Automatic Procedure to Determine Acidity in Fuel Ethanol by Photometric Titration Using Binary Search and Multicommuted Flow Analysis

Lima, Manoel J. A.; Reis, Boaventura F.

doi:10.21577/0103-5053.20180109

Manoel J. A. Lima

Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Avenida Centenário, 303, 13416-000 Piracicaba-SP, Brazil

Boaventura F. Reis ^**e-mail: reis@cena.usp.br

Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Avenida Centenário, 303, 13416-000 Piracicaba-SP, Brazil

*e-mail: reis@cena.usp.br

Figures (3)
Tables (4)

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Figure 1
Diagram of the flow analysis module for photometric titration. S₁ and S₂ are the glass syringes; x and y are the fluid lines joint machined in PTFE; V₁ and V₂ are 3-way solenoid valves (100 psi); V₃, V₄, V₅, and V₆ are 2-way solenoid valves (normally closed) (30 psi); L₁ and L₂ are the sampling loop and connecting line (50 and 10 cm long, respectively); W is a waste bottle; S, In, and Tit are the sample, indicators, and titrant solutions; and Ca is the carrier fluid (water). The insertion is the detection system employing an LED-based photometer.

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Figure 2
(A) and (B) correspond to the current procedures based on ABNT and ASTM methodologies, respectively. (b, d) Absorption spectra of the α-naphtholphthalein and phenolphthalein in alkaline medium, respectively (50% ethanol/water); (a, c) emission spectra of the red and green LEDs, respectively. These spectra were achieved with an Ocean Optics Spectrometer (USB4000-UV-Vis).

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Figure 3
Records of the signals related to the proposed titration procedure. Experimental conditions were: 50% (v/v) ethanol solution spiked with 5.05 mmol L^-1 acetic acid; titrant solution, 10.0 mmol L^-1 NaOH; solutions insertion step, six sampling cycles; and threshold (150 < ΔmV < 400 mV). (a-i) Records of the signals generated by mimicking the ANP recommendation at the first titration run; (g*, h*, i*) related to the second run. Inset shows the schlieren effect (x, y) and the analytical measurement (z).

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Table 1
Results of titration of ethanol standard solutions (50% v/v)

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Table 2
Values of the variables while performing a titration run

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Table 3
Results of determination of acidity in fuel ethanol

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Table 4
Performance comparison with existing procedures for acidity determination in ethanol fuel

Acetic acid added / (mmol L^-1)	Batch / (mmol L^-1)	MCFA / (mmol L^-1)	Recovery / %
10.0	10.45 ± 0.14^a a α -Naphtholphthalein was used as an indicator;.	10.65 ± 0.92^a a α -Naphtholphthalein was used as an indicator;.	104.5^a a α -Naphtholphthalein was used as an indicator;.	106.5^a a α -Naphtholphthalein was used as an indicator;.
50.0	50.65 ± 0.14^a a α -Naphtholphthalein was used as an indicator;.	46.80 ± 1.51^a a α -Naphtholphthalein was used as an indicator;.	101.3^a a α -Naphtholphthalein was used as an indicator;.	93.6^a a α -Naphtholphthalein was used as an indicator;.
5.0	5.30 ± 0.20^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	5.29 ± 0.35^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	106.0^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	105.8^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.
15.0	15.68 ± 0.15^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	14.15 ± 0.61^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	104.5^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.	94.3^b b phenolphthalein was used as an indicator. MCFA: multicommuted flow analysis.

Attempt	time titrand / s	time titrant / s	Variation / s	Titrand / µL	Titrant / µL	Signal / mV	Sample / (mmol L^-1)
a	1.0	1.0	-	150.00	150.00	> ΔmV	10.00
b	1.5	0.5	0.50	225.00	75.00	< ΔmV	3.33
c	1.25	0.75	0.250	187.50	112.50	> ΔmV	6.00
d	1.375	0.625	0.1250	206.25	93.75	< ΔmV	4.55
e	1.312	0.688	0.0625	196.88	103.13	> ΔmV	5.24
f	1.343	0.656	0.0313	201.56	98.44	< ΔmV	4.88
g	1.328	0.672	0.0156	199.22	100.78	< ΔmV	5.06
h	1.320	0.680	0.0078	198.05	101.95	< ΔmV	5.15
i	1.316	0.684	0.0039	197.46	102.54	ΔmV	5.19

Sample	Acidity / (mg of acetic acid per liter of ethanol)
Sample	MCFA^a a α -naphtholphthalein as color indicator;	Titration-batch (ANP)^a a α -naphtholphthalein as color indicator;	MCFA^b b phenolphthalein as color indicator. MCFA: multicommuted flow analysis.	Titration-batch (ASTM)^b b phenolphthalein as color indicator. MCFA: multicommuted flow analysis.
A	9.92 ± 0.28	10.08 ± 0.58	9.52 ± 0.09	10.82 ± 0.72
B	6.6 ± 0.9	6.9 ± 0.3	9.1 ± 0.1	8.98 ± 0.45
C	20.2 ± 0.1	17.2 ± 0.3	20.10 ± 0.05	21.32 ± 0.27
D	18.98 ± 0.41	16.58 ± 0.25	20.01 ± 0.05	19.29 ± 0.63
E	9.6 ± 0.1	9.1 ± 0.3	9.81 ± 0.09	10.76 ± 0.51

CV / %	Recovery / %	Waste^a a Waste generated per determination, / mL	Sampling rate^b b determinations per hour, / h^-1	r^c c linear correlation coefficient achieved by plotting the results of the proposed procedures versus those of the reference methods;	Reference
< 1 (n = 6)	93.6-106.5	≤ 11.5	≥ 10	0.9978^d d ANP methodology; 0.9880^e e ASTM methodology. CV: coefficient of variation.	this work
< 1 (n = 15)	97.6-102.7	15.5	120	0.9991	¹⁰10 Fatibello-Filho, O.; Borges, M. T. M. R.; Anal. Chim. Acta 1998, 366, 81.
< 3	98.7-102.7	> 100	-	-	¹¹11 Avelar, H. M.; Barbeira, P. J. S.; Fuel 2007, 86, 299.
< 1	-	> 12	-	-	¹²12 Gonzaga, F. B.; Gonçalves, M. A.; Sobral, S. P.; Ribeiro, C. M.; Fuel 2012, 94, 70.

[1] *e-mail: reis@cena.usp.br

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Automatic Procedure to Determine Acidity in Fuel Ethanol by Photometric Titration Using Binary Search and Multicommuted Flow Analysis