A titration curve plotting pH against volume of titrant added, producing the characteristic S-shaped (sigmoidal) curve used in analytical chemistry. The plot reveals buffer regions, equivalence points, and acid/base strength at a glance. Essential for chemistry education and laboratory analysis, it helps identify when a reaction reaches completion and which indicators are appropriate.
#' anyplot.ai
#' titration-curve: Acid-Base Titration Curve
#' Library: ggplot2 3.5.1 | R 4.4.1
#' Quality: 90/100 | Created: 2026-06-24
library(ggplot2)
library(ragg)
# Theme tokens (Imprint palette, theme-adaptive chrome)
THEME <- Sys.getenv("ANYPLOT_THEME", "light")
PAGE_BG <- if (THEME == "light") "#FAF8F1" else "#1A1A17"
ELEVATED_BG <- if (THEME == "light") "#FFFDF6" else "#242420"
INK <- if (THEME == "light") "#1A1A17" else "#F0EFE8"
INK_SOFT <- if (THEME == "light") "#4A4A44" else "#B8B7B0"
INK_MUTED <- if (THEME == "light") "#6B6A63" else "#A8A79F"
GRID_COLOR <- if (THEME == "light") "#C5C4BD" else "#494945"
IMPRINT_PALETTE <- c(
"#009E73", # 1 brand green — titration curve (first series, always)
"#C475FD", # 2 lavender — derivative overlay
"#4467A3", # 3 blue
"#BD8233", # 4 ochre
"#AE3030", # 5 matte red
"#2ABCCD", # 6 cyan
"#954477", # 7 rose
"#99B314" # 8 lime
)
# Data: 25 mL of 0.1 M HCl titrated with 0.1 M NaOH
c_acid <- 0.1 # mol/L
v_acid <- 25.0 # mL
c_base <- 0.1 # mol/L
vol_step <- 0.25 # mL
volume_ml <- seq(0, 50, by = vol_step)
mmol_acid <- c_acid * v_acid # 2.5 mmol
mmol_base <- c_base * volume_ml
total_vol <- v_acid + volume_ml
excess <- mmol_acid - mmol_base
pH <- ifelse(
abs(excess) < 1e-9,
7.0,
ifelse(excess > 0, -log10(abs(excess) / total_vol), 14 + log10(abs(excess) / total_vol))
)
# Derivative dpH/dV (forward finite difference; last point set to NA)
dphdv_raw <- c(diff(pH) / diff(volume_ml), NA_real_)
# Scale derivative to primary axis range [0, 12] — leaves headroom below pH 14
max_dphdv <- max(dphdv_raw, na.rm = TRUE)
dphdv_scaled <- dphdv_raw / max_dphdv * 12
df <- data.frame(
volume_ml = volume_ml,
pH = pH,
dphdv_scaled = dphdv_scaled
)
# Equivalence point: 25 mL NaOH, pH 7 for strong acid/base
equiv_vol <- v_acid
equiv_ph <- 7.0
# Secondary axis breaks in dpH/dV units
sec_breaks <- pretty(c(0, max_dphdv), n = 5)
# Title — 42 chars (< 67 baseline), no font-size scaling required
title_str <- "titration-curve · r · ggplot2 · anyplot.ai"
p <- ggplot(df, aes(x = volume_ml)) +
# Steep-slope transition zone (pH 4–10): key region around equivalence
annotate("rect",
xmin = -Inf, xmax = Inf,
ymin = 4, ymax = 10,
fill = IMPRINT_PALETTE[3], alpha = 0.07
) +
# Derivative overlay (scaled to primary axis, dotted)
geom_line(
aes(y = dphdv_scaled, color = "dpH/dV", linetype = "dpH/dV"),
linewidth = 1.0, na.rm = TRUE
) +
# Main titration curve
geom_line(
aes(y = pH, color = "pH", linetype = "pH"),
linewidth = 1.5
) +
# Equivalence point: vertical dashed line (capped at y=11 so derivative spike has breathing room)
annotate("segment",
x = equiv_vol, xend = equiv_vol,
y = 0, yend = 11,
linetype = "dashed",
color = INK_SOFT,
linewidth = 0.7
) +
# Equivalence point dot on the curve
annotate("point",
x = equiv_vol,
y = equiv_ph,
shape = 21,
size = 3.5,
fill = IMPRINT_PALETTE[1],
color = PAGE_BG
) +
# Equivalence point text annotation
annotate("text",
x = equiv_vol + 1.3,
y = 1.8,
label = paste0("EP: ", equiv_vol, " mL\npH = ", equiv_ph),
hjust = 0,
size = 3.0,
color = INK_MUTED
) +
# Transition zone label (right side, in empty area above plateau)
annotate("text",
x = 40,
y = 7.0,
label = "Transition\nzone",
hjust = 0.5,
size = 2.8,
color = IMPRINT_PALETTE[3],
alpha = 0.75
) +
# Color + linetype scales (merged into single legend)
scale_color_manual(
name = NULL,
values = c("pH" = IMPRINT_PALETTE[1], "dpH/dV" = IMPRINT_PALETTE[2]),
guide = guide_legend(override.aes = list(linewidth = c(1.5, 1.0)))
) +
scale_linetype_manual(
name = NULL,
values = c("pH" = "solid", "dpH/dV" = "dotted")
) +
# Dual y-axis: pH primary, dpH/dV secondary
scale_y_continuous(
name = "pH",
limits = c(0, 14),
breaks = seq(0, 14, by = 2),
expand = expansion(0, 0),
sec.axis = sec_axis(
~ . * max_dphdv / 12,
name = "dpH/dV (pH/mL)",
breaks = sec_breaks
)
) +
scale_x_continuous(
name = "Volume of NaOH added (mL)",
limits = c(0, 50),
breaks = seq(0, 50, by = 10),
expand = expansion(0.01, 0)
) +
labs(title = title_str) +
theme_minimal(base_size = 8) +
theme(
plot.background = element_rect(fill = PAGE_BG, color = PAGE_BG),
panel.background = element_rect(fill = PAGE_BG, color = NA),
panel.grid.major.x = element_blank(),
panel.grid.major.y = element_line(color = GRID_COLOR, linewidth = 0.3),
panel.grid.minor = element_blank(),
panel.border = element_blank(),
axis.title.x = element_text(color = INK, size = 10),
axis.title.y.left = element_text(color = INK, size = 10),
axis.title.y.right = element_text(color = IMPRINT_PALETTE[2], size = 9),
axis.text.x = element_text(color = INK_SOFT, size = 8),
axis.text.y.left = element_text(color = INK_SOFT, size = 8),
axis.text.y.right = element_text(color = IMPRINT_PALETTE[2], size = 7.5),
axis.line.x = element_line(color = INK_SOFT, linewidth = 0.4),
axis.line.y = element_line(color = INK_SOFT, linewidth = 0.4),
plot.title = element_text(color = INK, size = 12, face = "bold"),
legend.background = element_rect(fill = ELEVATED_BG, color = INK_SOFT, linewidth = 0.3),
legend.text = element_text(color = INK_SOFT, size = 8),
legend.key.width = unit(1.5, "cm"),
legend.position = "inside",
legend.position.inside = c(0.08, 0.88),
legend.justification = c(0, 1),
plot.margin = margin(15, 20, 12, 12)
)
ggsave(
filename = sprintf("plot-%s.png", THEME),
plot = p,
device = ragg::agg_png,
width = 8,
height = 4.5,
units = "in",
dpi = 400
)
Part of Acid-Base Titration Curve on anyplot.ai.