined by titrating with another solution with known concentration. The concentration is calculated according to the ratio of , the mass of primary standard substance or the concentration of stand solution obtained through titration.

The common basic primary standard substance used to standardize HCl solution include anhydrous Na2CO3 and borax (Na2B4O7·10H2O). Sodium carbonate is readily available in high purity form and is inexpensive. Since it absorbs moisture and CO2 from the air, however, it must be dried at 270℃~300℃ for about one hour and cooled down in a desiccator and kept in it. Borax has large molar mass but contains lattice water and must be kept in an environmental chamber kept at approximately 60% relative humidity.

The common acidic primary standards substance used to standardize NaOH solution include oxalic acid and potassium acid phthalate (KHC8H4O4). Potassium acid phthalate is the one most commonly used as primary standard substance because it has large molar mass, high stability, and readily available in high purity form.

The Standardization of HCl Concentration

Anhydrous sodium carbonate (Na2CO3) is often used to titrae HCl solution, the following is reaction：

Na2CO3 + 2HCl = 2NaCl + H2O +CO2↑

Here we have the titration of 0.1000mo1·L-1 Na2CO3 by 0.1000mo1·L-1HCl, the stoichiometric point is pH=3.9, the pH titration jump interval is 5.0 to 3.5, so methyl orange or methyl red fits reasonably well with its color change interval of pH=3.1~ 4.4 or pH=4.4 ~ 6.2. When the end point of titration is coming the solution should be boiled because of effect of CO2.

According to the reaction equations above, at stoichiometric point, we have the following equations:

n (HCl) = n (Na2CO3)

The accurate concentration of HCl solution can be calculated according to mass of Na2CO3 titrated and the volume of HCl solution consumed.

Apparatus and Reagents

Apparatus: analytic balance, Acidic burette (25mL), volumetric flask(100mL), volumetric pipet(20mL), conical flask(250mL×3), beaker(100mL), graduated cylinder(10mL, 1000mL) ,weighing bottle, burette holder, washing bottle, glass rod

Reagents: Anhydrous sodium carbonate (A.R), concentrated hydrochloric acid, 0.05% methyl orange indicator

Procedure

1. Preparation of 0.1mo1·L-1 HCl solutions 300mL

To calculate the volume of concentrated HCl needed to prepare 300mL 0.1 mo1·L-1 HCl solution, then measure the volume of concentrated HCl needed out by means of 10mL measuring cylinder and pour the acid into the liter measuring-cylinder （多空格）containing about 200ml of distilled water. Wash out the 10mL measuring cylinder 2～3 times with a small amount of distilled water and add the washings to the original solution. Make up to 300mL mark with distilled water, transfer the solution to liter reagent bottle, insert the stopper and thoroughly mix by shaking.

2. Preparation of standard solution of Na2CO3

Weigh out accurately from weighing bottle about pure Na2CO3 1.2g～1.4g (±0.0001g precisely) with analytic balance. Pour the anhydrous sodium carbonate into a 100mL beaker. Add 30mL distilled water. When stirring with a glass rod until all Na2CO3 has dissolved. Then transfer the contents to a 250mL volumetric flask. Wash out the beaker several times with a small amount of distilled water and add the washings to the original solution. Add water to the calibration mark and shake the flask sufficiently to homogenize the solution.

3. Standardization the concentration of HCl solution with anhydrous sodium carbonate

Transfer 20mL Na2CO3 standard solution with a 20mL pipet which is rinsed three times with a small amount of Na2CO3 solution before using it into a 250mL conical flask that . Wash the inner walls of the conical flask down with a little distilled water from a washing bottle. Add 2 drops of methyl orange indicator and the solution becomes yellow.

4. Rinse a clean burette three times with a small amount of the acid to be standardized，fill the burette to a point 2cm～3cm above the zero mark and open the stopcock momentarily in order to fill the tip of burette with liquid. Examine the jet to see that no air bubbles enclosed. If there are, more liquid mast be run out until the tip of burette is completely filled. Re-fill, if necessary, to bring the level is between the 0.00mL and 0.50mL marks. Read the position of the meniscus to 0.01mL. Place the conical flask containing the sodium carbonate solution upon a piece of unglazed white paper or a white tile beneath the acidic burette, and run in the acid slowly from the burette. During the addition of the acid, the flask mast be constantly rotated with one hand while the other hand controls the stopcock, continue the addition until the methyl orange becomes a very faint yellow. Wash the inner walls of the conical flask down with a little distilled water from a washing bottle, and continue the titration very carefully by adding the acid drop wise until the color of the methyl orange becomes orange or a faint pink. This marks the end point of the titration. Record the volume of the standard hydrochloric acid solution used in the titration.

5. Repeat the titration process 2～3 times until the relative error of the analytical results is 0.2% or lower.

Recording and treating data

1.Use mL concentrated HCl solution to prepare 300mL of 0.1 mol·L-1 HCl solution

2. Standardization the concentration of HCl solution

Table1 Standardization the concentration of HCl solution with anhydrous sodium carbonate

Questions

1. Is it necessary that volumetric flask is drying before using it to prepare Na2CO3 solution? Why?

2. Is it necessary that pipettes should be washed before using it to measure solution?

3. Why not take phenolphthalein as indicator, when Na2CO3 is used standardize HCl as a primary standard substance?

1

基础化学实验（英）