# What's the pH value for various liquids?丨DFRobot Science Lab EP03

### Objectives

In this chapter, the experiment will guide you to use a pH sensor to find out what kind of acidic or alkaline aqueous solutions are there in life. Moreover, the measured values can even be displayed on a small screen!

### Standards

NGSS - NEXT GENERATION SCIENCE STANDARDS - MIDDLE SCHOOL (MS)

From Molecules to Organisms: Structures and Processes - MS-LS1-5

Earth and Human Activity - MS-ESS3-3, MS-ESS3-4, MS-ESS3-5

Matter and its Interactions,MS-PS1-2

### Materials

##### BOSON Science Kit Materials

1、Battery Holder w/3x AAA batteries installed

2、MainBoard-3IO (m1)

3、Display Modulen(o11)

4、pH Sensor (i17)&pH Probe

5、BOSON cable

1 X short 1 X long

1、Clear Plastic Cups

2、Different liquids

### Activity

When eating food or drinking water, all kinds of tastes like sweet, sour, bitter and salty are experiences through our tongue. So, is there a method to measure the acidity or alkalinity of aqueous solution? Today we will introduce a unit used to express the degree of acidity or alkalinity - pH value.

pH value is an indicator of hydrogen ion concentration in aqueous solution, which is also commonly used to measure the acidity or alkalinity of solution. Under standard conditions (25 degrees Celsius and 1 atmosphere), the aqueous solution with pH=7 (e.g., pure water) is neutral; the solution with pH less than 7 indicates a elatively high concentration of hydrogen (H+)ions, so the solution tends to be acidic; while pH greater than 7 means a relatively low hydrogen ion concentration, so the solution tends to be alkaline. Therefore, the smaller the pH value, the stronger the acidity of the solution; the higher the pH value, the stronger the alkalinity of the solution.

Let's guess how acidic and alkaline these four liquids are.

●Explore this question by taking a poll of students in your class.

●Copy the chart below into your notebook.

●When completed, discuss the poll results with students sitting near you.

●List a few reasons from your discussion which support the results of the poll.

### Explore

In this activity, your group will use a pH sensor to detect the pH of each liquid.

#### Activity Procedure:

##### Setup:

Please connect the pH sensor, the main board and the OLED module as shown in the diagram, and then switch the OLED module to i17, so that the pH value measured by the pH sensor can be displayed on the OLED module.

#### Experiment

1. Pour the solution into the container

Take two containers, pour white vinegar and baking soda water into the containers respectively, and then stir them evenly. (The capacity shall be based on the height that can be measured by the pH sensor.)

2.Connect pH sensor and OLED module

Connect pH sensor and OLED module to mainboard. This means the value of pH sensor will be displayed on the OLED module.

3.Remove the front cover of pH sensor

Remove the cover with standard solution at the front end of the pH sensor.

Tips: Be careful to avoid pouring out the standard solution when taking it off.

4.Clean the front glass bead with pure water

Flush the front end of pH sensor with pure water for about 5 seconds. (Here, pure water is recommended, and the front glass bead is where to flush.)

5.Measure the pH value of white vinegar

Attach the pH sensor into the solution for about 30 seconds to measure the pH value of white vinegar and record in the table. As can be seen from the right picture, the pH value of white vinegar is 1.4, which belongs to a strong acid!

Tips: Before each measurement of different solutions, flush pH sensor with pure water again, otherwise the measurement results will be less accurate.

6.Measure the pH value of baking soda water

Using the same procedure, measure the pH value of baking soda water and then record it in the table.

Tips: Before measuring different solutions, flush the sensor again with pure water, otherwise the measurement results will be less accurate.

Tips:

Calibration method of pH sensor

The reason why the pH value of pure water under the standard condition is set at 7 is that the product of the concentration of hydrogen ions (H+) and hydroxide ions (OH-) naturally ionized by water under the standard temperature and pressure is always 1×1014, and the concentration of both ions is 1× 107 mol/L. However, in the case of non-water based solution or non-standard condition, the solution with pH 7 is not necessarily neutral.

The pH meter must be switched on in the standard solution (KCl solution of 3M).When the switchon and switch-off are stopped at the same number for two consecutive times, record the number as A, and then record the measured number as B. The figure obtained by using b× (7/a) is the calibrated pH value. PH meter can only be used in aqueous solution.

##### Data Record

Copy the Data Table below into your notebook.

#### Explain

Data Analysis:

In your groups discuss the following questions:

1.Which source of liquid had the highest pH?

2.Which source of liquid had the lowest pH?

3.Was there any correlation between a high or low pH and your observations of the liquid source? (For example, did a liquid source with a strong odor have a pH distinctly different from a source of liquid with no odor?)

### Elaborate

Vocabulary:

●Acid - a chemical substance with a pH less than 7. Acids can dissolve metals, turn blue litmus paper red, have a sour taste, are corrosive and can neutralize a base

●Base - a chemical substance with a pH greater than 7 - Bases can turn red litmus paper blue, have a bitter taste, feel slippery, and can neutralize an acid.

●Alkaline – synonyn for base; having the properties and/or conbtaining alkalin (metal);

●Neutral - with regards to acids and bases - a substance which is neither acidic or basic

●pH - a measurement of whether a substance is acidic or basic

Science Background:

The following information below will help you to understand characteristics of acids and bases, the pH scale, and how pH can be used as an indicator of water quality. Lemons taste sour. Soap is slippery. All substances have specific properties depending on their composition. Substances such as lemons are termed acidic. Including a sour taste, characteristics of acids include being able to dissolve metals, being able to turn blue litmus paper red. They can be corrosive and have the ability to neutralize bases.

On the other hand, besides being slippery, substances which are bases characteristically taste bitter, turn red litmus paper blue and have the ability to neutralize acids. How does a substance become classified as an acid or a base?

A small portion of water molecules, H2O interact and split apart to produce two types of ions; H+ (hydrogen ion) and OH- (hydroxide ion). Simply put, a substance becomes acidic when there are more hydrogen ions than hydroxide ions. A substance becomes basic when there are more hydroxide ions than hydrogen ions.

How can you tell is a substance is an acid or a base?

You can’t actually and count these ions but there is a way to tell whether a substance is an acid or base. A substance can be tested to determine whether it is an acid or a base by using what is known as litmus paper. Litmus paper is paper which has been prepared with either a red indicator dye or a blue indicator dye. An acid will turn blue litmus paper red and a base will turn red litmus paper blue.

Can you measure the strength or weakness of acids and bases?

The relative amounts of the ions producing acids and bases can be measured using what is known as the pH scale. A piece of paper known as pH paper, which has been prepared with an indicator dye will turn a certain color when dipped in a substance. This color is compared to a standard and the strength of the acid or base can be determined.

The pH of a substance can be measured on a scale from 0-14. pH readings less than 7 indicate the substance is an acid; 0 being the strongest acid. pH readings greater than 7 indicate the substance is a base (also know as alkaline); 14 being the strongest base. A pH reading of 7 means that the substance is neutral, neither acidic or basic.

#### Evaluate

Consider the following questions:

1.Will the temperature of solution affect the pH value? Try attaching the pH sensor into a solution at different temperatures, and then measure it.

2.20ml of white vinegar without water, then add 10ml of water, add 30ml water more, will pH value be different?