3.
Application of acid-base titration-determination of alkalinity
(1) Alkalinity composition
.
The alkalinity of water refers to the total amount of substances in the water that can quantitatively react with strong acids
.
More alkalinity sources of water, natural water alkalinity is substantially carbonates, bicarbonates function and hydroxide content, it can be divided into alkalinity hydroxide (OH - ) alkalinity, carbonate salt (CO.
's 2 3- ) alkalinity and bicarbonate (HC03 .
3 - ) alkalinity
.
We can not assume the presence of water while OH - and HC03 .
3 - , may constitute five kinds of combinations: (OH - ), (OH - , CO.
'S .
3 2- ), (CO.
'S .
3 2- ), (CO.
'S .
3 2- , HC03 .
3 - ), (HCO 3 - )
.
For example, a large number of algae thrive in natural water bodies, which strongly absorb CO 2 in the water, so that the water has a higher pH value, mainly carbonate alkalinity.
Generally, natural water with a pH value of <8.
3 mainly contains bicarbonate alkalinity, which is slightly higher than The weakly acidic natural water of 8.
3 can contain bicarbonate and carbonate alkalinity at the same time .
When the pH value is greater than 10, it is mainly hydroxide breakage
.
The total alkalinity is taken as the sum of the concentrations of these components
.
When the water contains borate, phosphate, or silicate , the measured value of total alkalinity also includes their role
.
(2) Determination of alkalinity
.
The alkalinity is determined by acid-base titration
.
Using HCl and H 2 SO 4 as standard solutions, phenolphthalein and methyl orange as indicators, according to the volume of acid consumed by the discoloration of different indicators, various alkalinity contained in water samples can be measured
.
Determination of alkalinity available continuous titration: taking up a volume of water sample, phenolphthalein indicator was added standard solution was titrated to a strong acid, to the solution from red to colorless, the amount of the standard solution of the acid represented by P
.
Then add methyl orange indicator to the water sample, continue to titrate the solution from yellow to orange, and use M to denote the volume of the standard solution for titration
.
According to the relative size of P and M, the composition of alkalinity in water can be judged and its content can be calculated
.
The reaction of various alkalinities in titration is as follows
.
Phenolphthalein discoloration:
Methyl orange discoloration:
The total alkalinity of water: T = OH - + CO.
'S .
3 2- + HC03 .
3 - = P + M
The relationship between various alkalinity and acid-base titration results is shown in Table 2.
2.
4
.
Table 2.
2.
4 Composition and calculation of alkalinity in water
(3) Alkalinity calculation
.
1) First judge the composition of alkalinity by the values of P and M (see Table 2.
2.
4)
.
For example, the result of the titration is P=11.
05ml, M=8.
90ml, check table 2.
2.
4, since P>M, the alkalinity components in the water are OH - and CO 3 2-
.
The OH-content is PM=2.
15ml, meaning that the OH - consumable acid standard solution in water is 2.
15ml, and the CO 3 2- content is 2M=19.
80ml, meaning that the CO 3 2- consumable acid standard solution in water is 19.
80ml
.
The total alkalinity is P+M=19.
95ml, which means that the total alkalinity in the water can consume 19.
95ml of the acid standard solution
.
2) The expression method of alkalinity
.
In mg/L, in mol/L or mmol/L, in mgCaO/L or mgCaCO 3 /L
.
3) Commonly used molar mass for alkalinity calculation
.
4) Expression for alkalinity calculation
.
[Example 2.
2.
1] The titration result of continuous titration is P=11.
05ml, M=8.
90ml; known V water sample=100ml, CHCl=0.
1mol/L
.
After analysis, the alkalinity components in the water are OH - and CO 3 2- , the content of OH- is PM =2.
15ml, the content of CO 3 2- is 2M=17.
80ml, and the total alkalinity is P+M=19.
95ml
.
The result of alkalinity calculation can be expressed as
Related links: Titration analysis method-acid-base titration (2)
